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Illustration of Newton's Third Law of Motion

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Author: Jerry Silver

The Idea

Newton's third law states that for every action (force), there is an equal but opposite reaction (force). This project illustrates how this concept can be applied to a particular physical situation. The outcome may be different than what many people expect.

What You Need

  • 1 ping-pong ball attached to a string
  • 2 beakers (or jars) filled with enough water to immerse the ping-pong ball
  • balance scale
  • counterweights

Method

  1. Set each of the beakers on the opposing pans of the scale and establish a balance, as shown in Figure 28-1.
  2. Predict what will happen when the ping-pong ball is lowered into the beaker of water. Will the side with the ping pong ball
    1. Rise?
    2. Fall?
    3. Remain balanced?
  3. Lower the ping-pong ball into the beaker and observe what happens.
  4. Remove the ping-pong ball. What is the effect on the balance?

Expected Results

Lowering the ping-pong ball into the beaker forces that side of the balance down, as shown in Figure 28-2.

When the ping-pong ball is withdrawn, the balance is restored.

Why It Works

There is a buoyant force on any object immersed in water (or partially immersed in water such as a floating ping-pong ball). For every action there is an equal and opposite reaction. In this case if the action (the buoyant force) is up, the reaction must be down, causing the observed effect.

 

Pushing water. Birds flying inside a truck.

Other Things to Try

This is similar to the enigma: if birds are in a truck, will the truck weigh less if the birds are flying, instead of at rest on the floor of the truck bed? It turns out that the force exerted by the birds' wings exerts the same downward pressure on the truck bed as the weight of the birds at rest. (As with the previous experiment, this is also addressed by a Mythbusters episode.)

The Point

This experiment shows how a reaction force is established by Newton's third law.

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