Question

A cyclists is travelling at 15ms-1 she applies brakes so that she does not collide with a wall 18m away. What acceleration must she have

Answer 1

In this question we will calculate the negative acceleration/deceleration, as follows:

Kinematic equations:

SUVAT-- s-displacement, u-initial velocity, v-final velocity,a- acceleration, t-timev=u + atv2 = u2 + 2ass=1/2(u+v)ts=ut + 1/2at2

Her initial velocity, u is 15m/s
Her final velocity is, v 0m/s (since she stops)
Displacement is, s 18m
Hence the acceleration, a :

v2=u2 + 2as

0=15^2 + 2 x a x 18
0=225 + 36a
36a= -225

a =  -225/36
a= -6.25 m/s/s

Therefore she has to decelerate at the rate of 6.25m/s2

Answer 2

Answer:

u=15m/s

v=0m/s (since she stops)

s=18m

a:

v2=u2+2as

0 = 15^2 + 2 x a x 18

0 = 225 + 36a

36a = -225

a = -225/36

a = -6.25m/s2