# A-level Physics (Advancing Physics)/Conservation of Momentum/Worked Solutions

1. A ball of mass 0.5 kg collides with a stationary ball of 0.6 kg at a velocity of 3ms−1. If the stationary ball moves off at a speed of 2ms−1, what is the new velocity of the first ball?

${\displaystyle 0.5\times 3=0.5v+(0.6\times 2)}$

${\displaystyle 1.5=0.5v+1.2}$

${\displaystyle 0.5v=0.3}$

${\displaystyle v=0.6{\mbox{ ms}}^{-1}}$

2. Two balls have masses 6 kg and 3 kg and velocities 4 m/s and 2 m/s respectively

Since momentum must be conserved:

${\displaystyle mu=mv_{1}+mv_{2}}$

${\displaystyle u=v_{1}+v_{2}}$ (1)

Since kinetic energy must be conserved:

${\displaystyle {\frac {mu^{2}}{2}}={\frac {m{v_{1}}^{2}}{2}}+{\frac {m{v_{2}}^{2}}{2}}}$

${\displaystyle u^{2}={v_{1}}^{2}+{v_{2}}^{2}}$

Substitute in the value of u from (1):

${\displaystyle (v_{1}+v_{2})^{2}={v_{1}}^{2}+{v_{2}}^{2}}$

${\displaystyle {v_{1}}^{2}+2v_{1}v_{2}+{v_{2}}^{2}={v_{1}}^{2}+{v_{2}}^{2}}$

${\displaystyle 2v_{1}v_{2}=0}$

Therefore, either v1 or v2 is 0. Using equation (1), if v1 is zero, then v2 = u, and vice versa. However, v1 cannot be the same as u, as this would mean that the first ball had to move through the second ball! So, the only physical solution is that the first ball stops, and the second ball continues moving with the first ball's original velocity.

4. A particle explodes to become two particles with masses 1 kg and 2 kg. The 1 kg particle moves with velocity 45ms−1. With what velocity does the other particle move?

${\displaystyle 0=(1\times 45)+2v}$

${\displaystyle 2v=-45}$

${\displaystyle v=-22.5{\mbox{ ms}}^{-1}}$

i.e. in the opposite direction to the motion of the 1 kg particle.

5. A 3 kg ball moving at 3ms−1 collides with a 5 kg ball moving at -5ms−1. The collision is perfectly elastic. What are the new velocities of the balls?

Since momentum must be conserved:

${\displaystyle (3\times 3)+(5\times -5)=3v_{1}+5v_{2}=-16{\mbox{ kgms}}^{-1}}$ (1)

Since kinetic energy must be conserved:

${\displaystyle {\frac {3\times 3^{2}}{2}}+{\frac {5\times (-5)^{2}}{2}}={\frac {3\times {v_{1}}^{2}}{2}}+{\frac {5\times {v_{2}}^{2}}{2}}}$

${\displaystyle 3{v_{1}}^{2}+5{v_{2}}^{2}=152}$

From (1):

${\displaystyle v_{2}={\frac {-16-3v_{1}}{5}}}$

${\displaystyle 3{v_{1}}^{2}+5\left({\frac {-16-3v_{1}}{5}}\right)^{2}=152}$

${\displaystyle 3{v_{1}}^{2}+5\left({\frac {9{v_{1}}^{2}+96v_{1}+256}{25}}\right)=152}$

${\displaystyle 3{v_{1}}^{2}+{\frac {9{v_{1}}^{2}+96v_{1}+256}{5}}=152}$

${\displaystyle 15{v_{1}}^{2}+9{v_{1}}^{2}+96v_{1}+256=760}$

${\displaystyle 24{v_{1}}^{2}+96v_{1}-504=0}$

${\displaystyle {v_{1}}^{2}+4v_{1}-21=0}$

${\displaystyle (v_{1}+7)(v_{1}-3)=0}$

So, v1 is either -7 or 3 ms−1.

If v1 = -7ms−1, by (1):

${\displaystyle -21+5v_{2}=-16}$

${\displaystyle 5v_{2}=5}$

${\displaystyle v_{2}=1{\mbox{ ms}}^{-1}}$

If v1 = 3ms−1:

${\displaystyle 9+5v_{2}=-16}$

${\displaystyle 5v_{2}=-25}$

${\displaystyle v_{2}=-5{\mbox{ ms}}^{-1}}$

This last solution is non-physical since it requires the balls to move through each other. So, v1 = -7ms−1 and v2 = 1ms−1

6. A ball collides with a wall, and rebounds at the same velocity. Why doesn't the wall move?

Let the mass of the wall be M, and the mass of the ball be m:

${\displaystyle mu=Mv_{wall}-mu}$

${\displaystyle 2mu=Mv_{wall}}$

${\displaystyle v_{wall}={\frac {2mu}{M}}}$

The mass of the wall is large. As M tends to infinity, therefore, vwall tends to 0.