0.18 Phy1160: force and motion -- momentum, impulse, and conservation  (Page 4/14)

Combine and rearrange equations

Therefore, by substitution we can write:

F = m * (v2 - v1)/t

Multiplying both sides by t gives

F * t = m * (v2 - v1), or

F*t = m*v2 - m*v1

where

• F*t represents impulse
• m*v2 and m*v1 each represent momentum
• m*v2 - m*v1 represents a change in momentum

A change in momentum

At this point, you should recognize that the product of mass and a change invelocity is a change in momentum.

In physics, the product of force and time is given the name impulse . It follows, therefore, that

impulse = F*t = change in momentum

This equation is often referred to as the impulse-momentum change equation .

The physics of collisions

One area of physics where momentum plays a large part is the physics of collisions. Momentum and possible changes in momentum are involved in the interactionsamong all moving objects, even when the changes in momentum are not visually obvious.

For example, the passage of a moving iron object through the magnetic field of a moving magnet will cause changes in the momentum of both the iron objectand the magnet, but the result may not be obvious.

Collisions may be more obvious

The type of interaction that we call a collision may be the type of interaction that is the mostfamiliar to us. Collisions between objects happen all the time. This module will discuss several examples that involve collisions.

Collisions in everyday life

I may be wrong, but I suspect that as a blind student, you may be more attuned to collisions in everyday life than your fellow sighted students.For example, each time the end of your cane touches an object, a collision has occurred. Regardless of how light the touch, you probably recognize that collision and take appropriate action.

The impulse-momentum change equation

A law associated with the impulse-momentum change equation may be expressed in the following way (see (External Link) ):

In a collision, an object experiences a force for a specific amount of time that results in a change in momentum. The result of the force acting for thegiven amount of time is that the object's mass either speeds up or slows down (or changes direction). The impulse experienced by the object equals the changein momentum of the object.

In equation form,

F * t = m * (v2 - v1)

All objects in a collision experience an impulse

When a collision occurs, each object involved in the collision experiences an impulse. The impulse is equal to the change in momentum.

A hypothetical collision with a punching bag

In gyms where students practice boxing, there is usually a large object called a punching bag. Some punching bags are large cylindrical containers made of leather orsome other pliable material filled with something like sand. Typically, they hang from the ceiling or from an overhead beam.

A void below the punching bag

Often, the punching bag is not attached to the floor and the bottom of the punching bag is often several feet above the floor. This leaves a void betweenthe bottom of the punching bag and the floor.