Physics---urgent

Question

A boy was hit by a car when he was crossing a zebra-crossing.

Let d be the distance between the car and the boy at the moment the driver first observed the boy. The driver applied the brakes and skid mark 36.0 m long was left on the road. After hitting the boy, the car travelled a distance of 19.7 m before coming to rest. You may neglect the change in speed of the car during the impact.

更新:

1) Estimate the thinking distance and the value of d. (with steps)

2) The speed limit of the road is 50 km h^-1(i.e. 13.9 m s^-1). If the car is travelling at this speed, explain whether it would hit the boy.

Answer 1

最愛解答

This is a question in 2002 CE past paper 1

1. From the graph, since u = 21.5 m/s

So, think distance l = 15 m

From the figure,

19.7 + d = 36 + l

d = 36 + 15 - 19.7

d = 31.3 m

2. If the car is travelling at this speed (13.9 m/s),

the thinking distance will be approximately 15 m from the graph.

The braking distance would be approximately 10 m

And the stopping distance

= 15 + 10

= 25 m

The stopping distance is shorter than d and hence the car will not hit the boy.

資料來源： Myself~~~