Question

A car of mass 100kg travelling with a velocity of 20mls was suddenly brought to rest 20m after application of break, find the average retardating force and power expended by the engine.?

Answer 1

average speed during stop = 20/2 = 10 m/s
went 20 meters so it took 2 seconds
change in momentum/time = force
100 * 20 / 2 = 1,000 Newtons
power = change in energy/time = (1/2)(100) (20^2) /20= 10,000 watts
or power = force * distance /time = 1000*20 /2 = 10,000 watts

Answer 2

Thanks so much damon.

Answer 3

Given: M = 100 kg, Vo = 20 m/s.

V^2 = Vo^2 + 2a*d = 0.
20^2 + 2a*20 = 0,
400 + 40a = 0,
a = -10 m/s^2.

F = M*a = 100 * (-10) = -1,000 N. The negative sign means the force opposed the action.

V = Vo + a*t = 0.
20 + (-10)t = 0,
t = 2 s.

P = F*d/t = 1000 * 20/2 = 10,000 Joules/s = 10,000 Watts.