Motion and Forces

What is motion? How can you tell if an object is speeding up or slowing down?

All animations: www.exrx.net

Displacement in Time and Space

Focus Questions

Motion Vocabulary

Balanced Forces Unbalanced Forces Inertia Gravity Friction Force Mass Magnitude v=d/t

Total Distance Total time Motion Position Reference Point Direction Speed Average speed

Motion is a change in position of an object

with respect to time.

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The change in position is measured in the amount of distance an object

has moved from one position

(reference point) to another.

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Position - the location of an object.

Cowpens to Downtown Spartanburg: 14 miles

Examples of units of speed are:

“meters per second” (m/s)“kilometers per hour” (km/h)

and“miles per hour” (mph).

Direction - the relationship of the position of a moving object to

another position.

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Speed is the rate of

change of the position of an object, or how long it takes something to

move a distance.

Speed does not

necessarily mean that

something is

moving fast.

Speed - the distance traveled by an object in one unit of time.

The average speed of an object tells you the (average) time at

which it covers a given distance. While the speed of the object may

vary during the total time it is moving, the average speed is the result of the total distance divided

by the total time taken.

Speed can be calculated by dividing the distance the object travels by the amount of time it takes to travel that

distance. Speed measurements contain a unit of distance divided by a

unit of time.

Average speed can be calculated using the formula

v = d/twhere:

v is the average speed of the objectd is the distance or length of the path of the object

t is the time taken to cover the path

Calculate the average speed of an object in motion:

http://sunshine.chpc.utah.edu/javalabs/java12/fnm/act1/lab.htm

Snowmobile Distance (in km) Time (in hrs) Avg. Speed (in km/hr)

Mangler 500 15.82  

Otter Pop 15 0.45  

Slider 50 1.41  

Snowflake 240 6.38  

White Fang 30 0.75  

We can measure the distance and time of an object in motion. This data can be

represented in a data table.

For example: Time (s) Distance

(m)

0 0

1 5

2 10

3 15

4 15

5 15

6 10

7 5

This data can then be represented on a

time-distance graph

Time-Distance Graph

0

5

10

15

20

0 1 2 3 4 5 6 7

Time (s)

Dis

tan

ce (

m)

This graph can then be used to describe the position, direction and speed of the motion of the

object.

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Time-Distance Graph

0

5

10

15

20

0 1 2 3 4 5 6 7

Time (s)

Dis

tan

ce (

m)

Time-Distance Graph

0

5

10

15

20

0 1 2 3 4 5 6 7

Time (s)

Dis

tanc

e (m

)

Reference Point

Starting Place – Point of Origin

PositionRelative to the reference point (X-axis), the object at position A is 10 meters away, at position B the object is 15 meters away, and at position C the

object is 10 meters away.

A

B

C

Time-Distance Graph

0

5

10

15

20

0 1 2 3 4 5 6 7

Time (s)

Dis

tan

ce (

m)

A

B

C

A

B

C

The direction of the object is described as whether it is “moving away” from or “moving toward” the reference point. If the object is “moving away”

from the reference point, the line will go up (distance increasing) as in position A.

If the object is “moving toward” the reference point the line will go down (distance decreasing) as

in position C. Time-Distance Graph

0

5

10

15

20

0 1 2 3 4 5 6 7

Time (s)

Dis

tan

ce (

m)

A

B

C

The slope of the line can tell the relative speed of the object. When the slope of the line is steep, the speed is faster than if the slope were flatter. When the slope of the line is flatter, the speed is slower.

For example:

Steeper Slope = Faster Speed

Time (s)

Dis

tan

ce (

m)

Flatter Slope = Slower Speed

Time (s)D

ista

nce

(m

)

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Create a data table and graph the following:

Alex and Ed left home at 1:00 PM and walked to the movie theatre which is 2.5 miles away. This took them 60 minutes. The movie lasted two hours. The boys left the theatre and walked an additional 2 miles to the store. This took them 90 minutes because they met up with some friends and talked for a while. They stayed at the store 1 hour and then their dad picked them up to take them home. They arrived home at 7:30 PM.

Hour Time Distance

Activity

         

         

         

         

         

         

         

         

         

         

         

         

         

         

Describing Motion: Newton’s Laws

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

The behavior of all objects can be described by saying that objects tend to "keep on doing what they're doing" unless something interferes.

Newton’s First Law of Motion(also known as Law of Inertia)

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There are two forces that can affect the movement (speed and

direction) of an object. Gravity, which is a property of all matter, is a force that pulls objects toward each other

without direct contact or impact. Objects on Earth are pulled toward the center of Earth and when raised above the surface of Earth, they

fall “down” toward Earth. As objects fall toward Earth, their speed increases at a definite rate.

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Friction is a force that opposes motion. It can slow down or stop the motion of an object. The slowing force of friction always acts in the direction opposite to

the force causing the motion. For example, friction slows or stops the

motion of moving parts of machines. Most tires are designed to increase

friction for better traction on the road.

