Work Done and Acceleration (Mistaken Answers?)

[amandela]

Homework Statement

Q1) A ball is thrown and follows a parabolic path. Air friction is negligible. Point Q is the highest point on the path. What is the direction of the acceleration there?

Q2) A weightlifter lifts a mass m at constant speed to a height h in time t. How much work is done by the weightlifter?

Relevant Equations

Wnet = ΔKE + ΔPE + Wnc

So for Q1, I answered down (towards Earth) but the solution says there is no acceleration there.

For Q2, I answered mgh, but the solution says it's mgh/t, which is power, right?

I just want to make sure I'm not super confused.

Thank you.

 

[PeroK]

You are correct in both cases. Where are you getting the other answers from?

 

[haruspex]

You are correct in both cases. Where are you getting the other answers from?

Agreed, except that if the weight is lifted from rest at speed h/t then it reaches h with KE $\frac 12mh^2/t^2$.

 

[PeroK]

Agreed, except that if the weight is lifted from rest at speed h/t then it reaches h with KE $\frac 12mh^2/t^2$.

It says at constant speed.

 

[amandela]

You are correct in both cases. Where are you getting the other answers from?

Thank you. They're from a test bank for the AP mechanics exam.

 

[PeroK]

Thank you. They're from a test bank for the AP mechanics exam.

There seems to be a growing problem with dodgy questions and/or dodgy answers. That someone teaching physics might think the acceleration is zero when velocity is zero ought to shock me, but doesn't surprise me.

Note that for any motion you can always change your inertial frame of reference so that an object is instantaneously at rest. But, acceleration is the same across all inertial reference frames. Which is why Newton's laws deal with force and acceleration and not velocity.