force and motion - mrs. bhandari's grade 7 science€¦ · the motion. friction force...
TRANSCRIPT
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Force and Motion How is it moving? So…what is motion?
So…what is motion? So…what is motion? In math terms…
• A “change in position over time” is the same as saying:
How can we describe motion?• Motion can be described by:
– DISTANCE (how far did it travel?)
– TIME (how long did it travel?)
– SPEED (how fast did it travel?)
– DIRECTION (which way did it go?)
– ACCELERATION (does the motion change?)
What does “speed” mean?
• Some examples of speed:
60 miles/hour 100 meters/minute
• Let’s break it down…
If you travel 60 miles per hour, how far do you travel in 1 hour?
• 60 miles/hour is the same as 60 miles
1 hour
Let’s practice calculating speed
• If you travel 100 km in 2 hours, what is your speed?
• Speed = Distance
Time
• Distance = 100 km Time = 2 hours
• Speed = 100 km
2 h
• Speed = 50 km/h
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Try it out!• Time yourself moving across the 5 meter
tracks on the floor.
• What is the DISTANCE?
• What is your TIME?
• Speed = Distance
Time
What is your SPEED?
Speed, Distance & Time
Speed = Distance
Time
Time = Distance
Speed
Distance = Speed x Time
S
D
T
Speed Practice Problems
A family takes a car trip heading northeast from Durham, NC to Washington, DC. They travel for 4 hours and cover 360 km.
What was their average speed?
Speed = Distance
Time= 360 km
4 h
= 90 km/h
Speed vs. Velocity
SPEED – tells you have fast or slow something is moving (changing position).
Example = 25 km/h
VELOCITY – tells you speed AND DIRECTION! (changing position in a certain direction)
Example = 25 km/h EAST
Speed Practice Problems
A family takes a car trip heading northeast from Durham, NC to Washington, DC. Their speed was 90 km/h. What was their VELOCITY?
Velocity is SPEED and DIRECTION!
Velocity = 90 km/h NORTHEAST
Speed Practice Problems
After school, your teacher went for a jog along Cornwallis Rd. She ran for 30 minutes at a speed of 150 m/min. How far did she run?
Distance = Speed x Time
Distance = 150 m/min x 30 min
Distance = 4500 meters (4.5 km)
S
D
T
Speed Practice Problems
Mr. Sawyer goes on a long bike ride in the country. He rides his bike 35 km at a speed of 20 km/h. For how long was he riding his bike?
Time = Distance
Speed
Time = 35 km
20 km/h
S
D
T
= 1.75 h
Acceleration Practice Problems
Ms. Litwak buys a new car that can accelerate from rest (0 m/s) to 24 m/s in 8 seconds. What is the car’s rate of acceleration?
Acceleration = Final speed – Initial SpeedTime
Acceleration = 24 m/s – 0 m/s8 s
= 3 m/s2
What was the average speed of the runner in the first 5
seconds of the race?
Speed = 25 m5 s
Speed = 5 m/s
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A snail slowly slithers down the sidewalk. It travels at a speed of 3 cm/min. How far would it travel in 20 minutes?
Distance = Speed x Time
Distance = 3 cm/min x 20 min
Distance = 60 cm
The fastest man on earth, Usain Bolt, runs at a speed of 10 m/s. How long would it take him to run 160 meters?
Time = Distance
Speed
Time = 160 m
10 m/s
Time = 16 seconds!
Acceleration
Acceleration
• Acceleration is a CHANGE in motion
• An object is accelerating if it is:
– Speeding up
– Slowing down
– Changing direction
Using an Accelerometer
• An accelerometer measures acceleration.
• CAREFULLY push your accelerometer so it slides across the table and comes to rest.
• How does the paper clip show acceleration?
• In your notebook, complete the chart for each situation.
Taking a ride on the school bus…
• YOU act like an accelerometer when you ride the school bus.
• What happens to you,
when the bus:– Speeds up suddenly
– Turns a corner
– Stops quickly at a red light
– Rides along at
a steady speed
Taking a ride on the school bus…
• When your MOTION CHANGES, that’s a sign that the bus is ACCELERATING.
Is it accelerating?
• Your car speeds up when the light turns green.
• A racecar goes at a constant speed around a curved track.
• A toy car moves in a straight line across the room at a steady speed of 0.5 m/s.
• A roller coaster car slows down as it climbs a hill.
Graphing Acceleration
• On a position-time graph, changes in speed are shown by curved lines.
Straight line – Not
accelerating!
Curving up –
Speeding up
Curving down –
Slowing down
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• Look carefully at each car
below.
