Answer
The straps exert a force of 1100 N on the child opposite the original direction of motion.
Work Step by Step
First, we convert $v$ to meters per second:
$v = (45 ~km/h)(\frac{1000 ~m}{1 ~km})(\frac{1 ~h}{3600 ~s}) = 12.5 ~m/s$
Then, we can use kinematics to find the acceleration:
$v = v_0 + at$
$a = \frac{v-v_0}{t} = \frac{0 - 12.5 ~m/s}{0.20 ~s} = -62.5 ~m/s^2$
We can use the rate of deceleration to find the magnitude of the force:
$F = ma = (18 ~kg)(62.5 ~m/s^2) = 1100 ~N$
The straps exert a force of 1100 N on the child opposite the original direction of motion.
