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Physics:Balloon Rocket
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 he first object, but in an opposite direction. Since these forces are not opposed, they are said to be unbalanced. Unbalanced forces cause the objects to move in the direction of the force. The diagram 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. The 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.”
Discover for Yourself!
Demonstrate how unbalanced forces can cause a balloon to blast off, sending it flying along string. The diagram shows a string threaded through a straw. The string is stretched between two chairs. The balloon rocket is an inflated balloon taped to the straw. Note: I find it easier to first inflate the balloon, then while holding the neck closed, a helper tapes the balloon to the straw. Move the straw i to one end of the string, and then release the balloon. The straw with the attached balloon blasts off toward the opposite end of the string.
The action/reaction pair for the balloon rocket can be described as:
1. The force of the balloon (side opposite opening openign) pushes on the air inside the balloon.
Results: Air is pushed out the open mouth of the balloon.
2. The air inside the balloon pushes on the balloon (the side opposite the opening).
Results: The balloon is pushed in the direction of this force sending the balloon forward–it goes in a straight line because it is secured to the straw with the string threaded through it.
For more fun motion and other physics activities, see:
Janice VanCleave’s Physics for Every Kid