• Decide which car matches
graphs A, B, and C.
Graph B – Constant Speed
Graph C – Accelerating Slowly
Graph A – Accelerating Quickly
Calculating Acceleration
• Acceleration is the change in speed per unit of time.
• Acceleration is measured in units of meters per second per second, or m/s2.
Acceleration = Final speed – Initial Speed
Time
Calculating Acceleration
• A car leaving traffic changed its speed from 10 m/s to 25 m/s in 7.5 seconds. What was its acceleration?
Acceleration = Final speed – Initial Speed
Time
Acceleration = 25 m/s – 10m/s
7.5 s= 2.0 m/s2
Introduction to
FORCES
FORCES
When you ride a bike, your foot PUSHES against the
pedal. The push makes the wheels of the bike move.
When you drop something, it is PULLED to the ground
by gravity.
A FORCE is a PUSH or PULL in a
particular DIRECTION.
Forces can affect motion in the following ways:
They can make objects:
i) START MOVING
ii) MOVE FASTER
iii) MOVE SLOWER
iv) STOP MOVING
v) CHANGE DIRECTION
vi) CHANGE SHAPE
FORCESFORCES AFFECT HOW OBJECTS MOVE.
BIG
SCIENCE
IDEA
FORCESIdentify each picture as a PUSH or a PULL. Is the
force causing a change in speed or direction or both?
FORCES
Since forces cause changes in SPEED
or DIRECTION of an object, we can say
that forces change VELOCITY, so….
Forces cause
ACCELERATION.
FORCES
More than one force can act on an object
at one time. What happens to the object
when forces act depends on 2 things:
1) Strength of the Forces
2) Direction of the Forces
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FORCES
When 2 or more forces act on an object,
the forces combine to form a net force.
or OPPOSE each other.
Forces may WORK TOGETHER
FORCESIf the forces cancel each other out, and do
not cause the object to move, the forces
are said to be BALANCED.
If the forces don’t cancel each other out – 1
force is stronger than the others – the forces
are UNBALANCED and will cause a
CHANGE IN MOTION.
MEASURING
FORCE
The strength of a force is
measured in NEWTONS.
The symbol is (N).
We use a SPRING SCALE
to measure force.
MEASURING
FORCE
- Always “zero” your balance
before use.
- Pull gently and with
constant force.
-Practice using your spring
scale to drag items across
your desk.
COMBINING
FORCES
Two forces in the same direction can add
together to produce a larger net force.
5 N
right
5 N
right
+ =
10 N
right
COMBINING
FORCESTwo forces in opposite directions can
subtract to produce a smaller net force in
the direction of the larger force.
5 N
right10 N
left
- =
5 N
left
COMBINING
FORCESTwo forces may cancel each other out (if
equal and opposite) to produce NO NET
FORCE.
5 N
right
- =
5 N
left
0 N
(No Net
Force)
Circle the best answer:
1) The forces shown above are PUSHING / PULLING forces.
2) The forces shown above are WORKING TOGETHER / OPPOSITE FORCES.
3) The forces shown above are EQUAL / NOT EQUAL.
4) The forces DO / DO NOT balance each other.
5) The net force is 1000 N TO THE RIGHT / 1000 N TO THE LEFT / ZERO.
6) There IS / IS NO motion.
Circle the best answer:
7) The forces shown are PULLING / PUSHING forces.
8) The forces shown are WORKING TOGETHER / OPPOSITE FORCES.
9) The forces shown are EQUAL / NOT EQUAL.
10) The forces DO / DO NOT balance each other.
11) The stronger force is pulling RIGHT / LEFT.
12) Motion is the to the RIGHT / LEFT.
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13) Two movers are trying to move a heavy box. One mover pushes to
the right with a force of 150 N. The other mover pushes to the left
with a force of 200 N.
a) Draw & label the forces on the diagram.
b) What is the net force? 50 N LEFT
c) Will the box move? YES
d) If yes, in what direction? LEFT
150 N 200 N
50 N NET FORCE 14) Two movers are trying to move a heavy chair. One mover PULLS to the left with a force of 200 N. The other mover PUSHES to the left with a force of 200 N.
a) Draw & label the forces on the diagram.
b) What is the net force?
400 N LEFT
c) Will the chair move?
YES
d) If yes, in what direction?
LEFT200 N 200 N
400 N NET FORCE
15) Four children are fighting over the
same toy. Mike is pulling North with
a 50 N force, Justin is pulling East
with a 40 N force, Chantal is pulling
South with a 50 N force, and Tykera
is pulling West a 30 N force.
a) Draw & label the forces on the
diagram.
b) Is there a net force on the toy?
