# A model train, with a mass of #15 kg#, is moving on a circular track with a radius of #6 m#. If the train's kinetic energy changes from #81 j# to #27 j#, by how much will the centripetal force applied by the tracks change by?

##### 1 Answer

The centripetal force has decreased by

#### Explanation:

This question is about circular motion and how much centripetal force is being applied, which is given by the equation:

where

Now, instead of solving for the velocity (and needing to take a square root, just to have to square it again, let's solve for the term

substituting then gives

We are looking for the change in centripetal force which we can write as:

Therefore, the centripetal force has decreased by

**Note**: The question had given us more information than we actually used - the mass was redundant since the kinetic energy alone was sufficient information. Other versions of this question may not include the mass.