1. Motion, Speed, Acceleration
Chapter 8

2. 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?

3. 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

4. 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.

5. Speed

6. 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

7. 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 = 200 miles
Time is = 200 minutes
Distance/time = 200/150 = 1 miles/minute

8. Instantaneous Speed
Instantaneous speed describes the speed and any particular point in time
Very difficult to measure

9. 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

10. 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)

11. 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)

12. Calculating Acceleration
Acceleration is the change in velocity, so you have to calculate the change.
Acceleration = (Velocityfinal – Velocityinitial)/time
Also:
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

13. Velocity-Time Graphs
Velocity changes can be graphed on velocity/time graphs
Velocity on Y-axis
Time on X-axis
Constant Velocity

14. Velocity-Time Graphs
Negative Acceleration (slowing down)

15. 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

16. 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

17. Net Forces

18. 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?

19. 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

20. 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

21. 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.

22. Newton’s Laws of Motion
8.3 & 8.4

23. 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?

24. 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.

25. 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?

26. 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)

27. 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

28. 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)

29. 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