Question

what force would be needed in order to stop a 27 kg child sledding down a hill at 8 m/s if you have 5 s to stop it?
a $\frac{27}{40}n$
b $\frac{216}{5}n$
c 216 n
d 1080 n

Explanation:

Step1: Calculate the acceleration

Use the formula $a=\frac{v - u}{t}$, where the final velocity $v = 0\ m/s$, the initial velocity $u = 8\ m/s$ and time $t=5\ s$. So $a=\frac{0 - 8}{5}=\frac{- 8}{5}\ m/s^{2}$.

Step2: Calculate the force

According to Newton's second - law $F = ma$, with $m = 27\ kg$ and $a=\frac{-8}{5}\ m/s^{2}$. Then $F=27\times\frac{- 8}{5}=\frac{-216}{5}\ N$. The magnitude of the force is $\frac{216}{5}\ N$.

Answer:

B. $\frac{216}{5}\ N$