YES = 10 N EAST
c) In which direction will the toy
move?
EAST
d) Who gets the toy?
JUSTIN
50
N
50
N
30 N 40 N
10 N
Net Force
MIKE
JUSTIN
CHANTAL
TYKERA
FRICTION
FRICTION
• What will happen when the ball is released?
• When the ball reaches the bottom of the slope, will it keep moving forever?
Since the ball stops, there must be a force acting to slow the ball down.
The force that slows the ball to a stop is FRICTION.
What is Friction?
• Friction is a force that two surfaces exert on each other when they rub against each other.
• The direction of the friction force is always OPPOSITE to the direction of the motion.
Friction Force
Direction of Motion
Types of Friction
• Static Friction opposes the motion of an object that is at rest
• To make the object move, you have to exert a force larger than the force of static friction.
Direction of Intended Movement
Static Friction Force
Types of Friction
• Sliding friction occurs when two solids slide over each other.
• Sliding friction makes car brakes work and stops athletes from slipping.
Direction of Slide Sliding Friction Force
Types of Friction
• Rolling friction occurs when an object rolls across a surface.
• Rolling friction is easier to overcome than sliding friction for the same materials.
Friction Force
Direction of Motion
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Types of Friction
• Fluid friction occurs when a solid object moves through a liquid or gas.
• Air resistance is a type of fluid friction.
Friction Force
Direction of Motion
Which type of friction is slowing down the object in each situation?
• You are slipping down a waterslide at Emerald Pointe.
• You use a lot of force to slide a desk across the floor.
• You’re riding a skateboard down the street and it slowly rolls to a stop.
• You try to push the couch, but can’t seem to move it. Friction Force
SLIDING FRICTION
ROLLING FRICTION
FLUID FRICTION
STATIC FRICTION
Friction Thought Questions• Why would you add oil to a rusty bike
chain?
• Why would you add sand to an icy driveway or road?
• Why is it easier to move heavy furniture using a handcart rather than pushing it?
• Why would a shoe company be interested in studying friction?
• What would happen if we repeated the tug-of-war and one team had only socks on?
Review of Friction Forces
Review - What is Friction?
• Friction is a force that two surfaces exert on each other when they rub against each other.
• The direction of the friction force is always OPPOSITE to the direction of the motion.
• It SLOWS down moving objects!
Friction Force
Direction of Motion
What factors affect Friction?
• In your homework, you found out 2 factors affect friction:
– Types of surfaces involved
– How hard the surfaces push together
• Today, we’re going to investigate DIFFERENT SURFACES to see which ones create the most friction.
CARPET SANDPAPER DESK
What factors affect Friction?
• In your homework, you found out 2 factors affect friction:
– Types of surfaces involved
– How hard the surfaces push together
• Today, we’re going to investigate DIFFERENT SURFACES to see which ones create the most friction.
CARPET SANDPAPER DESK
Conclusions Questions1. Which surface material created the
MOST frictional force?
2. Which surface material created the LEAST frictional force?
3. Give an example of a situation where we use a certain surface material to:
a) REDUCE friction
b) INCREASE friction
GRAVITY& AIR RESISTANCEThe physics of falling
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The Force of Gravity
• Gravity is the force that pulls all objects down to the earth.
• Rain falls from the sky down to earth…
• If you drop a book, it falls to the ground…
• If you trip, you’ll fall down…
• Actually in science, gravity is a force ofattraction that acts between ALL objects(the earth, you, the desk, a book)
• The force of gravity is much STRONGERfor LARGER objects (more mass).
Universal Gravitation Universal Gravitation• Because the Earth is by far, the largest
and closest object around, it has the greatest force of attraction...
• So, no matter where you are on earth, all things fall to the ground due to gravity…
What is “free fall”?
• When gravity is the ONLY forceacting on an object, it is in free fall.
• In that case, gravity is an UNBALANCED FORCE which causes the object to accelerate.
Acceleration due to Gravity
• Calculate the acceleration of an object in free fall.
A = Final speed – initial speed
time
A = 50 m/s – 0 m/s
5 s
Acceleration = 10 m/s2
Objects in Free Fall
• Do all objects fall at the same rate?
• If we dropped a bowling ball and a tennis ball from the same height, which would land first?
• Let’s try it!
Mass and Gravity Oct. 24, 2008
Question: How does mass affect the speed of a falling object?
Hypothesis: (What do you think will happen AND WHY?)
