Question

A mass of 1.2kg travelling at 6 meter per second strike a wall at the right angle and rebounce with velocity of 3 metre per second .if the contact lasted 0.25 seconds find the force on the mass

Answer 1

Velocity of the mass $\displaystyle\sf {m=1.2\ kg}$ before hitting the wall is,

• $\displaystyle\sf {u=6\ m\,s^{-1}}$

The velocity of the mass after hitting the wall is,

• $\displaystyle\sf{v=-3\ m\,s^{-1}}$

Negative sign is because the mass is assumed to travel towards its initial position, i.e., in the opposite direction, after hitting wall.

The change in momentum of the mass, assuming there's no mass loss, is,

$\displaystyle\longrightarrow\sf{\Delta p=m(v-u)}$

$\displaystyle\longrightarrow\sf{\Delta p=1.2(-3-6)}$

$\displaystyle\longrightarrow\sf{\Delta p=1.2\times-9}$

$\displaystyle\longrightarrow\sf{\Delta p=-10.8\ N\,s}$

The time during which the mass is being in contact with the wall is,

• $\displaystyle\sf {\Delta t=0.25\ s}$

By second law of motion, the force acting on the mass is,

$\displaystyle\longrightarrow\sf{F=\dfrac {\Delta p}{\Delta t}}$

$\displaystyle\longrightarrow\sf{F=\dfrac {-10.8}{0.25}}$

$\displaystyle\longrightarrow\sf {\underline {\underline {F=-43.2\ N}}}$