Ball in the air conceptual question.

[Sentience]

Homework Statement

A golf ball is hit so that it leaves the ground at 60° above the horizontal and feels no air resistance as it travels. Which of the following statements about the subsequent motion of this ball while it is in the air is true?

a. Its acceleration is zero at its highest point.
b. Its speed is zero at its highest point.
c. Its velocity is zero at its highest point.
d. Its acceleration is always 9.8 m/s^2 downward.
e. Its forward acceleration is 9.8 m/s^2.

Homework Equations

The Attempt at a Solution

I know that A isn't true, as an object in free fall is always under the influence of gravity.

I'm not sure about B

C can't be true right? Because something always under the influence of some acceleration can never have a zero velocity?

D is wrong because gravity acts vertically, not horizontally.

E. Makes sense to me

E is the correct answer, but I'm wondering why b and c are not correct as well.

 

[supahman]

Hi there Sentience. I am no expert, but I have seen a question quite similar to your's quite a few times. The balls acceleration should be zero at its highest point for a very short period, and zero again at rest. Answer E is possible, however I would say the correct answer is A.

 

[Sentience]

A can't be true. Whether ball has just left the golf club or it is at its highest point, the acceleration due to gravity remains constant. (ignoring drag).

 

[ckutlu]

Hi there Sentience. I am no expert, but I have seen a question quite similar to your's quite a few times. The balls acceleration should be zero at its highest point for a very short period, and zero again at rest. Answer E is possible, however I would say the correct answer is A.

I think you might be confusing the acceleration being zero with the vertical velocity being zero. There is constant force (assumed constant) applied to the ball vertically, which is called the gravitational force, so A must be wrong since if there is a force there must be acceleration.

Also since there is no horizontal force applied to the ball while it is on air, there is no horizontal acceleration of the ball. (E is wrong)

B and C is also wrong because since there is no force applied horizontally, the ball's horizontal velocity never changes while in the air. So that even though it's upward velocity reaches zero at its highest point, the total value of velocity still contains the horizontal velocity.

D is true, because there is only gravitational force applied to the ball while it's in the air and it is Fg = mg. Newton's law can be stated as: Fnet = ma. Since Fg is the only force, it becomes: mg = ma. Thus a = g.

Hope that it helps

 

[supahman]

I think you might be confusing the acceleration being zero with the vertical velocity being zero. There is constant force (assumed constant) applied to the ball vertically, which is called the gravitational force, so A must be wrong since if there is a force there must be acceleration.

Also since there is no horizontal force applied to the ball while it is on air, there is no horizontal acceleration of the ball. (E is wrong)

B and C is also wrong because since there is no force applied horizontally, the ball's horizontal velocity never changes while in the air. So that even though it's upward velocity reaches zero at its highest point, the total value of velocity still contains the horizontal velocity.

D is true, because there is only gravitational force applied to the ball while it's in the air and it is Fg = mg. Newton's law can be stated as: Fnet = ma. Since Fg is the only force, it becomes: mg = ma. Thus a = g.

Hope that it helps

Ah, thanks for clearing that up. I had always assumed that as an object is in the air, there would be a short period when its acceleration would hit zero. Velocity makes more sense. :)

 

[Sentience]

I think you might be confusing the acceleration being zero with the vertical velocity being zero. There is constant force (assumed constant) applied to the ball vertically, which is called the gravitational force, so A must be wrong since if there is a force there must be acceleration.

Also since there is no horizontal force applied to the ball while it is on air, there is no horizontal acceleration of the ball. (E is wrong)

B and C is also wrong because since there is no force applied horizontally, the ball's horizontal velocity never changes while in the air. So that even though it's upward velocity reaches zero at its highest point, the total value of velocity still contains the horizontal velocity.

D is true, because there is only gravitational force applied to the ball while it's in the air and it is Fg = mg. Newton's law can be stated as: Fnet = ma. Since Fg is the only force, it becomes: mg = ma. Thus a = g.

Hope that it helps

That makes sense. Thank you.