Observations & Data Collection:Repeat each trial twice and record your observations.
Ping pong ball vs. Wooden ball:
Wooden ball vs. Metal ball:
Ping pong ball vs. Metal ball:
Conclusion: (One sentence)
Objects in Free Fall
• Do all objects fall at the same rate?
• ALL objects in free fall travel at the same rate, regardless of mass!
• In free fall, heavy objects and light objects fall at the same rate!
So, which will land first?• WHY does the penny
land first?
• Remember the force that opposes motion (slows things down)? FRICTION!
• Falling objects experience friction with the air called AIR RESISTANCE that slows them down.
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Air Resistance
• The larger the object (more surface area), the more air resistance.
• That’s why parachutes work! The upward force of the air acting on the LARGE parachute slows you down as you fall.
Air Resistance
• Draw a diagram showing the forces…
• Downward force of gravity is same on both.
• Upward force of air resistance is greater on the feather.
• The net force (down) is greater on the
penny.
Air
Resistance
Gravity
Net Force on Feather
Net Force on Penny
Without air resistance, all objects would fall at the same rate…
Galileo Drops the Ball
Hammer and Feather Drop on the Moon
Gravity on the moon?• The force of gravity is much weaker on
the moon because…– It is much farther away from earth.
– The moon is much smaller than earth.
That’s why astronauts weigh less on the moon!
Gravity Review
• Gravity is the force that pulls all objects down to the earth.
• When gravity is the ONLY force acting, ALL objects accelerate at a rate of 10 m/s2.
• Mass doesn’t matter – in free fall, heavy objects and light objects fall at the same rate!
Air Resistance Review
• Some objects take longer to fall – they are slowed down by FRICTION with the air called AIR RESISTANCE.
• The larger the surface area, the greater the force of air resistance pushing up.
• Without air resistance, all objects would fall at the same rate…
• Draw a free body diagram of the sky diver and label ALL the forces.Gravity = 1000 N
Air Resistance = 800 N
• What is the net force?
Net Force
= 200 N
Air
Resistance
= 800 N
Gravity =
1000 N
Newton’s 1st Law
Newton’s 1st Law of Motion
• After the ball is kicked, what forces are acting on it while it rolls?
• What if we could remove those forces? What would happen then if we kicked the ball?
Friction
GRAVITY
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Newton’s 1st Law of Motion
• Newton’s 1st law of motion states:
An object at rest will remain at rest,
-and-
an object moving at a constant velocity will continue moving at a constant velocity,
-UNLESS-
it is acted upon by an unbalanced force.
Newton’s 1st Law of Motion
That’s because larger objects have more inertia (more resistance to a change in
their motion)!
Objects resist any change to their motion!
This resistance is called INERTIA.
Which one would be easier to push?
or…
BASICALLY…
Newton’s 1st Law of Motion
• Unfortunately, your bed really doesn’t make itself…
• And dirty clothes won’t pick themselves up off the floor!
Make your bed!Do the laundry!
• That’s because things at rest will stay at rest until an unbalanced force (like your arms lifting the sheets) acts on them.
Newton’s 1st Law of Motion
• Inside a moving vehicle, everything is moving at the same velocity – your body, the objects in the car, and the car itself.
• What would happen to the things inside the car if the car hit a wall?
• That’s because things moving at a constant velocity will stay at a constant velocity unless acted upon by an unbalanced force.
• The car was acted upon by an outside force – the force of the wall hitting the car.
• The person inside the car wasn’t hit by the force of the wall, so he kept moving at the same speed and in the same direction.
Newton’s 1st Law of Motion
• Explain this animation:
• The truck is stopped by the force of the impact with the car, but the ladder continues to move at its original speed and in its original direction because of inertia.
Newton’s 1st Law of Motion
• So, how did the tablecloth demo work?
• The cloth experienced a pulling force that caused it to start moving. The dishes did not have a direct force applied to them, so they remained in their places because of inertia.
Newton’s 1st Law of Motion
• Following the instructions on your sheet, do each of the activities at your desk.
• In your notebook, write the title of each activity and explain in at least two complete sentences how it works, using Newton’s 1st Law. Newton’s 2nd Law
Let’s review Newton’s 1st Law
• This law deals with situations where forces are balanced…
• When forces are balanced, objects resist any changes in their motion… that’s called INERTIA.
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Newton’s 1st Law of Motion
• If no one kicks the ball, what will happen?
• Objects at rest will remain at rest…
Newton’s 1st Law of Motion
• If there was NO FRICTION (no unbalanced force) to slow down the ball, what would happen?
