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| Newton's Third Law of Motion |
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| Suppose you are watching the lift off of a space-shuttle. You hear a deafening roar and see burning gases shooting from the exhaust vents of the rockets. At that moment, the space shuttle system moves slowly upward. You can infer that the force for the lift off comes from the burning gases pushing against the shuttle rockets. Why does the shuttle system move in the opposite direction of the gases? |
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| Look carefully at the photo. Where is the interaction of forces the greatest in the rockets? |
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| The forces on the space-shuttle are similar to the forces in a collision between two tennis balls. When the balls collide, they are propelled in opposite directions. The rockets of the space-shuttle force burning gases downward through the exhaust vents. In response to these downward forces, the shuttle system moves upward. The motion of the space-shuttle demonstrates Newton's third law of motion. |
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| When one object exerts a force upon a second object, the second object exerts an equal and opposite force upon the first object. |
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| The third law of motion states that every
action has an equal and opposite reaction. You can see equal and opposite forces interact when you blow up a balloon and release it, it moves in the opposite direction. The force propelling the balloon is equal and opposite to the force of the air leaving the balloon. |
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| Notice when the diver jumps down on a diving board. The board springs back and forces the diver into the air. The action force exerted on the board by the diver causes a reaction force by the board equal and opposite to the force exerted by the diving board. |
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| These interacting forces are unbalanced and differ from the balanced forces you studied in Newton's first law. Recall that no motion occurs when the forces are balanced. If the diver stands quietly on the diving board, all the forces on the diver and the board are balanced. Neither the diver not the board moves. |
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| The crew team uses Newton's third law of motion to move its boat. When an oar is put into the water, the water exerts an equal force on both sides of the oars, the surface of the flat side of the oars pushes against the water. The water pushes back on the oars with an equal and opposite force. The boat moves in the opposite direction of the oars with a force that is equal to that of the oars as they push against the water. The boat moves because the forces against it are unbalanced. |
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| Newton's third law of motion deals with action and reaction. To understand it, do the following experiments. |
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| The two balances show the same reading because the action force equals the reaction force. |
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| Hook two spring balances as shown in the diagram and pull from either side. You will notice that the pointers on the spring balances read equally (say 100 N). This shows that both the spring balances are registering equal force. |
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| Blow a balloon and hold its neck tightly facing downward. When you release the balloon, you will see that the balloon moves up instead of falling to the ground. |
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| Both the above experiments show that when a body exerts force on another body, the second body too exerts an equal and opposite force
on the first body. The two forces are called action and reaction. The relation between an action and its reaction was given by Newton in the form of Newton's third law of motion. According to it, |
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| To every action there is an equal and opposite reaction. |
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| In the first experiment, when two spring balances are pulled in opposite directions, they experience the same force acting in the opposite directions. In case of the balloon when you release it, the air inside the balloon rushes out downward, exerting force on the air outside. The balloon experiences equal and opposite reaction. Therefore, the balloon is pushed upward. |
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| Rockets work on the same principle. The exhaust gases produced as the result of the combustion of the fuel are forced out at one end of the rocket. As a reaction, the main rocket moves in the opposite direction. It is important to note that action and reaction act on different bodies; action acts on one body and reaction on the other. Both begin and end at the same time. |
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| Some of the practical applications of Newton's third law of motion is illustrated in the diagram. |
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