Magnetic Acceleration and Linear Momentum

The Idea

This is a simple experiment with a very unexpected outcome. A steel ball is rolling in a track drawn by a magnet. The seemingly gentle force produces a powerful acceleration that propels the ball at high velocity. The results are quite amazing and provide an interesting insight into the nature of linear momentum, as well as magnetic fields.

What You Need

• 4 stainless steel balls
• 1 neodeum cylindrical magnet
• track to guide the steel balls (a grooved mounting bracket for curtains works well, and it has the added advantage of providing a handy end "bumper")

Method

1. Place the steel balls in the track.
2. Group three balls together.
3. Roll the fourth ball toward the other three.
4. Notice what happens. (This is not the surprising part, but it establishes a baseline of expectation.)
5. Place three balls on the track. Then, place the neodymium magnet to the right of the three balls.
6. Roll the fourth ball from the right side of the magnet with about the same speed as the ball in Number 3.

Expected Results

Without the magnet, the incoming steel ball stops and knocks out another ball. The dislodged ball continues with the same velocity of the incoming ball. This is the familiar case of conservation of momentum during an elastic collision, as shown in Figure 114-1.

With the magnet in place, a single ball is also knocked out, as shown in Figure 114-2. However, the ball that is knocked out surprisingly moves at turbo speed—much faster than the velocity of the incoming ball. The magnet increases the velocity of the incoming ball. This much higher momentum at the last instant is imparted to the outgoing ball, which shoots off at a surprisingly higher speed.

Why It Works

In both cases, linear momentum is conserved. With the magnet, the incoming ball is accelerated and achieves a very high instantaneous velocity just before it hits the magnet. Conservation of momentum requires that the outgoing ball moves at that same high velocity.

Linear momentum is always conserved if no force is doing work. In physics, work is force applied over a distance. A principle of physics called the work-energy theorem states that if a force is exerted over a distance, the kinetic energy of an object (and, as a result, its velocity) changes. In this case, a magnetic force is doing work, which accelerates the steel ball. Because the magnetic force increases as the ball approaches the magnet, the speed picks up at an even greater rate than a constant force.

Other Things to Try

Repeat with other combinations of balls on either side of the magnet.

The Point

Linear momentum is the same before and after a collision. Because the steel ball is accelerated rapidly by the magnet, the velocity of the ball (and its momentum) is very high just before the collision. Conservation of linear momentum requires the velocity of the ball after the collision also be very high.

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