• Objects in motion will remain in motion …
Newton’s 1st Law of Motion
• The ladder’s inertia keeps it moving forward after the unbalanced force (car) stops the truck…
Newton’s 2nd Law of Motion• Newton’s 2nd law of motion describes how
UNBALANCED FORCES and MASS affect the ACCELERATION of an object.
• Let’s try it!
Test 1 – Increasing Force• Small Force - Blow through the straw lightly
toward the marble. Observe the marble’s motion.
• Large Force – Blow the straw with more force toward the marble. Observe the marble’s motion.
How does increasing the force affect the marble’s acceleration?
Test 2 – Increasing Mass• Small Mass (marble) - Blow through the straw
as hard as you can. Observe the marble’s motion.
• Large Mass (golf ball) – Blow the straw as hard as you can. Observe the golf ball’s motion.
How does increasing the mass affect the the object’s acceleration?
So, how do unbalanced forces affect an object’s motion?
• How does a batter’s swing affect the acceleration of a baseball?
Sacrifice Bunt Homerun Hit
So, how do unbalanced forces affect an object’s motion?
• The harder you hit, the faster it goes!
• The greater the force, the greater the acceleration…
Sacrifice Bunt Homerun Hit
So, how does mass affect an object’s motion?
• Which shopping cart would move faster with a single push?
Empty Cart Full Cart
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So, how does mass affect an object’s motion?
• The fuller the cart, the slower it goes!
• The more mass, the less acceleration…
Empty Cart Full Cart
Newton’s 2nd Law of MotionIt states:
ACCELERATION depends on the object’s MASS, and the net FORCE acting on the object.
• We can also write it mathematically:
Force = Mass x Acceleration
Newton’s 2nd Law
Force = Mass x Acceleration
M F A
If you raise the mass but keep force the same,
acceleration will decrease.
If you lower the mass but keep force the same,
acceleration will increase.
If you want more acceleration with the same force, you must decrease
the mass.
If you want less acceleration with the same force, you must increase
the mass.
Newton’s 2nd Law
Force = Mass x Acceleration
If you want more acceleration with the same mass, you must increase
the force.
If you want less acceleration with the same mass, you must decrease
the force.
Newton’s 2nd Law
Force = Mass x Acceleration
If you raise the mass but want the same
acceleration, you must increase the force.
If you lower the mass but want the same
acceleration, you must decrease the force.
Newton’s 2nd Law of Motion
Force = Mass x AccelerationMr. Sawyer’s car ran out of gas. How much force does Mr. Sawyer need to push his 750kg car at an acceleration of 1 m/s2?
F = m x a
F = 750 kg x 1 m/s2
F = 750 N right
1 m/s2750 kg
Try one on your own…
Ms. Litwak’s van runs out of gas. How much force does she need to push the 2000kg van at an acceleration of 0.5 m/s2?
F = m x a
F = 2000 kg x 0.5 m/s2
F = 1000 N right
0.5 m/s2
Speed, Distance & Time
We can also write the formula like this:
Acceleration = Force
Mass
Mass = Force
Acceleration
Force = Mass x Acceleration
m
F
a
Newton’s 2nd Law of Motion Newton’s 2nd Law of Motion
Find the golf ball’s acceleration.
a) The putter hits the 0.05 kg golfball with a force of 1 N.
b) The driver hits the 0.05 kg golfball with a force of 8 N.
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Newton’s 2nd Law of Motion
• Use Newton’s 2nd law of motion to explain in words the difference in the motion of the golf balls.
Newton’s 3rd Law
Which forces are acting to get this guy up in the air?
His feet push DOWN on the ground.
• But wait… His downward push can’t be causing his upward motion.
Which forces are acting to get this guy up in the air?
His feet push DOWN on the ground.
• There must be a force pushing UP!• The force of the ground pushes him UP!
The ground pushes UP on the man.
Newton’s 3rd Law of Motion
• Newton’s 3rd law says that: For every action force, there is an equal and opposite reaction force.
Action Force: Man’s feet push DOWN on the ground.
Reaction Force: Ground pushes UP on the man.
ALL forces act in PAIRS!
Let’s Demonstrate…
• Stand up and face a partner with your palms touching.
• Push on your partners hands. Don’t move your feet. The first partner to step back loses…
• How can Newton’s third law explain what happens?
Newton’s 3rd Law of Motion
• Explain this animation using Newton’s third law.
Newton’s 3rd Law of Motion
• The man’s foot exerts a backward push on the boat (action force), while the boat exerts a forward push on the man (reaction force).
