Gravity Horse power

This problem is very simple! The tricky part in this problem is the units. If you read the problem carefully the force is not given in newton(N) buts its in kg-f i.e. Kilogram-force. $1 kg-f = 9.81 N (approx)$

So the force acting (F) here is , $F = 400 \times 9.81 = 3924 N$ Also velocity (v) of car is, $v = 100 km/hr = \dfrac{250}{9} ms^{-1}$ Now let the total power developed be P . 20% of it is wasted so remaining 80% is used to drive the car.

Also we know that $Power = F.v$

So here, $\dfrac{80}{100} \times P = F.v$ $P = \dfrac{3924 \times 250 \times 100}{9 \times 80}$ $P = 136250 watt$ $P = \dfrac{136250}{746} h.p.$ $P = 182.64 h.p.$