# Motion and Forces: Newton's Third Law of Motion

By Accessible Science on Dec 04, 2014

The activity described below works well for all students. The student with a visual impairment can best experience the activity as the person who has a chance to release the balloon*. Also beneficial is an opportunity to be at the end when the balloon rocket arrives. The whole class will want to do this activity again and again.

*NOTE: Balloons should NOT be used by students with latex allergies. If in a latex-free school, use mylar balloons.

### Vocabulary

￼Action force - force acting in one direction
Reaction force - force acting in the opposite direction

### Action and Reaction

Forces always act in pairs. The two forces act in opposite directions. When you push on an object, the object pushes back with an equal force. Imagine a person sitting in a rolling chair at a desk. When the person sitting in a rolling chair pushes on the desk, this push or force is the action force.

Now, the desk pushes back against the person with a force of the same size. This reaction force will cause the rolling chair to move backwards. Notice that the two forces act on different objects. The action force acts on the desk. The reaction force acts on the person.

### Newton’s Third Law

Newton’s third law of motion describes action and reaction forces. The law states that for every action force, there is an equal and opposite reaction force. Imagine hitting a tennis ball. The racket exerts a force on the ball. This is the action force. The ball exerts an equal and opposite force on the racket. This is the reaction force.

Newton’s third law explains how many sports injuries are caused. The more force you use to a hit a tennis ball, the more reaction force your arm receives from the racket. Every time your feet hit the ground when you are running, the ground hits your feet with an equal and opposite force.

### Balloons and Rockets

Newton’s third law explains how balloons and rocket engines work. When the neck of an inflated balloon is released, the stretched rubber material pushes against the air in the balloon. The air rushes out of the neck of the balloon. The action of the air rushing from the balloon pushes against the balloon, moving it in the opposite direction.

When rocket fuel is burned, hot gases are produced. These gases expand rapidly and are forced out of the back of the rocket. This is the action force. The gases exert an equal and opposite force on the rocket itself. This is the reaction force. This force pushes the rock upward.

## Materials

• String the length of the classroom
• Balloon
• Tape
• Straw

## Procedure

1. Feed one end of the string through the straw.
2. Tie the string to stationary objects on opposite sides of the room. Make sure to stretch the string tight.
3. Blow up the balloon and hold it so that no air can escape but don’t tie it.
4. Have someone hold the balloon under the straw and tape the balloon to the straw in two places.
5. Let go of the balloon and it will take off like a rocket.

### Questions and Conclusions

1. Forces always act in __.
2. If a book is sitting on a table, is the table exerting a force? If so in what direction?
3. What effect would blowing more air into a balloon have on the motion of the balloon when released?
4. For every action force, there is an equal and __ reaction force.
5. How are action and reaction forces different?
6. In a rocket engine, the __ force pushes the rocket upward.
7. Action forces and reaction forces always act on __ objects.
8. An object resting on a table weighs 100 N. With what force is the object pushing on the table? With what force is the table pushing on the object?
9. When you walk, your feet push against the ground. At the same time, the ground pushes against your feet. Which is the action force? Which is the reaction force?

## NGSS Standards:

PS2.A: Forces and motion: The role of the mass of an object must be qualitatively accounted for in any change of motion due to the application of a force. (Grades 6-8)

Article and activity adapted from Concepts and Challenges: Physical Science, Fourth Edition. Parsippany, NJ: Globe Fearon Inc., Pearson Learning Group, 2009, pages 280 to 281.

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