A PAIR OF FORCES:EQUAL FORCES, BUT
IN OPPOSITE DIRECTIONS
Let’s Demonstrate…
• Two people (the same size) in rolling chairs face each other with their feet touching.
• Only 1 student pushes. What will happen?
• How can Newton’s third law explain what happens?
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Identify the force pairs in each situation
•A person fires a rifle.
•Action Force – gun pushes the bullet out at high speed.
•Reaction Force – the bullet pushes back on the gun (recoil).
Identify the force pairs in each situation
•A space shuttle lifts off.
•Action Force – engine pushes gases down & out.
•Reaction Force – the gases push the rocket up.
*This upward force must be stronger than gravity pulling down on the rocket!
Identify the force pairs in each situation
•A person stands still.
•Action Force – gravity pulls the person down to the floor.
•Reaction Force – the floor pushes up on the person.
*You don’t need MOTION for force pairs. They are everywhere!
Let’s Demonstrate…
• The toy clackers work 2 different ways… (watch your teacher)….
• How can Newton’s third law explain what happens?
If forces are equal and in opposite directions, why don’t they cancel out (and balance)?
•Forces only cancel if they act on the same object.(Think about a tug of war –all forces act on the rope).
•These forces are acting on different objects!
These forces are acting on different objects!
•Action Force – Rocket engine pushing on gases.
•Reaction Force – Gases push on the rocket.
Think about it . . .
• Why does it hurt so much when you stub your toe?
• When your toe exerts a force on a table, the table exerts an equal force back on your toe.
• The harder you hit your toe against it, the more force the the table exerts back on your toe (and the more your toe hurts).
What is Momentum?• Momentum is a measure of how much motion
object has.
• It is affected by mass and velocity. The heavier an object is, the more momentum it has.
• It’s easier to stop soccer ball coming towards you at 20 m/s than a car coming at 20 m/s.
• It’s easier to stop car travelling at 1 km/h than a car travelling 60 km/h!
Conservation of Momentum• When objects collide, their total momentum is
conserved (stays the same), unless outside forces act.
• The total amount of motion coming into a collision will also come out of the collision.
1. Place 4 coins in a row, touching each other.
2. Place the 5th coin about 2 inches away from the end of the row, keeping it in line.
3. Lightly flick your finger forward, propelling the single coin against the others.
4.What do you observe?
5.Try it again, flicking 2 coins into a row of 3 coins. What do you observe?
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MomentumMomentum can be calculated using this formula:
Momentum = mass x velocity
A golf ball with a mass of 0.05 kg travels at 16 m/s.
A baseball with a mass of 0.15 kg travels at 7 m/s.
Which ball has the greater momentum?
Golf ball’s momentum = 0.05 kg x 16 m/s
Baseball’s momentum = 0.15 kg x 7 m/s
= 0.8 kg m/s
= 1.05 kg m/s
Work, Power & Simple Machines(Making work easier…phew!)
What is WORK?
If you put a lot of effort into doing something and are worn out at the end, you think you’ve done a lot of WORK, right?
Not necessarily….
If you haven’t exerted a force AND moved an object some distance, you haven’t done any WORK at all!
What is WORK?
In scientific terms, you do WORK when you exert a FORCE that causes an object to move some DISTANCE in the SAME DIRECTION of the force.
Examples:
– Pushing a lawn mower
– Lifting books out of your bag
– Pulling a suitcase on wheels
What 2 things must happenfor WORK to be done?
MOTION – The object must move.
If the object doesn’t move,
there is no work done.
FORCE & MOTION IN THE SAMEDIRECTION
Movement must be in the same direction as force.
If the motion is in a different direction than the force, there is no work.
Is WORK being done?
Pushing a car that’s stuck in snow.
NO! (No work because the car doesn’t move).
Lifting a baby out of his stroller.
YES! (Baby moves in same direction as you lift)
Carrying your bookbag to class.
NO! (Force is pulling up, but motion is sideways)
Pushing a lawn mower.
YES! (Mower goes in same direction as you push)
Calculating WORK
WORK = FORCE x DISTANCE
The greater the distance, the more you work.
Eg. Pushing a car 100 m vs.
Pushing a car 200 m (more work!)
The greater the force, the more you work.
Eg. Lifting 1 book onto a table vs.
Lifting 10 books onto a table (more work!)
Calculating WORK
You carry a baby that weighs 30 N upstairs to his room (3 meters above you). How much work is done?
WORK = FORCE X DISTANCE
WORK = 30 N x 3 meters
WORK = 90 N·m (90 J)
Work is measured in Joules (J)
1 Joule = 1 N·m
What is POWER?
