An object’s momentum is the product of its mass and velocity. An object can have a large momentum by having a large mass or a large velocity. Momentum is a vector, and it is especially important to pay attention to the signs of the components of momentum.

A force of short duration is an impulsive force. The impulse $J_x$ that this force delivers to an object is the area under the force-versus-time graph. For time-dependent forces, impulse and momentum are often more useful than Newton’s laws.

Working with momentum is similar to working with energy. It’s important to clearly define the system. The momentum principle says that a system’s momentum changes when an impulse is delivered:

A momentum bar chart, similar to an energy bar chart, shows this principle graphically.

《 LOOKING BACK Section 9.1 Energy overview

The total momentum of an isolated system is conserved. The particles of an isolated system interact with each other but not with the environment. Regardless of how intense the interactions are, the final momentum equals the initial momentum.

《 LOOKING BACK Section 10.4 Energy conservation

One important application of momentum conservation is the study of collisions.

• In a totally inelastic collision, the objects stick together. Momentum is conserved.

• In a perfectly elastic collision, the objects bounce apart. Both momentum and mechanical energy are conserved.

This chapter looks at two other important applications of momentum conservation:

• An explosion is a short interaction that drives two or more objects apart.

  

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• In rocket propulsion the object’s mass is changing continuously.