homework help??

Question

Newton’s 2nd law says that if there is no net force exerted on a system no acceleration is possible does it follow from this that no change in momentum can occur. Why or why not

Answer 1

Conservation of momentum requires that no external forces act on the system. So dp/dt = 0 = m*dv/dt + v*dm/dt where p =momentum, m = mass and v = velocity. Now assuming the mass does not change, then

dm/dt = 0 and thus dv/dt = acceleration, a = 0. Since force = ma, there is no force acting if momentum is conserved.

Now you can have an elastic collision between billard balls for example, where each ball's momentum, may change as a result of the collision, but the total momentum of all the balls invovled in teh collision is constant.