A 1500 $\mathrm{kg}$ car begins sliding down a $5.0^{\circ}$ inclined road
with a speed of 30 $\mathrm{km} / \mathrm{h}$ . The engine is turned off, and the only
forces acting on the car are a net frictional force from the road and
the gravitational force. After the car has traveled 50 $\mathrm{m}$ along the road, its speed is 40 $\mathrm{km} / \mathrm{h}$ (a) How much is the mechanical energy
of the car reduced because of the net frictional force? (b) What is
the magnitude of that net frictional force?