Power is the rate at which work is being done (or how much work is being done in a unit of time).
POWER = WORK ÷ TIME More power means less time to do the
same work OR more work done in the same amount of time.
Power is measured in Watts (W).
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POWER
Think about 2 cars – one with a 200 horsepower engine, and one with a 500 horsepower engine.
Which one has the more powerful
engine?
Which one will go further in 10 minutes?
Which one will go 10 miles in the shortest amount of time?
Calculating POWER
A motor exerts a force of 2,000 N to lift an elevator 8.0 m in 4.0 seconds. What is the power of the motor?
Power = Work = Force x Distance
Time Time
Power = 2,000N x 8 m
4 s
Power = 4,000 J/s (4,000 Watts)
= 16,000 J
4 s
What are MACHINES?
Most people think of complex, automated, technical, or electronic gadgets with motors…, but
machines can be much simpler.
A machine is any device that lets you do WORK in an easier or more effective way.
How do Machines do work?
Machines make work easier by changing3 things about the FORCE you exert to do work:
AMOUNT of force you exert
DISTANCE over which you exert force
DIRECTION in which you exert force
How do Machines work?
In other words, a machine changes the strength, distance, directionof your push or pull.
What is the mechanical advantage of a machine?
A machine’s mechanical advantage is the number of times a machine
increases a force exerted on it.
Mechanical = Output ForceAdvantage Input Force
What is the mechanical advantage of a machine?
You exert 10 N of force on a can opener. The can opener exerts 30 N
of force on the can. What is the mechanical advantage?
Mechanical = Output Force = 30 NAdvantage Input Force 10 N
Mechanical Advantage = 3
What are SIMPLE MACHINES?
There are only 6 basic kinds of simple machines that make work
easier.
These 6 simple machines make up all the other compound machines we use everyday.
SIX SIMPLE MACHINES
The six simple machines are:
Inclined Plane
Wedge
Screw
Lever
Wheel & Axle
Pulley
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INCLINED PLANE
An inclined plane is a flat, sloped surface connecting a lower level with a higher level.
INCLINED PLANE
It lets you use less force over a longer distance to raise a load to a higher level.
Input Force
Output Force
INCLINED PLANE:Examples
Ramps (Boat ramps, wheelchair ramps)
Propeller
Ladders/Stairs
WEDGE
A wedge has slanting slides that taper to a thin edge – it splits material apart. (A moving inclined plane!)
It converts motion in one direction, into a splitting motion that acts at right angles to the blade.
WEDGE
Input Force
Output ForceOutput Force
WEDGE:Examples & Uses
Ax, Knife, etc.
Zippers
Used in all cutting machines (to split materials apart)
Lifting machines may use wedges to slide under loads
SCREW
A screw has a “thread” or “groove” wrapped around a central cylinder. (Another inclined plane - wrapped around a cylinder!)
SCREW
While turning, it converts a twisting motion into a forward or backward motion.
Input Force
Output Force
SCREW:Examples & Uses
Screws can holds things together or lift materials.
Screws
Screw top lids for jars/bottles
Light bulb
Swivel stools/chairs
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LEVER
A lever is rigid bar that pivots/rotates on a fixed point. The fixed point is called the “fulcrum”.
LEVER
Levers may increase the size or distance of force or change direction of the force.
There are 3 types of levers.
LEVERS:Examples & Uses
First Class Levers:– Scissors, See-saws, Pliers
Second Class Levers:– Staplers, Nutcrackers,
Wheelbarrows
Third Class Levers– Shovels, baseball bats, tweezers
WHEEL & AXLE
A wheel and axle are 2 circles or cylinders attached together around a common axis.
The larger circle is the “wheel”, the smaller cylinder/rod is called the “axle”.
WHEEL & AXLE
The wheel is locked to the central axle –when one turns, so does the other one.
A short powerful force at the axle, will move the wheel’s edge a long distance.
A long motion at edge of wheel, moves the axle with great force.
Input
Force
Output
Force
Output
Force
Input
Force
WHEEL & AXLE:Examples & Uses
Screwdriver
Windmill
Cars/Bicycles
Rolling Pin
Door Knob
Fan
PULLEY
A pulley is a grooved wheel with a rope, used to raise/lower/move a load.
PULLEY
A simple fixed pulley only changes the direction of force.
Pulley systems decreases the input force, allowing you to move heavier loads.
