Justin, with a mass of 50 kg, is going down an 8.0-m-high water slide. he starts at rest, and his speed at the bottom is 12 m/s. part a how much thermal energy is created by friction during his descent? express your answer with the appropriate units.

We can use the law of conservation of energy to solve the problem.

At the top of the slide, Justin is still, so its velocity and its kinetic energy are zero. He only has gravitational potential energy U, so its total mechanical energy is:


At the bottom of the slide, the height is now zero, so there is no potential energy left. Instead, Justin acquired a speed v, so its kinetic energy K will be different from zero now, and it will be:


So, the energy lost by Justin when going from top to bottom is

And since energy cannot be destroyed, this energy must have converted into something else, and in fact it corresponds to the thermal energy created by the frictional force during the descent.