A man is driving a car with a speed of 36 km/hr (roughly 10 m/s). Suddenly a child appears in the middle of the road. He slams on the brakes, which in turn exerts a retarding force of 1000 N. The mass of the man and the car together is 500 kg. If the child is 27 metres away from the car when he first hit the brakes, will the child be saved?

Asked by Topperlearning User | 4th Jun, 2014, 08:19: AM

Expert Answer:

Total mass of driver + car = m = 500 kg

Initial velocity of the car is 

Final velocity is

Average retarding force is F = -1000 N.

From Newton's 2nd law, we have

So, the acceleration is

Now, using the equation of kinematics, we find the stopping distance d as

Thus, the child is saved as it was at a distance of 27 m.


Answered by  | 4th Jun, 2014, 10:19: AM