Laws of Physics: Newton's Third Law and Conservation of Linear Momentum

The Idea

If Sir Isaac Newton had a skateboard, it might have saved him some time in discovering his third law, although Newton might have had so much fun doing it, he wouldn't have had time to invent calculus. The laws of physics apply to all objects. Sports, in particular, can be thought of as intuitive applications of the principles of physics. This experiment takes advantage of the fact that all objects in the universe follow the laws of physics. We focus particularly on Newton's third law and conservation of linear momentum.

What You Need

• 2 rolling chairs
• or 2 people capable of keeping their balance on skateboards (each with helmets); rollerblades will also work
• medicine ball or a several pound mass, such as a bowling ball
• safe place to do this

Method

1. Two people face each other sitting in the chairs on rollers, a few feet apart.
2. One person tosses the medicine ball to the other (both are seated in chairs). Feet should be kept off the floor, so the chairs are free to move.
3. The two people again face each other. One tries to push the other. What happens?

Expected Results

The person who catches the ball, as well as the person who throws the ball, will move backward. Similarly, the person doing the pushing, as well as the person getting pushed, will recoil backwards.

Why It Works

Momentum is mass times velocity.

At the start of this, the two skateboarders have zero momentum (they have mass, but no velocity, so their momentum is zero).

The velocity of the ball transfers momentum from the first to the second person. The first person recoils backward. The second person also moves backward in the opposite direction.

Another principle illustrated here is Newton's third law: For every action (movement of the ball), there is an equal and opposite reaction (recoil of the person in the chair).

Other Things to Try

The previous demonstrations can be done by skateboarders or rollerbladers. (Please remember, although we are interested in horizontal action and reaction here, gravity is still active in the vertical direction, so keep your balance.)

Mousetrap and tennis ball

Conservation of linear momentum and Newton's third law can be demonstrated by attaching a mousetrap to a low-friction cart. The trap is set and a tennis ball is positioned in place of the cheese. When the mousetrap is released, the process of tossing the ball results in the equal and opposite reaction of the mousetrap recoiling in a backward motion. This is shown in Figure 26-1. Both the mousetrap and the ball initially have zero momentum. The momentum of the ball going to the left is equal but opposite to the momentum of the mousetrap and cart moving to the right.

Fan car

Putting a propeller on a cart with wheels, as shown in Figure 26-2, propels the cart forward (or backward if turning the other way).

What would you expect to happen if a sail is put in front of the propeller to catch the air, as shown in Figure 26-3? Some people would say the cart will move faster because the force from the fan will "push" the cart. However, what we find is this: with the sail in place, the cart does not move as it did without the sail. This is a surprising result for many people seeing this for the first time. The reason for this is, without the sail, the equal and opposite reaction of the propeller causes the cart to move forward. However, with the sail in place, the force of the propeller balances the reaction force. As a result, there is no net force and the cart does not move.

The Point

Linear momentum is conserved in the absence of external forces. For every action, there is an equal and opposite reaction.

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