Physics H/work, help please??!!..?

Question

A sports car, accelerating from rest, was timed over 400m and was found to reach a speed of 120km/h in 18.0seconds.

a) What was the average speed of the car in m/s?

b) Calculate the average acceleration of the car in km/h.

c) What was the average acceleration in m/s^-2 (per second/per second)?

d) If the driver of the car had a reaction time of 0.60 s, how far would the car travel while the driver was reacting to apply the brakes at this speed of 120 km/h?

if possible please can you show me you working out, as i don't entirely get how to do this question.

Thankyou

Answer 1

a.

The average speed is computed from v = s / t using total distance and time

v(average) = 400m / 18 sec = 22.22 m/s

b.

The average acceleration from rest may be computed from

s = 1/2 a t^2, or a = 2s/t^2 = 800 m / (18 sec)^2 = 2.47 m/s^2, approx

OK, that's the answer to part c.  Since acceleration cannot be expressed in km/h, I don't know what part b wants.

d.

s = v t = 120 km/h * .6 s / 3600 s/hr = .02 km = 20 m