1. Describing Motion
Power Point 02

2. Student Learning Objectives
Compare and contrast types of motion
Speed vs Velocity vs Acceleration
Acceleration vs Terminal Velocity
Circular Motion vs Parabolic motion

3. How do speed and velocity compare?
Average speed is the distance covered in some amount of time.
Velocity is speed in some direction.
Up, Down
Left, Right
N, S, E, W
–, +
s = d t

4. Practice An Olympic runner maintains 9 m/s during the 100 meter dash.
How long does the race take?
What is the runner’s speed in mph?
How does this speed compare to the speed of a car?

5. More Practice
When driving through a flat curve at a constant rate of speed, is there acceleration? Explain.

6. How is acceleration defined?
Acceleration is the change of velocity in some amount of time.
Deceleration is negative (opposite) acceleration.
Slowing Down
a = Dv t

7. Acceleration & Distance
A car that maintains a constant speed in a straight line has zero acceleration.
The distance traveled depends on how long the object has been accelerating.
d = (1/2)at2

8. Practice
On the freeway, it takes 3 seconds to increase your speed from 15 mph (6.7 m/s) to 65 mph (29 m/s).
What is the average acceleration in standard units?
What distance in meters will the car travel while accelerating? How many miles is this?

9. What causes vertical acceleration?
Objects are accelerated downward because of gravity.
The gravitational pull from the Earth is the same on all objects, so the downward acceleration is the same on all objects, 9.81 m/s2.
Any difference in the net downward acceleration is due to air resistance.

11. Practice
As a tennis player tosses the ball into the air, what is the ball’s rate of acceleration if there is no air resistance?
Just after the ball starts upward?
At the top of the trajectory?
As the ball travels downward to the ground (if missed)?

12. More Practice
A professional cliff diver falls from a cliff into the sea, falling for 2.7 seconds. What is the speed of the diver as the diver enters the water?

13. What is terminal velocity?
An object in free fall is only affected by gravity (no air resistance).
Terminal velocity occurs when air resistance balances gravity.

15. Practice What do the words terminal velocity mean?
What contributes to air resistance?
In what direction is gravity? In what direction is air resistance?

16. What causes circular motion?
Objects move in a straight line, unless an unbalanced force changes the motion.
A centripetal force causes a change in motion.
ac = v2 r
Fc = mac

17. Practice Describe some examples of centripetal motion.
If an object is moving in a circle, what is the direction of the force on the object?
In what direction is the change, the acceleration, when an object is traveling in a circular path?

18. More Practice
Which would have the greater centripetal force and acceleration? (Assume everything is identical accept the one thing being compared)
Small circle or large circle? Explain.
Heavy object or light object? Explain.

19. What causes parabolic motion?
The addition of the horizontal velocity and the vertical acceleration due to gravity results in a curved path.

20. Practice
At what angle relative to the ground should a football be thrown to have the greatest horizontal range?
What are some examples of projectile motion that you have observed?

21. More Practice
Which would hit the ground first, a fired bullet or a dropped bullet? Assume both are released from the same height above the ground. Also assume no air.

23. Projectile Motion t = 𝟐𝒚 𝒈 x = vt
The time of flight, the time the object has before it hits the ground, depends on maximum vertical height (y).
The distance a projectile travels, the horizontal range (x), depends on the object’s horizontal speed and how long it has to fall.
t = 𝟐𝒚 𝒈
x = vt

24. Practice
Two baseballs are released from the same height. Which baseball will travel farther? Assume no air.
Horizontal speed of 90 mph
Angled speed of 90 mph
Explain your answer

25. More Practice
A baseball is released from 1.5 meters above the ground, with vx = 96 mph (43 m/s). Assume no air.
After the is released, what will change the horizontal speed?
What is the horizontal speed the instant before the ball hits the ground?
What is the direction of acceleration after the release?
If the ball falls for 3 seconds before hitting the ground, what is the vertical speed of the ball?

26. A Math Example
What is the horizontal range of the baseball that is released from 1.5 meters above the ground, with vx = 96 mph (43 m/s)? Assume no air.
Would you expect the baseball to travel more or less distance if it was released from y = 1.8 meters? Why?