Forces come in pairs When an object collides with another object, the two objects exert an equal and opposite force on each other for the same length of time. Force = change in momentum Time Force x time = change in momentum So the change in momentum of one body is the same as the change in momentum of the other body, in the opposite direction

Momentum is conserved In any collision or explosion momentum is always conserved total momentum after = total momentum before m 1 v 1 + m 2 v 2 = m 1 u 1 + m 2 u 2 This can be used to analyse the collision of two objects

5 kg3 kg5 kg3 kg 4 m/s2 m/s ? m/s Before After total momentum after = total momentum before m1v1 + m2v2 = m1u1 + m2u2 5v + 3 v = (5 x 4) + (3 x 2) 8v = 26 v = 26 / 8 = 3.25 m/s

The conservation of momentum explains the recoil of a gun A stationary tank of mass kg fires a 250 kg shell at a velocity of 200 m/s. Calculate the recoil velocity of the tank. total momentum after = total momentum before m1v1 + m2v2 = m1u1 + m2u v x 200 = x x v = v1 = v1 = - 5 m/s Recoil velocity of the tank is 5 m/s in the opposite direction

Rockets A rocket or jet moves forward because the exhaust gases go backwards. The momentum of the rocket forward= momentum of the gases backwards