0.18 Phy1160: force and motion -- momentum, impulse, and conservation  (Page 2/14)

Supplemental material

I recommend that you also study the other lessons in my extensive collection of online programming tutorials. You will find a consolidated index at www.DickBaldwin.com .

Discussion

This module will describe and discuss some scenarios which, never having been seen, may be hard for a blind student to imagine. Some of those scenarios can be difficult tobelieve even when you can see them.

Newton's cradle

The behavior of Newton's cradle is somewhat difficult to believe even when you see it in operation. Newton's cradle is a gadget that is often found in novelty shops. Ittypically consists of an open wood or metal frame with about five steel balls suspended by strings on parallel beams that run from one end to the other alongthe top.

Several steel balls in a row

Each ball is suspended by two strings so that the ball forms the lower vertex of a triangle and the two equal-length strings form the sides of the triangle.Each string is attached at the upper end to a beam. The purpose of suspending each ball by two strings instead of suspending them on a single string is to cause all of the ballsto swing back and forth along the same straight line.

A collision

When the system is in equilibrium, the balls are lined up in a row and each ball barely touches the one next to it. If you pull one of the balls at the endback and then release it, allowing it to swing down, it will strike its neighbor on the downswing.

Only the ball on the other end appears to move

Surprisingly, the neighboring ball doesn't appear to move when struck, nor does its neighbor, nor does that neighbor's neighbor. The only ball that appears to move isthe ball at the far end of the line. That ball will be sent off in an upswing.

The process is reversed

When that ball reaches the top of its upswing, it will reverse direction, swing back down, and collide with its neighbor. This causes the ball that wasused to start the process to be sent off in an upswing.

The process continues

Left alone, this process will continue until all of the energy in the systemhas been dissipated, which can be many minutes or even hours later.

Newton's cradle illustrates the conservation of momentum. You will find an interestingarticle that explains some of the technical details at (External Link)

Momentum

You learned in an earlier module that momentum is the product of the mass of an object and the velocity of the object. Because velocity is a vector quantity,momentum is also a vector quantity. The direction of the momentum vector is the same as the direction of the underlying velocity vector.

A common symbol for momentum is p

A common symbol for momentum is p . Momentum is a derived item in the SI system of units. In the SI system, momentum is defined as kg*m/s.

In equation form , therefore:

• momentum = mass * velocity, or
• p = m * v in units of kg*m/s

Mass in action

Momentum can also be thought of as "mass in motion." Since all objects have mass, if an object is moving, its mass is in motion. Therefore, the object has momentum.