Question

If a ball rolling down a hill is half way between the top and bottom, how much potential energy does the ball have compared to kinetic energy?

Answer 1

Answer:

The gravitational potential energy and kinetic energy of this ball should be equal (assuming that there is no energy loss due to friction.)

Explanation:

The ball loses gravitational potential energy as it rolls down the hill. At the same time, the speed of the ball increases, such that the ball gains kinetic energy.

If there is no friction on this ball (and that the ball did not deshape,) all the gravitational potential energy that this ball lost would be converted to kinetic energy.

If the gravitational field strength $g$ is constant throughout, the gravitational potential energy of an object in that gravitational field would be proportional to its height.

If $m$ denote the mass of this ball, the gravitational potential energy ($\rm GPE$) of this ball at height $h$ would be ${\rm GPE} = (m \cdot g) \cdot h$, which is proportional to $h\!$.

The value of $g$ near the surface of the earth is indeed approximately constant (typically $g \approx 9.8\; \rm m \cdot s^{-2}$.)

At halfway between the top and bottom of this hill, the height of this ball would be $(1/2)$ of its initial value (the value when the ball was at the top of the hill.) Because the $\rm GPE$ of this ball is proportional to its height, at halfway down the hill, the $\rm GPE\!$ of this ball would also be $(1/2)\!$ its initial value.

However, if there was no friction on this ball (and that the ball did not deshape,) that $(1/2)$ of the initial $\rm GPE\!$ of this ball was not lost. Rather, these $(1/2)\!$ of the initial $\rm GPE$ would have been converted to the kinetic energy ($\rm KE$) of this ball.

Hence, when the ball is halfway down the hill:

$\displaystyle \text{GPE halfway down the hill} = \frac{1}{2}\, \text{Initial GPE}$.

$\begin{aligned}& \text{KE halfway down the hill}\\ &= \text{Initial GPE} - \text{GPE halfway down the hill}\\ &= \text{Initial GPE} - \frac{1}{2}\, \text{initial GPE}\\ &= \frac{1}{2}\, \text{Initial GPE}\end{aligned}$.

Therefore:

$\begin{aligned}& \text{GPE halfway down the hill} \\ &= \frac{1}{2}\, \text{Initial GPE} \\ &= \text{KE halfway down the hill}\end{aligned}$.

In other words, under these assumptions, when this ball is halfway down the hill, the gravitational potential energy and the kinetic energy of this ball would be equal.