6.3 Other examples of momentum effects

The above examples involve collision but there are all sorts of momentum situations e.g.

When a gun is fired the bullet goes in the forward direction (positive momentum), but the gun recoils in the complete opposite direction (negative momentum).

From Newton's 3rd law, you get an equal and opposite force reaction.

The bullet goes one way and the gun the other way from the force of the explosion.

numerically, ignoring +/- signs: m_gunv_gun = m_bulletv_bullet     (see Q2 in part 6.4)

Note that the combined momentum = zero, which it is at the start and immediately after the gun is fired.

Before a rocket is fired it has zero velocity and zero momentum.

When the rocket's propellant is fired, the rocket goes one way (positive momentum) and the exhaust gases go in the opposite direction (negative momentum).

So, in conserving momentum, we can say ..

numerically, ignoring +/- signs: m_rocketv_rocket = m_{exhaust gases}v_{exhaust gases} 

Because the rocket produces gases with great force and velocity, the rocket must respond according to Newton's 3rd law of motion - equal and opposite forces in dramatic action!

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INDEX physics notes: Conservation of momentum, elastic and non-elastic collisions, Newton's 2nd law calculations - problem solving