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# Newton's Third Law of Motion

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Author: Janice VanCleave

### Action-Reaction

Isaac Newton (1642–1727), the famous British scientist credited with discovering gravity, also gave us three laws describing motion. Newton's first law of motion states that a force is needed to change the motion of an object. In other words, a force either starts an object moving or causes a moving object to stop. His second law of motion explains how the force needed to accelerate (a change in velocity) an object depends on the mass of the object. His third law explains that forces act in pairs.

In this project, you will demonstrate Newton's third law of motion, that every action has an equal and opposite reaction due to the action of paired forces. You will also determine how pairs of forces that are equal but acting in opposite directions can produce motion.

### Getting Started

Purpose: To demonstrate Newton's third law of motion.

### Materials

• pencil
• 5-ounce (150-ml) paper cup
• 40 to 50 pennies
• 12-inch (30-cm) piece of string
• 1 handheld spring scale

### Procedure

1. Use the pencil to make two holes across from each other just beneath the rim of the cup. Place the coins in the cup.
2. Loop the string through the holes, then tie the ends of the between the holes.
3. Hold the scale and adjust it so that it reads zero.
4. While holding the scale, attach the cup so that the cup hangs freely. Observe the reading on the scale.

### Results

The cup pulls the scale down so that the measurement on the scale indicates the weight of the cup and the coins.q

### Why?

Newton's third law of motion states that for every action there is an equal and opposite reaction due to pairs of forces. In other words, Newton realized that if one object exerts a force on another, the second object exerts an equal force but in the opposite direction on the first object. You can be sure that two forces are action-reaction pairs of forces if the reverse description of one force describes the other force. In Figure 54.1, the three identified action-reaction pairs of forces are A/A1; B/B1; C/C1. The description of force A is "the hand acts on the scale," and the description of force A1 is "the scale acts on the hand." One description is the reverse of the other, so the forces are equal in magnitude, opposite in direction, and act on different objects. Thus forces A and A1 are an action-reaction pair.

### Try New Approaches

The scale attached to the cup measures the downward force of the cup (the action). How can the upward force of the hand be measured (the reaction)? Repeat the experiment using two scales. First hang one scale from the other. So that the weights of the scales are not considered, while holding the top scale (called A), adjust the scales so each reads zero. Let the top scale be A and the bottom scale be B. Attach the cup to the bottom scale (called B) as before. The reading on scale A measures the upward force (the reaction force) and scale B measures the downward force (the action force). Science Fair Hint: A diagram showing the action-reaction pairs can be used as part of a project display.

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