1. Chapter 10: Motion

2. Motion Problem: We need a reference point... Is your desk moving?
nonmoving point from which motion is measured

3. Motion Reference point Motion Motion
Change in position in relation to a reference point.
Reference point
Motion

4. You have mistakenly set yourself as the reference point.
Motion
Problem:
You are a passenger in a car stopped at a stop sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward.
You have mistakenly set yourself as the reference point.

5. Displacement
Displacement: the change in position (location) of an object
Differences between distance and displacement
Distance can be a straight line but doesn’t have to be
Displacement MUST be a straight line
Displacement must be in a particular direction

6. v d t Speed & Velocity Speed rate of motion
distance traveled per unit time
SI Unit: m/s

7. Speed & Velocity Instantaneous Speed Average Speed
speed at a given instant
Average Speed

8. Speed can be measured at a certain point in time.
Instantaneous Speed
Speed can be measured at a certain point in time.
For instance, if you are driving, your speedometer may read 30 mi/hr.
You don’t have to drive for an hour for your car to know this – your speed is instantaneous.

9. It depends on the storm’s direction!
Speed & Velocity
Problem:
A storm is 10 km away and is moving at a speed of 60 km/h. Should you be worried?
It depends on the storm’s direction!

10. Speed & Velocity Velocity speed in a given direction
can change even when the speed is constant!
= DISPLACEMENT / TIME

11. velocity = displacement ÷ time
Velocity is a measure of an object’s SPEED and DIRECTION.
Velocity is measured by:
velocity = displacement ÷ time
v = d/t

12. Velocity & Displacement
If you run around a circular track and end up in the same position you started, your displacement is 0.
So is your velocity: v = 0/time, so v=0

13. Acceleration is the rate of change of velocity
Acceleration occurs when an object:
Speeds up
Slows down
Changes direction
Acceleration is the rate of change of velocity

14. t a a: acceleration vf: final velocity vi: initial velocity t: time
vf - vi t
Acceleration
Acceleration
the rate of change of velocity
change in speed or direction
SI UNIT: m/s2
a: acceleration
vf: final velocity
vi: initial velocity
t: time

15. Positive acceleration “speeding up”
Negative acceleration
“slowing down”

16. t d v d = 100 m v = d ÷ t t = 20 s v = (100 m) ÷ (20 s) v = ?
Calculations
Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster?
GIVEN:
d = 100 m
t = 20 s
v = ?
WORK:
v = d ÷ t
v = (100 m) ÷ (20 s)
v = 5 m/s
You skate faster! v d t

17. t a a = (vf - vi) ÷ t t = 3 s a = (32m/s - 10m/s) ÷ (3s) vf = 32 m/s
Calculations
A roller coaster starts down a hill at 10 m/s. Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration?
GIVEN:
vi = 10 m/s
t = 3 s
vf = 32 m/s
a = ?
WORK:
a = (vf - vi) ÷ t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2 a
vf - vi t

18. t d v v = 330 m/s t = d ÷ v d = 1km = 1000m t = (1000 m) ÷ (330 m/s)
Calculations
Sound travels 330 m/s. If a lightning bolt strikes the ground 1 km away from you, how long will it take for you to hear it?
GIVEN:
v = 330 m/s
d = 1km = 1000m
t = ?
WORK:
t = d ÷ v
t = (1000 m) ÷ (330 m/s)
t = 3.03 s v d t

19. t a t = ? t = (vf - vi) ÷ a t = (0m/s-30m/s)÷(-3m/s2) vf = 0 m/s
Calculations
How long will it take a car traveling 30 m/s to come to a stop if its acceleration is m/s2?
GIVEN:
t = ?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2
WORK:
t = (vf - vi) ÷ a
t = (0m/s-30m/s)÷(-3m/s2)
t = -30 m/s ÷ -3m/s2
t = 10 s a
vf - vi t

20. Distance and Time are variables!
Graphing Speed
Distance and Time are variables!
Independent Variable: time
Dependent Variable: distance

21. Graphing Constant Speed

22. Graphing Changing Speed

23. Graphing Changing Speed

24. Graphing Changing Speed

25. slope = speed steeper slope = straight line = faster speed flat line =*
07/16/96
Graphing Motion
Distance-Time Graph A B
slope =
steeper slope =
straight line =
flat line =
speed
faster speed
constant speed
no motion *

26. Graphing Motion Who started out faster? A (steeper slope)
Distance-Time Graph A B
Who started out faster?
A (steeper slope)
Who had a constant speed? A
Describe B from min.
B stopped moving
Find their average speeds.
A = (2400m) ÷ (30min) A = 80 m/min
B = (1200m) ÷ (30min) B = 40 m/min

27. Acceleration is indicated by a curve on a Distance-Time graph.
Graphing Motion
Distance-Time Graph
Acceleration is indicated by a curve on a Distance-Time graph.
Changing slope = changing velocity

28. no accel. (constant velocity)
Graphing Motion
Speed-Time Graph
slope =
straight line =
flat line =
acceleration
+ve = speeds up
-ve = slows down
constant accel.
no accel. (constant velocity)

29. Graphing Motion Specify the time period when the object was...
Speed-Time Graph
Specify the time period when the object was...
slowing down
5 to 10 seconds
speeding up
0 to 3 seconds
moving at a constant speed
3 to 5 seconds
not moving
0 & 10 seconds

