Newton's Third Law of Motion

This experiment comes from my hotel room. I was traveling do some consulting for a science museum, and was sitting at the desk, thinking of ideas for experiments. As I sat, I swiveled back and forth in the chair. As I swung back and forth, I decided that it would make an interesting experiment.

To explore this, you will need:

  • a swivel chair that will let you spin around
  • pillows or other weights that will not cause damage if you throw them

Start by placing the chair in an open space, with plenty of clearance in all directions. This will prevent broken lamps and e-mails to me from upset parents. Sit in the chair and lift your feet off the ground. Now, without touching the floor, the base of the chair, or other objects, try to get the chair to turn. Not very easy, right? With some experimentation, you should be able to get it to move some. If you twist the upper part of your body quickly to the right, the chair should turn slightly to the left. If you twist back to the left, the chair should turn to the right.

Next, get a heavy pillow or something similar. Again, making sure that your chair can turn easily, hold the pillow away from your body. Holding your arm straight, swing the pillow from the side and release it. As you throw the pillow in one direction, your chair should turn in the other direction. Try this again with something heavier. I used a bag of laundry. The heavier the weight that you throw, the more your chair should spin. Also, the farther you throw the weight, the more you will turn.

Why does this happen? There are some very basic laws that govern how things move. Isaac Newton boiled them down into three basic laws. Newton's third law of motion tells us that for every force, there will be an equal force in the opposite direction. As you saw when you first tried to get the chair to turn, you need something to push against. If you can push against the floor, it is easy. As you push against the floor, an equal amount of force pushes you away, turning the chair.

The same thing happens when you twist your body. As your muscles push your upper body to the right, they also push your lower body to the left. Instead of pushing against the floor, you are pushing against your own body. In the same way, when you throw the pillow in one direction, an equal amount of force pushes you in the opposite direction. That is why throwing a heavier weight makes you spin more. The more force it takes for you to throw the pillow, the more force you get to push you in the opposite direction. (This is actually part of Newton's Second Law of Motion.)

Since a heavier weight takes more force to move it, what would happen if we used a very heavy object, like a refrigerator? Do not try to throw the refrigerator across the room. Instead, move your chair near your refrigerator. Sitting in the chair, reach out and push against the refrigerator. What happens? Your chair turns, but the refrigerator stays still.

If there is an equal force in both directions, why doesn't the refrigerator move too? With your chair beside the refrigerator, reach out and place your hand against it. Use a tiny amount of pressure to push against it. Nothing happens. Neither you nor the refrigerator moves. As you push a little harder, you will reach the point where your chair begins to move. At that point, there is enough force in your direction to move the chair, but not enough force in the opposite direction to move the refrigerator. I tried making the refrigerator lighter by removing some of the ice cream, but it did not seem to make a difference in my results. Even after I added the weight of the ice cream to my body, the results were still the same, but you might want to try that yourself, just to make sure.