Conversion of energy

[Godric]

You have an incline plane above the ground at a height H, and you have a ball at the top of the plane, you release the ball, it falls down the the ground.

Q: THe best description of the conversion of energy from start to finish for the ball in the experiment is from?

A. KE to KE
B. PE to KE
C. KE to PE
D. PE to PE

My reasoning: you start at the top, your not moving, all you have is PE, then once released you have KE (your PE is converting to KE). but once you hit the ground you have PE again? No outside forces acted on the ball and when it reaches the bottom it has a v=0 and so I thought it'd PE once again, but the ground could be h=0 so it cant have PE either.
So then figured that my viewing frame must be incorrect for this problem, so am I to view the problem 1 second right before the the ball hits the ground? ? Then I would have PE at the top, turning into KE as it reaches the bottom (for the viewing frame "1 sec before hitting the ground?)

The correct answer is B

Thanks to anyone that can understand my convoluted reasoning and explain to me the right answer

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[milski]

Yes, the question seems to be about the ball falling but not yet stopping.

Potential energy is continuously decreasing, as the object moves/falls down. The positing where it is 0 is completely arbitrary. In most cases you only care about changes in the PE and not it's total value, which is what allows you to set the 0 wherever you want.

In the example below you start with some potential energy and zero kinetic energy. Once the ball starts dropping the PE starts to decrease and the KE to increase. If you ignore the air resistance, PE+KE=const during the whole fall.

At the end of the fall PE has decreased by some amount and KE has increased by the same amount, which is why we say that PE was converted to KE.

The ball hitting the ground and stopping is a whole different story, which the question does not seem to consider. In that case the KE at the end is converted to heat + PE'. PE' here is potential energy due to deformations, not related in any way to gravity.

In your case when the ball stops, it is not correct to say that there are no forces acting on it. The normal force from the ground does act on the ball. This is the force which accelerates it from whatever velocity it has to zero (aka stops it).

 

[sciencebooks]

Yes, the question seems to be about the ball falling but not yet stopping.

Potential energy is continuously decreasing, as the object moves/falls down. The positing where it is 0 is completely arbitrary. In most cases you only care about changes in the PE and not it's total value, which is what allows you to set the 0 wherever you want.

In the example below you start with some potential energy and zero kinetic energy. Once the ball starts dropping the PE starts to decrease and the KE to increase. If you ignore the air resistance, PE+KE=const during the whole fall.

At the end of the fall PE has decreased by some amount and KE has increased by the same amount, which is why we say that PE was converted to KE.

The ball hitting the ground and stopping is a whole different story, which the question does not seem to consider. In that case the KE at the end is converted to heat + PE'. PE' here is potential energy due to deformations, not related in any way to gravity.

In your case when the ball stops, it is not correct to say that there are no forces acting on it. The normal force from the ground does act on the ball. This is the force which accelerates it from whatever velocity it has to zero (aka stops it).

I also went with this assumption and got PE to KE. I generally consider energy differences from the top of the inclined plane to the bottom of the inclined plane and don't assume that it's stopped unless explicitly told. Is that an okay way to be interpreting the questions?

 

[milski]

I also went with this assumption and got PE to KE. I generally consider energy differences from the top of the inclined plane to the bottom of the inclined plane and don't assume that it's stopped unless explicitly told. Is that an okay way to be interpreting the questions?

It's a fair assumption. The process of stopping involves forces/interactions which were not present until that moment and it makes sense to call them out explicitly.

 

[mehc012]

If it helps, remember that it's an inclined plane, not a drop. If you roll a ball down a ramp, it continues rolling once it reaches the bottom. So it goes from stationary (no KE) to moving (KE) and high (PE) to low (less PE). In a frictionless world with no other inclines, the ball would continue moving after it reached the end of the ramp.