Output
Force
Input
Force
Output Force
Input
Force
PULLEY:Examples & Uses
Cranes
Raising a flag on a pole
Window Blinds
Raising a sail on a boat
Clothesline
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COMPOUND MACHINES
Most machines are combinations of 2 or more simple machines.
For example, a simple can opener is a combination of 3 simple machines:
– Lever
– Wheel & axle
– Wedge
Simple Machines(Making work easier…phew!)
Machines make work easier by changing 3 things about the FORCE:
The amount of force
The distance of the force
The direction of the force
Machines make work easier by changing 3 things about the FORCE:
The amount of force(eg. A ramp lets you lift a heavy object
with LESS force)
Machines make work easier by changing 3 things about the FORCE:
The distance of the force(eg. A baseball bat lets you move your
arms a short distance, but move the end of the bat a large distance).
Machines make work easier by changing 3 things about the FORCE:
The direction of the force(eg. The pulley on a set of window blinds
lets you move the blinds UP with a DOWNWARD pull.
What is the mechanical advantage of a machine?
A machine’s mechanical advantageis the number of times a machine increases a force exerted on it.
Mechanical = Output ForceAdvantage Input Force
What is the mechanical advantage of a machine?
You exert 10 N of force on a can opener. The can opener exerts 30 N of force on the can. What is the mechanical advantage?
Mechanical = Output Force = 30 N
Advantage Input Force 10 N
Mechanical Advantage = 3
What is the efficiency of a machine?
The EFFICIENCY compares:
– the work you put IN to
– the work the machine puts OUT.
An IDEAL machine is 100% efficient.INPUT WORK = OUTPUT WORK
In the real world, some input work is always lost due to FRICTION between the moving parts of the machine.
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What is the efficiency of a machine?
EFFICIENCY = Output Work x 100%Input Work
You mow the lawn with a rusty lawn mower. You do 50,000 J of work on the lawn mower but only 25,000 J go to cutting the lawn. What is the efficiency of the lawn mower?
What is the efficiency of a machine?
You mow the lawn with a rusty lawn mower. You do 50,000 J of work on the lawn mower but only 25,000 J go to cutting the lawn. What is the efficiency of the lawn mower?
EFFICIENCY = Output Work x 100%Input Work
Efficiency = 25,000 J x 100%
50,000 J
Efficiency = 50%
Try the rest of the practice problems on your own…
Mechanical = Output ForceAdvantage Input Force
EFFICIENCY = Output Work x 100% Input Work
The Physics of
Rollercoasters
The Early Days of Rollercoasters
The first American
rollercoaster, the
Mauch Chunk
Switchback Railway,
c. 1870
Length: 40 miles
Speed: over 100 mph
Wooden Coasters
Racer at King’s Island
1972
Length: 3415 feet
Speed: 61 mph
Steel Coasters
Revolution at Magic Mountain
1977
First Loop of
the Modern Era
Classic RollercoastersViper at Magic Mountain
1990
3 loops, 4 corkscrews
Speed: 73 mph
Suspension Coasters
Batman The Ride
Magic Mountain
1994
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Stand-Up Coasters
Riddler’s Revenge
Magic Mountain
1998
Tallest, Fastest
Stand-Up Coaster
SupermanSuperman
Magic Mountain
1997
100 mph
415 feet tall
Six Flags Great
Adventure
New Jersey
2005
Speed: 128 mph
Height: 456 feet
Kingda Ka
Rollercoaster Forces
• There are two main
forces which need to be
considered when
designing rollercoasters:
• Gravity
• Friction
How Rollercoasters Work
• Rollercoasters are
driven by gravity
• There is no engine
attached to the cars
• The cars are pulled to
the top of a hill and
released
Potential and Kinetic Energy
• Every rollercoaster relies on conservation of
energy
• At the top of the hill, the rollercoaster has
potential energy
• At the bottom of the hill, the potential
energy has been converted into kinetic
energy
Potential Energy
• Potential Energy = mass x gravity x height
• Heavier objects have more potential energy
• The higher an object, the greater its potential
energy
Kinetic Energy
• Kinetic Energy = 1/2 x mass x velocity2
• Heavier objects have more kinetic energy
• The faster an object is moving, the greater
its kinetic energy
Hills
• K.E. = 1/2 mv2, P.E. = mgh
• When cars go up a hill, their height
increases and their velocity decreases
• When cars go down a hill, their height
decreases and their velocity increases
• A rollercoaster can never go higher than the
top of the first hill
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Loops
• Loops are treated like hills, with one
difference
• Cars must have enough energy to reach the
top of the loop
• Cars must have a certain speed at the top of
a loop so that they don’t fall