Magnetic Linear Accelerator

Have you ever seen one of those roller coasters that shoots out of thestation at an insanely high speed? These roller coasters don’t need toclimb hills first to use gravitational potential energy—their power comesfrom magnetism and energy conservation.

A series of elect romagnets (magnets made by pumping electricalcurrent through coils of wire) alternately push and pull on therollercoaster, pumping up its speed pretty quickly. Some engineers haveimagined using the same idea to launch objects into space (from, say, abase on the Moon) without using rockets.

In this project, you’re going to build a very simple magnetic accelerator tolaunch steel balls at targets. What could possibly go wrong?

Problem: Build a simple magnetic linear accelerator.

Materials

Wooden ruler with groove along the middleFour small, powerful magnets (e.g., neodymium magnets)Nine steel balls, roughly 5/8” in diameterTapeHobby knifeSafety Goggles

Procedure

1. Place the ruler (flat side down) on a table.

2. Lay one magnet in the ruler groove, about 2.5” from the ruler’s end. Use the tape to secure themagnet to the ruler and the knife to trim the tape to the size of the magnet.

3. Repeat step 2 with each of the remaining three magnets, placing each about 2.5” away from thepreceding magnet.

4 . To the right side of each magnet, place two steel balls in the groove.

5. Place a “target” a few inches to the right of the ruler. Your tape dispenser will work fine.

6. Place the ninth ball in the groove on the far left end of the ruler (opposite the target).

7. Put your safety goggles on.8. Let the ball go and stand back!

Results

The ball will be attracted to the first magnet and set off a chain reaction of balls firing between themagnets until the last one flies off the ruler at high speed to strike its target.

Why?

What you just saw is a fantastic example of energy conservat ion. Energy from one ball getstransferred to the next, and then to the next, and so on. But where is all the energy in the last ballcoming from if the first ball starts off from rest?

The answer is in the magnets.

Before the starting ball is released, there is potent ial energy stored up between the ball and the firstmagnet. The magnet and ball feel an attractive force, but your finger is preventing anything fromhappening. Once you let go of the ball, it gets drawn towards the magnet (which won’t move becauseit’s taped down). Potential energy gets converted to kinet ic energy—the energy of motion. This is nodifferent then holding a ball in the air and letting it go.

Eventually the ball strikes the magnet—but where does all that energy go? Well, it gets transferred tothe balls on the other side of the magnet. The ball closest to the magnet is held pretty tight, but thesecond ball is farther away and doesn’t feel as strong an attraction to the magnet. This means there’senough kinetic energy from the first ball to send this ball flying off with nearly the same amount ofenergy. (That’s why we need two balls stuck to the other side of the magnet: to lessen the attractive

force a bit. Try getting it to work with only one ball loaded up next to each magnet and see whathappens.)

This second ball is launched at roughly the same velocity as the first ball achieved. As this second ballgets drawn to the second magnet, the attractive force causes it to accelerate and hit the secondmagnet at a higher velocity than the first ball hit the first magnet. The third ball takes off with the highestvelocity achieved by the second ball, and since it gets accelerated by the third magnet in turn, it strikesthird magnet faster and harder than the first two balls struck their respective magnets.

Are you seeing a pattern begin to emerge? With each added magnet, more kinetic energyaccumulates in each launched ball. The last ball takes off with the combined kinetic energies of all theballs that came before it!

In principle, you can add more rulers and magnets and get the final ball moving as fast as you like—upto a point. Eventually, the balls would be moving fast enough to break the magnets, a limit for which I’msure your target is very thankful.