1. Chapter 2
Motion & Speed

2. Motion & Speed motion – occurs when an object changes position
in order for you to know if something has changed position you need a reference point
relative motion - movement relative to another object, even if you do not move yourself
example: you are moving right now relative to the other planets

3. Motion & Speed distance – describes how far an object has moved
the SI unit for distance is the meter (m)
displacement – the distance and direction an object moves in relation to the starting point
must include distance as well as in what direction the object moved

4. Motion & Speed
example: if you drive 10 km east from your home to the store and then 7 km west to a friend’s house, what is his displacement from his starting point at his home?

5. Motion & Speed speed – the distance an object travels per unit of time
equation:

6. Motion & Speed
Suppose you ran a 4 km race in 20 min. Calculate your speed.
Sound travels at a speed of 330 m/s. If a lightning bolt strikes the ground 1000 m away from you, how long will it take for the sound to reach you?

7. Motion & Speed speed – the distance an object travels per unit of time
motion at a constant speed does not slow down or speed up, such as when a car is set on cruise control
most of the time, speeds that you experience are not constant. For example, when driving in a car you speed up, slow down, or stop for a red light
So, when speed is changing you use average speed

8. Motion & Speed
average speed – the total distance traveled divided by the total time
describes the speed of motion when speed is changing
example: when riding a bike, you start going 0 km/hr and then speed up to about 20 km/hr. Then you slow down when going up a hill. The 5 km trip took you 0.5 hr. What is your average speed?

9. Motion & Speed
instantaneous speed – the speed at a given point in time
when something speeds up or slows down its instantaneous speed changes

10. Motion & Speed
velocity – describes the speed of an object and the direction of the motion
speed describes only how fast something is moving & DOES NOT take into account direction
example: if 2 escalators both going at the same speed but in different directions their speed of the passengers on them are the same but their velocity is different

11. Motion & Speed
velocity – describes the speed of an object and the direction of the motion

12. Motion & Speed Graphing Motion use a distance / time graph
time is on the X axis and distance is on the Y axis
this will show the objects movement over time
the slope of a distance time graph = speed

13. Motion & Speed
Graphing Motion

14. Motion & Speed Graphing Motion
A flat horizontal line indicates that the object or person is NOT moving
A diagonal line indicates that the object or person IS moving
Example #1: Jake was on his way to his soccer game when he realized he left his shin guards at home. He turned around and went home to pick them up. On his way back to his game he had to stop and wait for a train. After the train passed he was able to drive the rest of the way to his soccer game. Explain what is occurring at each event.

15. Motion & Speed
Example #2: You’re at the park playing with your remote control helicopter and trying to land it on a picnic table. You descend the helicopter toward the table. During the descent you hover above the table while your friend clears off the food. When they are done you finish the descent. Explain what is occurring during each segment of the graph.

16. Motion & Speed
Example #3: You leave your house and walk 2 blocks in 3 minutes to the nearest gas station. You spend 5 minutes at the gas station before heading home. You walk 1 block in 1 minute before you stop at the crosswalk. You wait 1 minute before being able to cross the street and walk the remaining 1 block home. Explain what is occurring during each segment of the graph.

17. Motion & Speed
Example 4: Draw a distance vs. time graph for the following scenario.
Sandy left her house to go to work. She travels 1 mile in 2 minutes before she is stopped and has to wait 1 minute at a red light. She then travels another 5 miles in 7 minutes to get to work. Once she parks her car she realizes that she forgot her purse. It takes her 5 minutes to drive 6 miles back to her home.

18. Motion & Speed
Example 5: Draw a distance vs. time graph for the following scenario.
Joe leaves his house to walk his dog. It takes him 5 minutes to walk 5 blocks to the park. When Joe and his dog get to the park they stop at the fire hydrant for 1 minute so the dog can do his business. They then continue to walk another 3 blocks through the park in 2 minutes towards a pet shop. After staying in the pet shop for 3 minutes to buy a bone, Joe and his dog turn around and walk home in only 7 minutes.

