How is Lenz's Law Used in Drop Tower Rides?

Imagine you’re sitting at the top of a drop tower ride at an amusement park. You anxiously wait for the free fall to begin. But before it does, the power goes out. Panicked thoughts begin to run through your head. How am I going to get down safely? How will the braking system work without electricity?

Did you know?

The tallest and fastest drop tower ride is Zumanjaro: Drop of Doom. It is over 125 m tall and drops at a speed of 140 km/h. 

Luckily for you, the braking system at the bottom of the ride doesn’t need to be plugged in. It also doesn't rely on a backup battery or generator during a power outage. Instead, drop tower rides use magnetic brakes in conjunction with hydraulic cylinders or air-pressurized brakes. Magnetic brakes work based on electromagnetic induction, a system that can be explained using Lenz’s law.

What is electromagnetic induction?

In 1831, Michael Faraday discovered that he could create an electric current in a loop of wire by placing a magnet through it. He also found that he could produce an electric current by moving the coil of wire over the magnet when it was stationary. 

This was the principle of electromagnetic induction at work. This principle states that a change in a magnetic field produces an electric current. Specifically, when a conductor is in the presence of a dynamic (changing) magnetic field, an electric current is produced. In Faraday's experiment, the wire was the conductor. The magnet he moved through and over the wire created the dynamic magnetic field.

Electromagnetic induction occurs when an alternating current flowing through a circuit generates a current in another circuit simply by being placed near it. Do you or anyone you know have an induction cooktop in your home? If so, you have seen an example of electromagnetic induction. Beneath the heating element there are coils that generate an alternating current. When you cook with induction cookware such as iron, it acts as a second conductor. A current is induced on your cookware without the actual coils beneath the element touching it. Inside your cookware, the induced current converts to heat. The heat is used to cook your food. 

Today, many everyday items rely on the principle of electromagnetic induction - even your cell phone batteries! 

What is Lenz’s Law?

Lenz’s Law states that an induced electric current flows in such a direction that its own magnetic field opposes the change that produced it. It was formulated by Russian physicist Heinrich Lenz in 1834, just a few years after Faraday’s discovery.

You can observe Lenz's law in the video below. If you can't watch it, here is what happens. The presenter drops a magnet into a copper tube. The magnet is not touching the sides of the tube. And yet, the magnet moves through the tube very slowly.

Why does this happen? Well, copper is a conductor. We know from Faraday's law that as the magnet passes through the tube, it creates an electric current in the copper. And as Lenz's Law explains, that current will generate a magnetic field that will oppose the magnet. The magnet is moving downwards, so the generated magnetic field will move upwards. As a result, the magnet moves slowly.

The presenter then drops a non-magnetic object through the same copper tube. Because there is no magnet, no current and no opposing magnetic field, the object falls through the tube at normal speed.

Did you know?

The Shanghai Maglev Train uses electromagnetic induction to levitate above its track and reach speeds up to 430 km/h. 

Why does a changing magnetic field create an electric current in the first place? To understand, remember the Law of Conservation of Energy. Energy cannot be created or destroyed, but one type of energy can be transformed into another. Lenz's Law and electromagnetic induction demonstrate how kinetic energy is transformed into electrical energy.

Let’s think back to the eddy current tube example shown in the video. When you move the magnet into a coil (in this case, a tube wrapped in aluminum), kinetic energy exists in the movement of the magnet. The kinetic energy is transformed into electrical energy, which creates an induced current in the coil. The electric current in the coil also produces a magnetic field. The magnetic field opposes the direction of the magnet's motion. The magnet is going downward, so the magnetic field is going upward. 

How does Lenz’s Law work in drop tower rides?

Let’s get back to the drop tower ride, where you’ve been stranded all this time. Each cart in the ride has permanent magnets under its seat. Copper strips are mounted vertically in the lower third of the tower. When the cart falls, the fall generates kinetic energy. That kinetic energy is transferred to the cart, including the magnets below the seats. When the magnets move past the copper conductor, the kinetic energy is transformed into electrical energy. This induces an electric current. The induced current in the copper strips also creates a magnetic field. And as Lenz’s law states, that magnetic field opposes the motion of the magnets. As a result, the magnetic field pushes up against the seat, causing the cart to slow down. This is how a drop tower ride’s magnetic brakes work. These magnetic brakes are used with hydraulic cylinders to further slow the falling cart down. The result is a reliable, no-friction braking system.

So relax! Even if the power goes out, you’ll get back to the bottom safely!