Energy Stealer

If you’ve knocked me, then I’ve knocked you,
Like the wall that struck the ball, or the floor that struck the shoe.

Challenge: Level 180-degree turns.

Hint: Use three adjustable supports for each 180-degree turn. Clip two of them to one vertical tube, one right on top of the other. The bottom one with a tall support clip and the top one with a short clip. With a ball on the turn, slide the adjustable supports up or down to adjust the 180-degree turn so that the ball stays put.

Build the following:

Build the top section of track first. It has three parts: a long straight, a flex-track, and a 180-degree turn, in that order.

Attach the whole section of track with adjustable supports. All three adjustable supports on the upper left side of the framework should clip to the 180-degree turn.

Now build four identical sections of track. Each section has four parts: a flex-track, a short straight, a flex-track and a 180-degree turn, in that order.

Connect the four sections to the framework. Start by connecting the first section to the 180-degree turn of the top section. Attach each section to the framework by snapping three adjustable supports into the 180-degree turns at the end of each section.

Now add a flex-track and a long straight at the very end of the course.

Test each 180-degree turn to see that it is level. Place a ball in the middle of the turn. If it stays put, the turn passed the test.

When all the turns are level, place one ball on each turn.

You’re done! Now place a ball on the long straight at the top of the course. Before you release it, make a prediction. What will happen to the ball you release? What will happen to the other balls?

Surprise! The ball you released started a chain reaction, making each ball move down one level. Why? Because energy can be neither created nor destroyed. It has to be transferred or it has to change forms, or both.

When two objects with equal mass collide, the transfer of energy becomes quite interesting. The energy of the moving ball moves to the resting ball, making it move, and making the moving ball slow or stop. You can tell that not all of the energy was transferred though – if you use your ears. Some of the energy in the first ball changed to sound. Some of that energy also changed to heat, but it is difficult to detect. Meanwhile, the energy of the moving ball falls to zero, while the resting ball absorbs the energy and goes!

Where did the first ball get the energy that made the second ball move? Gravity is pulling all the time on both balls. When you released the ball at the top of the course, the many forces acting on it were no longer in equilibrium – they were out of balance. So something had to happen. What happened was that the ball began to move in response to the pull of gravity. Before you released it, its energy was potential energy – the energy of an object that has a distance to fall. When the ball began rolling, that potential energy started to become kinetic energy – the energy of motion. This was the energy that was transferred to the next ball.

Easy Variations

1. As soon as a ball reaches the bottom, grab it and place it at the top again to keep the energy moving.

2. Add the chain-lift (and a few sections of track) to do the heavy lifting for you.

3. If you have more than six balls available, start a new ball at the top as soon as the first ball hits the second ball. Can you keep balls in motion on very section of track?

Stage II

Let’s examine the change in energy of a ball as it moves from the top of the course to the bottom. For this experiment, remove all of the intermediate balls so that a ball can roll from the top of the course to the bottom without any collisions.

In this activity gravity pulls on the ball to make it roll down the track. As it rolls down the track, the work gravity does on the ball converts potential energy to kinetic energy. (You can read more about kinetic and potential energy in the physics tutorial.)

Compare the situation where the ball rolls down the track to one where you simply drop a ball from the starting height and let it fall to the bottom. How fast is the ball going when it reaches the bottom? How does this speed compare to the final speed of a ball rolling down the track? (The falling ball has a higher final speed than the one rolling down the track.)

Which ball loses more energy along the way? The rolling ball loses more energy. This is why it is going slower at the end. It has less kinetic energy. Most of this energy loss has been due to friction as it rolls down the track.

Stage III

Let’s do some measurements to estimate how much energy is lost as the ball travels down the track.

First, let’s calculate the energy of a ball that simply falls. Using the equations from the physics tutorial for potential and kinetic energy:

U = mgh (potential energy)

K = (mv²)/2 (kinetic energy)

For a falling ball all of the potential energy is converted to kinetic energy. So the kinetic energy is equal to:

K= mgh

For the ball rolling down the track, we can estimate it kinetic energy by measuring its speed. Measure the speed of the ball on the last long straight section of track. Do this by using a stopwatch to measure the time it takes to roll down this section of track. Use a ruler to measure the distance of the long straight piece of track.

The speed is computed using the formula:

V = distance/time

Put in your distance (in meters) and time (in seconds) to compute the average speed of the ball along this final piece of track.

Now we can estimate the kinetic energy of the ball using the formula above.

K = (mv²)/2

Compare the energy of the rolling ball to that of the falling ball? Which has more kinetic energy? Are you surprised at how much energy was lost along the way? (By lost, we mean converted through vibrations and friction into heat.)

This isn’t a perfect result. It’s just an estimate. See the last exercise to learn where we lost some accuracy!

Exercises

Stage I

1. Whenever you push on something, it pushes back. What’s pushing back on you right now?

Answer: The chair you’re sitting in is pushing back on you. Also, the floor is pushing back on your feet.

Stage II

1. Compare the energy lost as the ball rolls down the track to the energy lost when you walk down a flight of stairs. When you’ve reached the bottom of the stairs, how fast are you going? (Not very fast – at least I hope not) What would happen if none of the energy was lost (or dissipated) as you walked down the stairs?

Answer: You would be moving very fast and would probably get hurt when you reached the bottom and collided with the floor!

Stage III

1. In measuring the kinetic energy of the rolling ball, we measured its speed. We didn’t measure all of its kinetic energy, however. Can you figure out what energy we forgot?

Answer: The ball is rotating, and we didn’t count the energy that this requires. So the ball actually has more kinetic energy than we computed.

You can use what you learned from energy stealer in your own projects. Build some level track between some steeper sections of track. Then place a ball on the level section and predict what will happen when another ball hits it.