Projectile Motion Practice Test  Answers                        

Conceptual Physics

 

This section of the practice test is designed to test your skills in understanding the concepts of projectile motion. None of these problems require you to do any math to solve them.

 

1.      A vector is a quantity that has magnitude (the number) and direction.

a. give some examples so vector quantities that we are using in class right now.

      distance 50m south

      velocity 20m/s north

2.      A scalar is a quantity that has just magnitude

  1. give some examples of some scalar quantities that we are using in class right now.

time 30 seconds

speed 50m/s

3.      When does an object in motion become a projectile? When it is moving freely through the air.

4.      What are the two components (independent parts) of a projectiles motion? the horizontal and vertical motion

5.      For up and down motion what is happening to the projectile? On the way up gravity is slowing the projectile down. On the way down Gravity is speed ing the projectile up. Because gravity is causing the acceleration both up and down the two halves (up and down) are symmetrical.

6.      For horizontal motion what is happening to the projectile? It is moving at a constant speed, assuming there is not air resistance.

7.      At the instant a ball is thrown horizontally with a large force, an identical ball is dropped from the same height. Which ball hits the ground first? The will both reach the ground at the same time, because gravity is accelerating them both at the same rate downward.

8.      A ball is thrown into the air at some angle. At the very top of the ball’s path what is its velocity in the horizontal direction? The same as the velocity in the horizontal direction was when it was launched.

9.      A ball is thrown into the air at some angle. At the very top of the ball’s path what is its velocity in the vertical direction? zero

10.  In the absence of air resistance, the angle at which a thrown ball will go the farthest is 45

11.  A ball thrown in the air will never go as far as physics ideally would predict because of air resistance slowing down the horizontal speed.

12.     At what part of an angled projectiles path is the combined horizontal and vertical speeds going to be the greatest? At the beginning and the end. That is when the vertical component is the very fastest, the horizontal component doesn’t matter since it is the same speed the whole time.

13.     A cannonball is lunched from the ground at an angle of 30 degrees above the horizontal at a combined horizontal and vertical speed of 20m/s. If there is no air resistance.

  1. How fast will the ball be going when it lands (assuming it lands at the same height) 20m/s
  2. At what angle will the ball strike the ground assuming it lands at the same height. 30

14.  What is the difference between the vertical acceleration of a ball when it is going up compared to when it is coming down? The acceleration is negative (slowing down) on the way up and positive (speeding up) on the way down.

15.  A ball is thrown into the air at an angle of 30 degrees and lands at the same angle 10 meters away. What other angle could you launch it at to get the ball to also land at 10meters away (assuming you launch it with the same speed both times). 60

16.  How does gravity affect a satellite in orbit around the earth? A satellite is actually continually falling towards earth because of earth gravity. It just can’t reach earth because the curve of its path as it falls just happen to math the curve of the earth. So the satellite falls around the earth.

17.  “Hang time” is the term used to describe the amount of time a person is in the air when they leap directly upwards. Does this time change is the person is also running horizontally when they jump straight up? No it stays the same.

 

This section is to test your ability to do the word problems that describe the motion of the ball. For each question I have already given you the answer. This way you will know if you have done the problem right or wrong.

 

18. Zac accidentally falls out of a helicopter that is traveling horizontally at 60m/s. He plunges into the water below 3 seconds later. Assuming to air resistance, what is the horizontal distance he travels while falling? (180m)

This is really easy because I give you the horizontal speed and the time it takes to fall. Just plug the numbers in and you have the horizontal distance he will travel before he hits the water.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

19. Elliot jumps horizontally from the top of a building that is 20m high. He hopes to reach a swimming pool that is at the bottom of the building, 14m horizontally from the edge of the building. To make it to the pool what horizontal speed does he need to jump with? (7m/s)

For this problem I want you to find the initial horizontal speed Elliot will need to travel to reach the pool 14 m out from the building. First you need to find the time it will take him to fall 20m.

20. A ball is thrown horizontally from the top of a tall cliff. Neglecting air drag, what vertical distance has the ball fallen 3 seconds later? (45m)

For this problem you are finding the vertical distance. Since you know the time it takes to fall this will be a one step problem.

a.       Without air resistance does it matter what is thrown off the cliff will they all be in the same place after 3 seconds of falling. No it doesn’t matter all objects fall 44.1m in 3 seconds (neglecting the effects of air resistance).

21. A ball rolls off the edge of a horizontal roof at a velocity of 10m/s. What is the horizontal speed of the ball one second later? (10m/s) 10m/s (the same the horizontal speed does not change)

22. A ball is thrown straight up. Its initial speed is 41m/s

a.       What is the maximum height the ball will reach (assuming no air resistance).

I am asking for the vertical distance, but to answer this you will first need to find the time the ball was in the air. This is an up problem. This time we will use the other vertical time equation. The starting speed will be s1 and the speed at the top of the path will be s2.

b.      What will the balls speed be at the top of its path. 0m/s

c.       What will the balls speed be when it lands (assuming it lands at the same spot). 41m/s

d.      How long total will the ball be in the air? tup = 4.1s so I will double this to find the total time it was in the air = 8.2s

a.        85.8m b. 0 c. 41m/s d. 8.36s

23. In a standing jump the hang time for Jessica is 0.6 seconds. A) What is her hang time when she jumps the same height while moving horizontally? .6s (the same her horizontal speed will not affect her vertical motion) B) What is her maximum height? I am asking for the vertical distance.A) .6s B) .441m

This is an up problem so remember that 0.6seconds is the total time up and down, you need to half this to find the time up 0.3s and use this number in the vertical distance equation.

 

Challenge Problem:

If gravity on the moon is 1/6 of gravity here on earth compare the height of a rocket that launches straight up with an initial speed of 30m/s here on earth to what it would be on the moon. To solve this problem you will need to find the height the ball reaches here on earth and on the moon (for the moon problem you will need to find gravity on the moon.) For each problem you will be finding the vertical distance, but first you will need to find the time the ball is in the air on both the earth and the moon.

Earth

Moon

 

On the moon the ball would go nearly twice as high.

 

 

 

 

 

 

 

 

Drew claims that he can throw a dart at a dartboard from a distance of 2.0m and hit the 5.0cm wide bulls-eye if he throws the dart horizontally with a speed of 15m/s. He starts the throw at the same height as the tops of the bulls-eye. See if Drew is able to hit the bulls-eye by calculating how far his shot falls from the bulls-eye’s lower edge. To solve this problem I want to find the vertical distance that the dart has fallen. What makes this problem tricky is that you do not know the time it is falling. What you do know is that it can travel a distance of 2m before hitting the dart board. Assuming that the speed of the dart is a constant 14m/s for the whole flight you can find the time the dart is in the air using the horizontal speed equation.

.133s is not just the time to travel two meters. It is also the amount of time that the dart has to fall because of gravity. Use this time to find the vertical distance the dart has fallen and compare it to the diameter of the target.

.087m is 8.7cm. The bulls-eye was 5cm in diameter is the dart would have fallen 8.7cm-5cm = 3.7cm to far and it would miss the bulls-eye.

 

I didn’t get questions about orbital motion on this review but I will expect you to understand orbital motion for the exam.

Important vocabulary: scalar, vector, components, horizontal, vertical, range