Peoples Physics Concepts

http://www.ck12.org Chapter 8. Energy Conservation

Solution

Since the centripetal force you exert on the ball in order to make it spin is perpendicular to the ball’s path, you do

not work on the ball while spinning it in a circle. Therefore, the only work you do on the ball is when you are

pulling it in a straight line.

W=F d

W=10N∗5m

W=50J

Example 2

A block of mass 5kg is sliding down a ramp inclined at 45 degrees. If the coefficient of kinetic friction between the

ramp and the block is 0.3, how much work does the force of friction do as the block slides 3m down to the bottom

of the ramp.

Solution

In order to find the work done by friction, we first want to find out the magnitude of the force of friction.

f=μkN start with the equation for the force of friction

f=μkmgcos( 30 ) substitute the y-component of the weight of the block for the normal force

Now that we have the magnitude of the force of friction, we can plug that into the equation for work.

W=F d

W=μkmgcos( 45 )

W= 0. 3 ∗5kg∗ 9 .8m/s^2 ∗cos( 45 )

W= 10. 4 J

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