19. Motion & Speed acceleration – the rate of change of velocity
acceleration occurs if an object changes its speed, its direction, or both

20. Motion & Speed acceleration – the rate of change of velocity
positive acceleration
acceleration is in the same direction as the velocity (direction)
car speeds up (speed)

21. Motion & Speed acceleration – the rate of change of velocity
negative acceleration
acceleration is in the opposite direction of the velocity (direction)
car slows down (speed)

22. Motion & Speed acceleration – the rate of change of velocity
changing direction
any time a moving object changes direction, its velocity changes and it is accelerating
example: a horse on a carousel’s speed is always constant but the horse is accelerating because it is constantly changing direction

23. Motion & Speed acceleration – the rate of change of velocity
calculating acceleration
a =acceleration
vf = final velocity
vi = initial velocity
t = time
measured in m/s2

24. Motion & Speed
A car is driving 10 meters per second. The driver sees a police vehicle and slows down to 5 meters per second in 5 seconds. Has the car accelerated or decelerated? By how much has the car changed in acceleration?

25. Motion & Speed
A jet airliner starts at rest at the end of a runway and reaches a speed of 80 m/s in 20 seconds. What is the acceleration for the airliner?

26. Motion & Speed
A skateboarder is moving in a straight line at a speed of 3 m/s and comes to a stop in 2 seconds. What is the skateboarder’s acceleration?

27. Motion & Speed Interpreting velocity vs. time graphs
Velocity goes on the y-axis 
Time goes on the x-axis 
The slope of the line represents the object’s acceleration

28. Motion & Speed
Velocity vs. Time graph

29. Motion & Speed Velocity vs. Time graph
A flat horizontal line indicates that the object or person is NOT accelerating
A diagonal line indicates that the object or person IS accelerating or decelerating

30. Motion & Speed
force – a push or a pull that one thing exerts on another
examples: force of the floor on your feet
force of gravity pulling you down
force of someone pushing on your arm
if you exert a force on an object, it causes the motion of the object to change
example: a tennis racket striking a ball- causes the motion to change

31. Motion & Speed
net force – the sum of the forces (positive & negative) acting on an object
the net force includes the size of the force and the direction of the force

32. Motion & Speed Two Types of Net Forces
1. balanced forces – forces on an object that are equal in size but opposite in direction
the forces “cancel each other out”
example: if two students push on a box with the same amount of force but in opposite directions

33. Two Types of Forces
Balanced forces: Forces on an object that are equal in size and opposite in direction +
= 0
Net Force = 0

34. Motion & Speed Two Types of Net Forces
balanced forces – forces on an object that are equal in size but opposite in direction

35. Motion & Speed Two Types of Net Forces
2. unbalanced forces – occurs if two objects are pushing with unequal forces in the
opposite direction or if two forces are pushing in the same direction
example: two students pushing a box on opposite sides but one student is pushing with more force
two students pushing a box on the same side

36. Motion & Speed Two Types of Net Forces
2. unbalanced forces – occurs if two objects are pushing with unequal forces in the

37. Motion & Speed
inertia – the tendency of an object to resist any change in its motion
if an object is moving, it will keep moving in that direction and at that speed unless acted on by another force
in other words, the velocity of an object remains the same unless another force changes it

38. Motion & Speed
inertia – the tendency of an object to resist any change in its motion
The Law of Inertia is also called Newton’s First Law
example: a ball moving in a certain direction with a certain speed will continue moving until another ball hits it

39. Motion & Speed
inertia – the tendency of an object to resist any change in its motion
the greater the force – the greater the inertia
inertia is related to mass – so, the greater the mass, the greater the inertia (this is because there is more resistance to change its motion.)

40. Motion & Speed
inertia – the tendency of an object to resist any change in its motion
Newton’s 1st Law, or the Law of inertia can explain what happens in a crash. Explain why it is beneficial to wear a seatbelt.