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Inertia is the tendency of objects to resist any change in motion. It is the tendency for objects to stay in motion if they are moving or to stay at rest if they are not moving unless acted on by an outside force.

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/newtlaws/il.html

Inertia causes a passenger in a car to continue to move forward even though the car stops.

Inertia is why seat belts are so important for the safety of passengers in vehicles.

Inertia is why it is impossible for vehicles to stop instantaneously.

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Inertia is a property of the object;

it is not a force.

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A plThe force of gravity, in combination with the property of inertia,

is responsible for the orbits of moons and planets.

http://liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/orbit/orbit.html

Varying the amount of force or mass will affect the motion of an object. ForceThe greater the force exerted on an

object, the faster an object will move. For example, racecars have very large engines to produce the force needed to move the cars so fast.

The smaller the force, the slower the object will move.

MassThe greater the mass of an object with the

same force exerted on it, the slower the object will move. Less massive objects can move faster with less force.

For example, in football, backfield players who must move faster are often less massive than linemen who do not have to move fast.

A tennis ball vs. bowling ball is another example. The same force on the small mass of a tennis ball will make it move much faster than the same force on the larger mass of a bowling ball.

Forces have a magnitude (strength) and a direction. Think of forces as arrows with the length of the arrow representing the magnitude (strength) of the force and the head of the arrow pointing in the direction of the force. Using such arrows, the resulting size and direction of the force can be predicted.

Forces occur in pairs and can be balanced or unbalanced. They affect the magnitude (speed) (illustrated by the length of the arrow) and direction (illustrated by the direction of the arrow point) of moving objects.

Balanced

Unbalanced

Balanced forces

Balanced forces act on an object in opposite directions and are equal in size as shown in the arrows below. Balanced forces do not cause a change in the magnitude or direction of a moving object. Objects that are not moving will not start moving if acted on by balanced forces. Balanced forces will cause no change in the motion of an object.

Examples:

In a tug of war, if there is no movement in the rope, the two teams are exerting equal, but opposite forces that are balanced.

In arm wrestling, the force exerted by each person is equal, but they are pushing in opposite directions.

Draw each of these forces using arrows

Unbalanced forces

Unbalanced forces are not equal, and they always cause a change in the magnitude and direction of a moving object. When two unbalanced forces are exerted in opposite directions, their combined force is equal to the difference between the two forces and is exerted in the direction of the larger force.

For example, if a soccer ball (small arrow) is kicked as it moves toward a player (long arrow), it will move in the opposite direction because of the force of the kick (smaller arrow to the right of the =) as shown below:

Or, if in a tug of war, one team pulls harder than the other, the rope will move in that direction as shown below:

If unbalanced forces are exerted in the same direction, the resulting force will be the sum of the forces in the direction the forces are applied. For example, if two people pull on an object at the same time, the applied force on the object will be the result of their combined forces (resulting force) as shown below:

When forces act in the same direction, their forces are added. When forces act in opposite directions, their forces are subtracted from each other.

Unbalanced forces cause a nonmoving object to start moving.

and

2nd Law

Newton’s Second Law of Motion states that if an unbalanced force

acts on a body, that body will experience acceleration ( or

deceleration), that is, a change of speed.

One can say that a body at rest is considered to have zero speed, ( a constant speed). So any force that

causes a body to move is an unbalanced force. Also, any force, such as friction, or gravity, that

causes a body to slow down or speed up, is an unbalanced force.

2nd Law

When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N).

One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

2nd Law (F = m x a)

How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?

Write the formula F = m x a Fill in given numbers and units F = 1400 kg x 2 meters per second/second Solve for the unknown

2800 kg-meters/second/second or 2800 N

If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

3rd Law

For every action, there is an equal and opposite reaction.

3rd LawAccording to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

3rd Law

There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

Newton’s 3rd Law in Nature Consider the propulsion of a

fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.

The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

3rd Law

Flying gracefully Flying gracefully through the air, through the air, birds depend on birds depend on Newton’s third Newton’s third law of motion. As law of motion. As the birds push the birds push down on the air down on the air with their wings, with their wings, the air pushes the air pushes their wings up their wings up and gives them and gives them lift.lift.

Other examples of Newton’s Third Law The baseball forces

the bat to the left (an action); the bat forces the ball to the right (the reaction).

3rd Law

Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

3rd Law

The reaction of a rocket The reaction of a rocket is an application of the is an application of the third law of motion. third law of motion. Various fuels are burned Various fuels are burned in the engine, producing in the engine, producing hot gases. hot gases.

The hot gases push The hot gases push against the inside tube of against the inside tube of the rocket and escape out the rocket and escape out the bottom of the tube. the bottom of the tube. As the gases move As the gases move downward, the rocket downward, the rocket moves in the opposite moves in the opposite direction.direction.