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# Rockets and Newton's Laws

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Author: Janice VanCleave

### What You Need to Know

A law is a scientific statement that is generally accepted as true. Newton's third law of motion says that for every action force there is an equal and opposite reaction force.

### How Does Newton's Third Law of Motion Work?

Sir Isaac Newton (1642–1727) realized that if one object applies a force on another, the second object applies the same amount of force on the first object but in an opposite direction. Since these forces are not opposed, they are said to be unbalanced. Unbalanced forces cause the objects they push against to move in the direction of the force. The diagram on the left shows Newton's third law in action. When the boy hits the golf ball with the golf club, the ball is pushed forward, and the golf club is pushed backward. Two forces, A and B as shown, are needed to make these movements. Notice that the arrows for the forces are equal in size, are in opposite directions, and are on different objects. Force A is from the club hitting the ball, and force B is from the ball hitting the club. You can be sure that two forces are action-reaction pairs of forces if the reverse description of one force describes the other. In the figure, the identified action-reaction pair is A/B. The description of force A is "the club pushes against the ball," and the description of force B is "the ball pushes against the club."

### What Does This Have to Do with Creating a Balloon Rocket?

If an inflated balloon with an open end is released, the balloon will fly through the air due to the unbalanced forces making up action/reaction pairs of forces. In the diagram below, one set of action/reaction forces (A/B) is shown. Force A is on the balloon due to the gas inside the balloon pushing on the wall of the balloon. Force B is on the gas due to the balloon pushing on the gas inside the balloon. Because of these unbalanced forces, the gas is pushed out of the balloon and the balloon is pushed forward.

### Fun Fact

Action/reaction force pairs also make it possible for birds to fly. The wings of a bird push air downward (action). In turn, the air pushes the bird upward with an equal force (reaction).

### Real-Life Science Challenge

How does a rocket fly through empty space? Like the balloon, real rockets do not move because the exhaust gas pushes against the ground or air surrounding the craft. Instead, a rocket moves because of the unbalanced forces in action/reaction pairs. That's why rockets move in space where there is nothing for the exhaust gas to push against.

### Experiment

Now, start experimenting with your balloon rocket. What kind of balloon rocket goes the fastest? What kind goes the farthest? What effect do balloon size and shape have on the rocket's flight?

Hints

• Design a way to control the direction of the balloon rocket's flight. For example, you could run a string through a straw, then stretch the string between two objects, such as chairs. Tape the balloon to the straw.
• What else can make your balloon go farther, faster, or straighter?