1. Motion Notes
Physical Science

2. Physics of Motion We will look at:
Position
Distance
Displacement
Speed
Velocity
Acceleration
First you need to realize that motion is relative…

3. Motion is Relative
Motion is relative to the observer’s position and their reference point
Sometimes called a “frame of reference”
Consider the picture…
If this man is driving at 15 mph, how fast is his coffee cup moving?
Does the man feel like the cup is moving?
Why?

4. Motion is relative
An object is in motion if it changes position relative to a stationary reference point.
Use this to establish a COORDINATE SYSTEM
pick an origin or 0 point
decide which direction is positive

5. Draw an Example:

6. Direction
We use North, South, East, West or left & right to describe the direction of movement.
We can also use POSITIVE and NEGATIVE to describe direction.

7. POSITION
A location with respect to the origin or zero point.

8. Finding Position
4 miles
6 miles 4 10 -4 6
-10 -6
Position depends on where you put ‘0’.

9. Distance and Displacement
Distance measures the actual path an object takes
Displacement measures your overall distance from the initial position to final position in a STRAIGHT LINE.
DISPLACEMENT values must include a DIRECTION!
Which color line represents distance?
Displacement?

10. Scalar vs Vector Quantities
All measured quantities can be classified as being either a scalar or a vector.
Scalar
_________ only (size of the quantity ….a number)
Vector
_________ and _________
Magnitude
Magnitude
Direction

12. Distance vs. Displacement

13. Use the diagram to determine the resulting displacement and the distance traveled by the skier during these three minutes.

14. Answer The skier covers a distance of
(180 m m m) = 420 m and has a displacement of 140 m, right or east.

15. Write in your own words- what is the difference between distance and displacement?

16. Formula for Displacement
∆X = Xf-Xi
Xf= final position
Xi= initial position
Remember- displacement can be positive or negative! Why?

17. What is the coach's resulting displacement and distance of travel?

18. Answer The coach covers a distance of
(35 yds + 20 yds + 40 yds) = 95 yards and has a displacement of 55 yards, left or -55 yards.

19. Speed Speed is the distance traveled in an amount of time
We know some things move faster than others…but how do we measure it?

20. What two quantities must you know to determine speed?
Speed= distance/time
units- miles/hr, km/hr m/s

21. There are three types of speed you must know…
Constant speed
Instantaneous speed
Average speed

22. Constant Speed
When an object covers equal distances in equal amounts of time
Ex- if a race car travels at a CONSTANT SPEED of 96m/s, it will travel a DISTANCE of 96 meters EVERY SECOND.

23. But most objects do not travel at a constant speed.
The speed of an object can change from one minute to another.
So we can use AVERAGE SPEED to describe its motion-overall distance moved over the entire time
Use this equation…
Average Speed = total Distance / total Time

24. Instantaneous Speed
Instantaneous speed- measures speed at an instant in time

25. Does a speedometer of a car read instantaneous or average speed?

26. What 2 controls on a car enable a change in speed?

27. What if I want to describe speed AND direction?
For example…what if you wanted to find a plane. Knowing the speed would only tell you how far away to look but not in what direction. For that we need…
VELOCITY

28. Let’s get back to the car example…
Name another control that enables a change in velocity.

29. So…What is the difference between speed and velocity?
SPEED- reports the magnitude of distance over time (just the number).
VELOCITY- reports the speed AND direction of motion.

30. Are speed and velocity always going to be the same?
NO!
Only if the object is moving in the SAME direction the whole time.
What is an example of a motion where avg speed is not equal to average velocity? (demo)

31. Constant Velocity
Same displacement per unit of time

32. So how do you calculate speed vs velocity?
Mathematically:
Velocity = displacement / time (Includes a direction!)

33. Vavg= (xf-xi) / (tf-ti)
Let’s look closer…
Average velocity is calculated by the equation:
Vavg= (xf-xi) / (tf-ti)

34. Use the diagram to determine the average speed and average velocity between the following points. A. Going from A and B B. Going from A to B and ending at C

35. Answers:
A. Between A and B, speed and velocity will be the same because they are in the same direction the whole time.
Speed = 180 m/min
Velocity = 180 m/min east
B. When the motion continues to point C, the speed and velocity are different because the distance traveled and displacement are different.
Speed= 320 m/2 min= 160 m/min
Velocity= 40 m/2 min= 20 m/min east

36. Acceleration
Chapter

37. Acceleration is a change in velocity in a period of time.
So…Is it a vector or a scalar quantity?
VECTOR
It must include a direction (like velocity)
This also means that you can change acceleration by changing what?
Speed or Direction

38. Does travel at a constant speed mean you are not accelerating?
Remember that you can change velocity by changing direction, thus CHANGING ACCELERATION!

