Bowling Ball of Faith

Physics:	Energy Transfers, Laws of Motion
Grade Range:	Middle School, High School
Format:	Stage

The Bowling Ball of Faith is easy to do, easy to setup and easy to amaze with! Be sure to practice this demonstration a few times before presenting it so you know how to stand when you let go.

Materials

• Bowling Ball with Ceiling Hook
• Wire or strong twine/string
• A Place to Hang it

Safety Precautions

Please read the Physical Demonstration section of the Demonstration Safety page before performing this demonstration.

This demonstration requires: Safety Glasses

Demonstration

1. Loop the wire on a bar or hook in the ceiling of the presentation space. Give it enough slack so that, when you hang the ball from it, the ball will be about 4 feet off the ground.
2. Explain to the audience that the ball is currently hanging like a pendulum, and briefly talk about pendulum motion.
3. Grab the ball and pull it back, walking backwards until you can have the ball level with your face. Hold it up against your face, and ask the audience if they think it might hit you when you let go and let it swing.
4. Release the ball. On the return swing, it will get close to your face, but won't hit you!
   • Note: When you release the ball, make sure that you don't lean forward. Otherwise, you might end up leaning into a face tap with the bowling ball!

Why This Works

Pendulum Motion is a common topic in physics classes. When you have a weight attached to a string, you can track how it is moving and the pattern of movement quite easily. Our bowling ball is acting just like a pendulum, and so we can look at all the steps that happen during the swing, and see how the energy in the system is being transferred. When we first let go, the ball has Potential Energy (PE), or the potential to move, due to the pull of gravity on it and the distance between its height and the lowest point of the swing. When the ball gets to the lowest point in the swing, all of the PE has been transferred into Kinetic Energy (KE), or energy of movement. When it reaches the top of the swing, the KE has now converted back to PE, which results in the ball coming back into a swing. But, some of the energy converts into other forms. Some will become Thermal Energy (TE), due to the friction between the loop of string and the bar on the ceiling, or because of friction with the air. Some will convert into Sound Energy, which we hear as the ball swings back and forth. Because of some energy being converted into these other forms, the ball will not be as high on the return swing as it was before, and won't hit the presenter!

Our pendulum can also be used to show us how Newton's Laws of Motion work.

• The first law is the law of inertia: An object in motion will stay in motion unless acted on by an outside force. Likewise, an object at rest stays at rest unless acted on by an outside force. When we start the pendulum, we put the bowling ball in motion. The bowling ball will continue that motion, and will continue to swing on the string, until an outside force acts upon it. In this case, we know that there is friction acting on the pendulum, so over time it will eventually come to rest because of the friction.
• The second law is the force equation: a force is equal to an object's mass multiplied by the acceleration, or F=m*a. In the case of a pendulum, we can calculate the force it has by multiplying the mass of the bowling ball by the acceleration of gravity.
• The third law is force conservation law: every action has an equal and opposite reaction. When we pull back the bowling ball to the start position, we are providing a force on the bowling ball that is counteracting the force of gravity. By applying a force to the bowling ball to raise it higher, the bowling ball will be able to accelerate over a larger distance, balancing out to the force we applied.

Additional Information

• This demonstration pairs well with the Angular Momentum Chair and Free Falling Demonstrations