Recoil & Momentu

m

Recoil/Explosion: Action-ReactionInitial momentum of entire system is zero (neither object has KE – neither is moving)

Before a cannon fires a cannonball, both are at rest, so total momentum is zero

Example:

Recoil/Explosion: Action-ReactionThe interaction between the two objects must be equal and opposite

After the cannon fires the cannonball, both have the same momentum, but in opposite directions, so the total momentum still is zero

Note the velocity vectors – why does the cannonball have a greater magnitude of velocity than the cannon?

Its mass is less, so its velocity has to be more, than the cannon’s

When the astronaut throws the ball, she goes in the opposite direction

Recoil Formula

0 = m1v1f + m2v2f

You Try It!A 2 kg rocket zooms upward at5 m/s. It is propelled by 1 kg of water shooting downward. How fast does the water shoot out? Momentum Rocket =

Momentum Exhaust

0 = m1v1f + m2v2f

v2f = = -10 m/s

0 = (2 kg)(5 m/s) + (1 kg)v2f