Conservation of Momentum

Consider two objects that "interact" or collide or have some effect on each other:

From Newton's Third Law of motion, we know

F12 = - F21

In terms of momentum, this means

F12 = dp1/dt

F21 = dp2/dt

so that

dp1/dt = - dp2/dt


dp1/dt + dp2/dt = 0

d ( p1 + p2) /dt = 0

dpTot/dt = 0


pTot = p1 + p2

That is, the total momentum of the system remains constant. This is Conservation of Momentum

Conservation of Momentum is essentially a restatement of Newton's Third Law of Motion.

Linear Momentum

Return to ToC, Ch9, Linear Momentum

(c) Doug Davis, 2001; all rights reserved