*Reference:** Beginning Physics I*

**CHAPTER 8: IMPULSE AND LINEAR MOMENTUM**

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**KEY WORD LIST**

**Impulse, Momentum, Conservation of Momentum, Elastic Collision, Inelastic Collision, Coefficient of Restitution, Ballistic Pendulum, Center of Mass**

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## GLOSSARY

For details on the following concepts, please consult **CHAPTER 8****.**

**IMPULSE**

Impulse is the product of the average force acting upon a body and the time during which it acts, equivalent to the change in the momentum of the body produced by such a force.

*I = F**t*

** I **may be resolved into

*x*and

*y*components. It is equal to change in momentum,

**MOMENTUM**

If an object of mass *m* is moving at a given instant of time with velocity ** v** then,

The concept of linear momentum can be generalized to two or three dimensions.

**CONSERVATION OF MOMENTUM**

When there are no external forces acting on a system (or when the resultant of external forces acting on a system is zero), the total momentum of the system is conserved. In other words, total final momentum = total initial momentum.

**ELASTIC COLLISION**

An *elastic collision* is one in which the total kinetic energy of the colliding objects is the same just before and just after the collision.

**INELASTIC COLLISION**

An *inelastic collision* is characterized by a certain disappearance of kinetic energy in the collision process.

**COEFFICIENT OF RESTITUTION**

The *coefficient of restitution (e)* is defined as the ratio of the magnitude of the relative velocity after the collision to that before the collision.

For an elastic collision, e = 1. Generally speaking, the smaller the e value, the more thermal energy is generated and hence the more kinetic energy is lost.

**BALLISTIC PENDULUM**

A *ballistic pendulum* is a device that is used to measure the velocities of small swift projectiles such as bullets. See the sketch above.

**CENTER OF MASS**

The *center of mass* is the point at which the entire mass of a body may be considered concentrated for some purposes. It is defined as the position of the average displacement of the particles of the body, weighted according to mass.

For a rigid body, the *center of mass* of the body moves as if it was a particle having a mass equal to the total mass of the body acted on by the resultant force on the body. The *center of mass* is a geometric point fixed in relation to a rigid body, but it is not necessarily in the body.

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