BR Projectile Motion Question

[aspiringdoc09]

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I'm confused! I tried searching to see if this question had been answered so sorry if it was. I just started using the BR Physics for my upcoming test.

For Section I in Review Passage I Question 5: When the white cannon ball reaches its apex, the:

C. acceleration is not zero and its velocity is zero. I chose this answer
D. acceleration is not zero and it velocity is not zero. This is the correct answer

Now for Practice Test Discrete Question 18: Consider a ball that is thrown straight up with a speed v initial. What are the magnitudes of its acceleration and speed when it reaches its maximum altitude?

C. a=g, v= v initial I chose this answer
D. a=g, v=0 This is the correct choice

1) What are the differences between these two questions? 2) Why are velocities defined differently in each question type? 3) How can I differentiate these questions in the future to decrease my likelihood of falling for a trap?

I forgot initially that velocity will still exist during a projectile in the x-direction while v=0 at the apex in the y-direction. However, now it seems like the 2nd question is contradicting the 1st question. What am I missing?

 

[milski]

In one question the ball is thrown straight up - that would be zero x-velocity at any time. The cannonball is shot at some angle and will have some constant non-zero velocity.


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[howlovely]

Like milski said, the difference between the two questions is that the first question is about projectile motion while the second question is about free fall.

In projectile motion, the object has both an x- and y-component for the velocity. The x-component stays constant no matter what while the y-component changes. At the top of its path, the object will have zero velocity in the y-direction, but will still have the same velocity in the x-direction as it did initially.

However in free fall, there is only a y-component for the velocity. Thus at the top of its path, the object has no velocity.

You can differentiate projectile motion and free fall because usually in projectile motion questions, objects are "launched", while in free fall questions, objects are "thrown straight up".

 

[UTsksk]

Like milski said, the difference between the two questions is that the first question is about projectile motion while the second question is about free fall.

In projectile motion, the object has both an x- and y-component for the velocity. The x-component stays constant no matter what while the y-component changes. At the top of its path, the object will have zero velocity in the y-direction, but will still have the same velocity in the x-direction as it did initially.

However in free fall, there is only a y-component for the velocity. Thus at the top of its path, the object has no velocity.

You can differentiate projectile motion and free fall because usually in projectile motion questions, objects are "launched", while in free fall questions, objects are "thrown straight up".

Good job. I actually knew that one 😀

 

[Morsetlis]

I'm confused! I tried searching to see if this question had been answered so sorry if it was. I just started using the BR Physics for my upcoming test.

For Section I in Review Passage I Question 5: When the white cannon ball reaches its apex, the:

C. acceleration is not zero and its velocity is zero. I chose this answer
D. acceleration is not zero and it velocity is not zero. This is the correct answer

Now for Practice Test Discrete Question 18: Consider a ball that is thrown straight up with a speed v initial. What are the magnitudes of its acceleration and speed when it reaches its maximum altitude?

C. a=g, v= v initial I chose this answer
D. a=g, v=0 This is the correct choice

1) What are the differences between these two questions? 2) Why are velocities defined differently in each question type? 3) How can I differentiate these questions in the future to decrease my likelihood of falling for a trap?

I forgot initially that velocity will still exist during a projectile in the x-direction while v=0 at the apex in the y-direction. However, now it seems like the 2nd question is contradicting the 1st question. What am I missing?

The ball that goes straight up has no x component (horizontal), so its velocity AND speed will be 0 at the apex.

The ball that is launched at an angle has an x component, so neither velocity NOR speed will be 0 at the apex (there is a non-zero horizontal component). However, the y component of velocity and speed will be 0 at the apex.

Since this is some sort of science you may have to be more precise than you want to be 😎