# A model train, with a mass of #4 kg#, is moving on a circular track with a radius of #15 m#. If the train's kinetic energy changes from #32 j# to #12 j#, by how much will the centripetal force applied by the tracks change by?

##### 1 Answer

Jun 10, 2017

Or using correct significant figures:

#### Explanation:

We know that the centripetal force on an object moving in a circle is:

#F_"cent" = m*a_"cent" = (mv^2)/r#

And kinetic energy of an object is:

#KE = 1/2mv^2#

#therefore 2KE= mv^2#

We can substitute this into our first equation to give us:

#F_"cent" = (2KE)/r#

And therefore,

#DeltaF_"cent" = (2DeltaKE)/r = (2(12J - 32J))/(15m) = -40/15 N = -8/3N#

Now, since the problem gives us 2 sig. figs, the answer needs 2 sig. figs as well.

#-2.7 N#

*Final Answer*