39. Causes of acceleration
Increasing velocity
Example: Car speeds up at green light
Decreasing velocity
Example: Car slows down at stop light
Changing Direction
Example: Car takes turn (can be at constant speed)
screeeeech

40. What do the numbers mean?
Small acceleration – speed is increasing slowly
Large acceleration- speed is increasing rapidly

41. Positive vs. Negative Acceleration
Acceleration and velocity are in the same direction when speeding up and opposite directions when slowing down

42. Calculating Acceleration
If an object is moving in a straight line…
∆V= change in velocity
(Final Velocity – Initial Velocity) (m/s)
A= acceleration
∆ T= time (s)
(final time- initial time) ∆V
So what are the units of acceleration?
If A= V/T…
Acceleration is in m/s2

43. Calculating Acceleration
Acceleration = (Vf-Vi) t
Let’s look at the picture below…what is this car’s avg acceleration?
0 s
1 s
2 s
3 s
4 s
0 m/s
4 m/s
8 m/s
12 m/s
16 m/s

44. Question
A skydiver accelerates from 20 m/s to 40 m/s in 2 seconds. What is the skydiver’s average acceleration?

45. Constant velocity

46. Now consider a car moving with a rightward (+), changing velocity - that is, a car that is moving rightward but speeding up or accelerating.

47. Acceleration

48. Distance time graphs Draw 2 graphs One showing a slow constant speed
One showing a faster constant speed

49. Position-Time Graphs Slow, Rightward(+) Constant Velocity
Fast, Rightward(+) Constant Velocity
                                                                                       
                                                                                      

50. More Position-Time Graphs
Slow, Leftward(-) Constant Velocity
Fast, Leftward(-) Constant Velocity
                                                                                     
                                                                                      

51. Consider a car moving with a constant, rightward (+) velocity - say of +10 m/s. A car moving with a constant velocity is a car with zero acceleration.
Draw a graph!

52. Constant velocity = zero acceleration

53. Positive Acceleration
Now consider a car moving with a rightward (+), changing velocity - that is, a car that is moving rightward but speeding up or accelerating. Since the car is moving in the positive direction and speeding up, the car is said to have a positive acceleration.
                                                                                     

54. Positive acceleration

55. Describe this graph!

56. Does the velocity of the wind affect such things as a sprinter’s speed or an airplane’s flight time?

57. Resultant Velocity animation
vectors/plane.html

58. “Adding Vectors” Example:
A small airplane heads east with a speed of mph with respect to the air (the “air speed”). This would be the plane’s speed if the air was NOT moving – no wind)
If the wind/jet stream is moving east at 50 mph, what is the plane’s resulting velocity with respect to the ground (the “ground speed”)?
200 50
250 mph, east
with the wind

59. If, later, the airplane is flying west into the 50 mph wind with an “air speed” of 200 mph, now what is the plane’s resulting velocity with respect to the ground (the “ground speed”)?
200 50
150 mph, west
against the wind

60. Find the velocity in m/s of a swimmer who swims 110 m toward the shore in 72 s.

61. 1.5 m/s toward the shore

62. Imagine that you could ride a baseball that is hit high enough and far enough for a home run. Using the baseball as a reference frame, what does the Earth appear to do?

63. Calculate the displacement in meters a cyclist would travel in 5
Calculate the displacement in meters a cyclist would travel in 5.00 h at an average velocity of 12.0 km/h to the southwest.

64. But first let’s look at some graphs…
If I wanted to graph speed, what should I label my axes???
So the slope of the line=SPEED
Distance
Speed
Time

65. Constant Speed
What would a position-time graph look like for a constant speed?