Using Magnetic Balls to Show Conservation of Energy

As a steel ball rolls down a ramp to collide with three other balls, momentum and energy are conserved.

When a magnetic ball rolls down the same ramp, it launches the other ball completely off the ramp!

Teachable Topics:

• Conservation of Energy
• Conservation of Momentum
• Magnetism

Theory:

Two of the fundamental principals in physics and in life are that energy and momentum are conserved. In this experiment, those principles are demonstrated, as energy is converted from one form to another.

By splitting the motion of the ball into two parts, it becomes easier to analyze. The first part will be from when the ball is released at the top of the ramp, to immediately before the collision. The second part will be from the collision to the point where the next ball reaches its maximum potential energy (at the maximum height the ball reaches).

When the steel ball is at rest at the top of the ramp, it has zero kinetic energy, but it has potential energy that is equal to its mass multiplied by the height above the bottom of the ramp, multiplied by the acceleration due to gravity (E=mgh). The balls at the bottom of the ramp have zero kinetic energy and zero potential energy.

As the ball at the top is released, its potential energy is converted into potential energy, and immediately before the collision with the other balls, it has kinetic energy of 1/2*m*v² and zero potential energy. This final kinetic energy is equal to the initial potential energy (or if friction is taken into account, kinetic + energy lost due to friction = initial potential energy)

As the ball reaches its maximum height, potential energy is at a maximum, and kinetic energy is zero. If energy is conserved the ball will have reached the same height the original ball was released from.

For the magnetic ball, the situation is a little different. The ball's energy still changes from potential to kinetic, but as the magnetic ball rolls towards the steel ball, the magnetic force causes the ball to accelerate a small amount, so that the kinetic energy just before the impact is slightly larger than it was for the steel ball in the first situation. Since the kinetic energy right before the collision was larger, the kinetic energy right after the collision must be too. This is why the ball launches off the ramp. If the ramp had been higher, the second ball would have reached a higher point than the magnetic ball was released from.