30. Fkick Fgrav Force Force a push or pull that one body exerts on another
What forces are being exerted on the football?
Fkick
Fgrav

31. Fnet Ffriction Fpull N W
Force
Net Force
unbalanced forces that are not opposite and equal
velocity changes (object accelerates)
Fnet
Ffriction
Fpull N W

32. Net Force
The net force on an object is the sum of all the forces on the object
Net force = F1 + F2 + F3 + …
When gravity acts on an object, but the object does not fall, there is a normal force pushing up on the object
Fnormal + Fgravity = 0

34. Force
Balanced Forces
forces acting on an object that are opposite in direction and equal in size
no change in velocity

35. Balanced Forces If an object is not accelerating, either:
1. no forces are acting on it, or
2. all the forces on it balance
2 forces are balanced if they have equal sizes but opposite directions

36. Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.
ConcepTest 1
TRUE or FALSE?
The object shown in the diagram must be at rest since there is no net force acting on it.
FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.
Taken from “The Physics Classroom” © Tom Henderson,

37. Forces
Remember – an object can be moving with CONSTANT VELOCITY and the net force would equal 0.
The object must ACCELERATE, or CHANGE VELOCITY when a net force is applied

38. Unbalanced Forces
If an object is being pushed or pulled, the changing motion is caused by forces
The forces are unbalanced
They may have different sizes, or be in the same direction

39. Force Diagrams
Force diagrams use arrows to represent the sizes, directions, and types of forces on an object
The object is usually represented by a box

40. force that opposes motion between 2 surfaces depends on the:
Friction
Friction
force that opposes motion between 2 surfaces
depends on the:
types of surfaces
force between the surfaces

41. between rough surfaces
Friction
Friction is greater...
between rough surfaces
when there’s a greater force between the surfaces (e.g. more weight)
Pros and Cons?

42. Static vs Kinetic Friction
Static Friction:
The force that resists the initiation of sliding motion between two surfaces that are in contact and at rest
Friction between stationary (non-moving) surfaces
The force that prevents a car wheel from slipping as it rolls on the ground. Even though the wheel is in motion, the patch of the tire in contact with the ground is stationary relative to the ground
Kinetic Friction:
The force that opposes the movement of two surfaces that are in contact and are sliding over each other
Friction between moving surfaces

44. Types of Kinetic Friction
Sliding Friction
Rolling Friction
When objects slide past each other
Examples:
Sliding glass or screen door
If a round object rolls over a flat surface
Usually less than sliding friction

47. Fluid Friction Any object moving through a fluid like air
Example: when air slides past a car; can be minimized by smooth surfaces
Air resistance opposes motion
Air resistance to the car’s motion increases as the car travels faster because more of air must be moved each second

49. Friction Ways to Reduce Ways to Increase
Ways to Reduce Harmful/ Unwanted Friction
Ways to Increase Helpful Friction
Ways to Reduce
Use of lubricants (oil, wax, and grease)
Motor oil on engine parts
Replacing sliding friction with rolling friction
Ball bearing in between wheels and axels of bikes
Make Surfaces Smoother
Ways to Increase
Make surfaces rougher
Sand on ice
Increase the force between surfaces
Pressing down on sponge to clean dishes

50. a.k.a. “fluid friction” or “drag”
Air Resistance
Air Resistance
a.k.a. “fluid friction” or “drag”
force that air exerts on a moving object to oppose its motion
depends on:
speed
surface area
shape
density of fluid

51. Fair Fgrav Terminal Velocity
Air Resistance
Terminal Velocity
maximum velocity reached by a falling object
reached when… Fgrav = Fair
Fair
no net force
 no acceleration
 constant velocity
Fgrav

52. Animation from “Multimedia Physics Studios.”
Air Resistance
Terminal Velocity
increasing speed  increasing air resistance until…
Fair = Fgrav
Animation from “Multimedia Physics Studios.”

53. Animation from “Multimedia Physics Studios.”
Air Resistance
Falling with air resistance
heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity
Fgrav = Fair
larger Fgrav
 need larger Fair
 need higher speed
Animation from “Multimedia Physics Studios.”

54. Without air resistance, everything would fall at the same speed!
A bowling ball would hit the same time as a ping pong ball; a car and a skateboard; etc

55. follows a parabolic path called a trajectory
Projectile Motion
Projectile
any object thrown in the air
acted upon only by gravity
follows a parabolic path called a trajectory
has horizontal and vertical velocities
PROJECTILE MINI-LAB

56. Projectile Velocities
Projectile Motion
Projectile Velocities
Horizontal and vertical velocities are independent of each other!

57. Projectile Motion
Horizontal Velocity
depends on inertia
remains constant
Vertical Velocity
depends on gravity
accelerates downward at 9.8 m/s2

58. Animation from “Multimedia Physics Studios.”
ConcepTest
A moving truck launches a ball vertically (relative to the truck). If the truck maintains a constant horizontal velocity after the launch, where will the ball land (ignore air resistance)?
A) In front of the truck
B) Behind the truck
C) In the truck
C) In the truck. The horizontal velocity of the ball remains constant and is unaffected by its vertical motion.
Animation from “Multimedia Physics Studios.”