Question

A bicycle is travelling at 20m/s along a road. A child runs out into the road 50m ahead and the
bicycle rider steps on the brake pedal.What must the deceleration be to stop before it reaches the child?

Answer 1

u=20m/s
v=0m/s(he applied brakes)
s=50m
a=????(to be calculated)
$v {}^{2} = u {}^{2} + 2as \\ 0 {}^{2} = 20 {}^{2} + 2 \times a \times 50 \\ - 400 = 100a \\ a = \frac{ - 400}{100} \\ a = - 4 \: m \: per \: second {}^{2}$
So, the deceleration is 4 m/s^2.

Answer 2

u = 20m/s
v = 0m/s
s = 50m
a = a

v² - u² = 2as

0² - 20² = 2(a)(50);
-400 = 100 a
a = -400 / 100 = -4m/s²

∴Acceleration = -4m/s²
Deceleration = 4m/s²