Berkeley Review Physics Chapter 1 Question 18

[pencap]

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In the book it states that at the apex the acceleration = g and that velocity is not zero as most people assume. However, the answer for 18 says otherwise.

 

[milski]

At the apex the vertical component of velocity is zero and the horizontal is whatever it is anywhere else. So if the object was thrown/shot straight up, velocity at the apex will be zero, otherwise it won't.

 

[eradicator]

At the apex the vertical component of velocity is zero and the horizontal is whatever it is anywhere else. So if the object was thrown/shot straight up, velocity at the apex will be zero, otherwise it won't.

agree with milski, the vertical velocity is zero only....the acceleration cant be 0 anytime during free fall.

 

[pencap]

agree with milski, the vertical velocity is zero only....the acceleration cant be 0 anytime during free fall.

I understand vertical velocity is zero, yet horizontal isn't. But the question asks for velocity in general. Also number 5 in the 25 question pretest, asks basically the same question asks when the white cannon ball reaches its apex, and the answer is the acceleration is not zero and its speed is not zero.

Is it because the ball was launched with a horizontal velocity while in 18, it was simply thrown up vertically?

 

[pencap]

Also, I have a question on questions 30 and 34 in that same chapter exam.

In 30 it says that air resistance causes range to decrease, but time of flight to stay the same, in 34 it says both decrease.

What am I missing here?

 

[eradicator]

I understand vertical velocity is zero, yet horizontal isn't. But the question asks for velocity in general. Also number 5 in the 25 question pretest, asks basically the same question asks when the white cannon ball reaches its apex, and the answer is the acceleration is not zero and its speed is not zero.

Is it because the ball was launched with a horizontal velocity while in 18, it was simply thrown up vertically?

When they ask for speed/velocity they are asking for both the sum of horizontal and vertical velocity. And since horizontal velocity is nonzero, the velocity/speed arent zero.

 

[eradicator]

Also, I have a question on questions 30 and 34 in that same chapter exam.

In 30 it says that air resistance causes range to decrease, but time of flight to stay the same, in 34 it says both decrease.

What am I missing here?

For 30, the ball travels in a parabolic path...therefore the effects of air resistance on time cancels out...since it acts in opposite directions as the ball travels up and as the ball travels down. (review answer explaination at the back).

For 34 the ball is thrown horizontally...therefore the path is not symmetric like that of a parabola. Also the answer should be that, the ball has a longer flight time since it is traveling at a lower speed due to air resistance.