Unbalanced Forces and Momentum

We've defined momentum as mass times velocity. We also know that a force is a push or pull. A force can be applied to an object to change its momentum. 
You can imagine this concept as American tennis player Andy Roddick applies a force as he hits a tennis ball with a baseball bat during practice. That force will definitely change the momentum of the tennis ball!

Newton's second law of motion defines force as the product of mass times acceleration.