Question

a motor car is moving with a velocity of 180 km/h and it takes 4s to stop after the brakes applied.calculate the force exerted by the brakes on the motocar if its mass along with passengers is 1000kg.

Answer 1

Initial velocity (u) = 180 km/h = 180*5/18 = 50 m/s.
Final velocity (v) = 0 m/s. [As object is coming to rest].
Time (t) = 4 s.
Mass (m) = 1000 kg.

F = mv-mu / t.
F = 1000*0 - 1000*50 / 4.
F = -50000 / 4.
F = -12500 N.

Therefore, he force exerted by the brakes = -12500 N [Negative as the force applied is called retardation force that is it is the force applied to stop the object].

Answer 2

u = 180 km/hr
= 180 x 5/18
= 50 m/s
v = 0 m/s t = 4s
m = 1000 kg

F = m ( v - u ) /t
= 1000 ( 0 - 50 ) / 4
= - 1000 x 50 / 4
= - 50000 / 4
= - 12500 N

Hope it helps you.