Motion, Speed, Acceleration
Chapter 8
Motion
When you are riding in a car, why don’t the people in the car look like they are moving? Why do the things outside the car look like
they ARE moving? Are YOU moving right now?
Are you sure? When riding in a car, some people go
around you and it looks like you’re going backwards. Why?
Motion
Motion is a change in RELATIVE position: Motion requires a reference point, usually
stationary. Distance measure the path taken:
To measure how far you went, you have to follow the actual path
Displacement is the distance moved, regardless of path Straight line from start to finish
Speed
Speed measures how fast a motion occurs Requires distance and time Speed = distance/time
Miles per hour (mph = miles/hr) Feet per second (feet/second) PER means divided by (usually using time)
SI unit of measure for speed is meters per second (m/s)
Constant speed means same distance is covered in the same amount of time.
Speed
Graphing Speed
Graph Time on X-axis Graph Distance on Y-
axis Slope of line is speed
Assumes constant speed
Faster speed = more slope to line
Slower speed = less slope to line
Average Speed
Average speed is calculated based on starting and ending points (distance) and total time elapsed. Used to describe objects that don’t move at constant
speed Exercise: You travel 100 miles in 50 minutes, then 50
miles in 50 minutes, then 50 miles in 100 minutes. What is your average speed?
Answer: Distance is 100 + 50 + 50 = 200 miles Time is 50 + 50 + 100 = 200 minutes Distance/time = 200/150 = 1 miles/minute
Instantaneous Speed
Instantaneous speed describes the speed and any particular point in time Very difficult to measure
Velocity
VELOCITY measure how fast in a particular direction Velocity is speed with direction 60 mph north 4 m/s South
Velocity is a very important concept for physics The DIRECTION portion makes things act
differently If one direction is considered ‘positive’ The opposite direction is ‘negative’ The velocities can cancel out based on direction
Velocity
When you combine velocities, you get a ‘resultant velocity’: What is resultant velocity of 15 m/s east and 1 m/s
east? Going same direction, so add them 15 m/s + 1 m/s = 16 m/s
What is resultant velocity of 15 m/s east and 5 m/s west?
Going opposite directions, so west is negative direction 15 m/s east + (– 5 m/s west) = 10 m/s east
What is resultant velocity of 10 m/s east and 10 m/s south?
14 m/s southeast (use Pythagorean theorem)
Acceleration
When you accelerate your car, what are you doing to the motion?
Acceleration is a change in velocity Acceleration can be a change in speed
Either up or down (positive or negative) Acceleration can be a change in direction
Velocity has direction, so acceleration does also A uniform circular motion is constant acceleration
(always changing direction)
Calculating Acceleration
Acceleration is the change in velocity, so you have to calculate the change. Acceleration = (Velocityfinal – Velocityinitial)/time Also: v
at
Because final velocity can be less than initial velocity (slowing down), acceleration can be negative
Negative acceleration is called deceleration in normal speech
In science, it is still acceleration with a negative value
Velocity-Time Graphs
Velocity changes can be graphed on velocity/time graphs Velocity on Y-axis Time on X-axis
Constant Velocity
Velocity-Time Graphs
Negative Acceleration (slowing down)
Forces
Force – a push or a pull Science definition: an action on a body
(object) in order to change the body’s motion Many forces act at the same time on any
object Combination results from many forces acting
on an object is called the net force
Forces
Balanced Forces do not cause changes in motion This includes from rest (no motion) to motion
or motion to rest Example: Tug of War with equal teams
Unbalanced Forces result in changes in motion The net force is larger in one direction Example: Tug of War with one side stronger
Net Forces
Friction
Friction is a Force Caused by objects rubbing together
Like hands in winter Acts in direction OPPOSITE the applied force
Why your bike stops rolling! Why a ball stops rolling! Because of friction, a constant force must be
applied to your car to keep it rolling (= gas) Rougher surfaces have more friction
Why do tires have grooves & knobs? Why do court shoes have tread?
Types of Friction
Static Friction - between stationary objects Like brakes on a car when it’s not moving
Kinetic Friction – between moving objects Sliding Friction – two (flat) surfaces rubbing Rolling Friction – round object to flat surface
Like tires on bike or car Fluid Friction – in liquids or gases
Air Resistance is a type of fluid friction
Force of Friction: Static > Sliding > Rolling > Fluid
Friction and Motion
Friction between 2 objects results in HEAT buildup and low efficiency
Reduce Friction by: Make surfaces smoother Use of lubricants (turn it to fluid friction) (oil, grease,
wax. graphite Convert Sliding Friction to Rolling Friction
Increase Friction by: Make surfaces rougher Make more surface area rubbing Increase force pushing objects together
Assignment: Friction and Motion
You are riding your bike to school. Is friction good or bad. Justify your answer using the principles of force, motion, acceleration, air resistance, sliding friction rolling friction, and fluid friction. At least 3 paragraphs At least 3 sentences per paragraph.
Newton’s Laws of Motion
8.3 & 8.4
Newton’s First Law: inertia
Which is easier? Pushing a car from start to moving or keeping
it moving? Starting your bicycle moving or keeping it
moving?
Newton’s First Law: inertia
An object at rest remains at rest and an object in motion remains in motion unless acted upon by an unbalanced force. Inertia is the tendency of an object to maintain
the same motion.
Newton’s Second Law: Force
The unbalanced force acting on an object equals the objects mass times its acceleration F = mass * acceleration (F=m*a) The harder you push, the faster it accelerates It takes more force to move a larger mass
than a smaller mass. Which would you rather try to stop (-
acceleration) a freight train moving slowly or a baseball moving 100mph?
Free-Fall and Weight
Free-fall – when the only force acting on an object is gravity
Free-fall acceleration near the earth’s surface is constant – 9.8 m/s2
After 1st second it is moving 9.8 m/s After 2nd second it is moving 19.6 m/s After 3rd second it is moving 28.4 m/s Etc.
Weight is Free-fall acceleration times mass Weight is a force (mass * acceleration)
Weight and Mass
Mass is the amount of matter in an object It doesn’t change, no matter where you go.
Weight is mass times acceleration due to gravity. It changes depending upon gravity On the moon, your mass is the same as on
earth, but 1/6th of the weight
Terminal Velocity
Terminal velocity is when air resistance (friction due to air) exactly balances the weight. A sky-diver will eventually reach terminal
velocity and not fall any faster About 320 km/h (200 mi/h)
Newton’s Third Law
For every force, there is an equal and opposite reaction force They are on different objects When you kick a ball:
Your foot applies a force to the ball The ball also applies a force to your foot.
Applications: Rockets Jet-skis Ram-jet engines