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?