Newton's Third Law of Motion

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4. If so, why is he slipping? No runner is his able to run on a slippery track in spite of his muscle power. The force that drives the runner forward comes from the ground when there is sufficient friction between his feet and the ground. 5. This car requires 40,000 N to speed up at the rate of 10m/s 2 . This sprinter requires 300 N to speed up at the rate of 5m/s 2 . How is the road able to judge the individual force requirements for the car and the sprinter? 6. Answer to this comes from Newton’s Third Law of Motion: • Nothing in the universe can act without being acted upon. • To every action there is an equal and opposite reaction. The first statement is somewhat philosophical. The second one is vague. In exact terms what the first statement means is that whenever any object A offers a force F, it itself experiences the same force back on itself in the reverse direction. 214 Physics Education • July − September 2009
7. The second statement can be interpreted as follows. Assume two objects A and B. A offers a force on B directed from A to B. We denote this force by F AB . The 3rd Law says B will simultaneously offer a force F BA on A which is directed from B to A. These two forces are equal and opposite, even if the objects A and B are grossly unequal in size and mass. Written as vectors, F BA = −F AB . (1) We can refer to any one of the pair of forces, say the force F AB , as the action force and the second one, i.e. F BA , as the reaction force. Then Eq. (1) is a mathematical expression of the second statement. In the particular example shown these two forces are repulsive forces. 8. Here they are attractive forces. A pulls B with a force F AB . As a result A is itself being pulled towards B with a force F BA , and F BA = −F AB . 9. A runner can run only if the ground kicks him forward. To get this kick the runner must kick the ground backward. It is possible to kick a solid and firm object like football or grass with sufficient force, but it is difficult to kick a loose object like air or slippery road with sufficient force. Therefore the runner is unable to run on a slippery road. Physics Education • July − September 2009 215
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Newton's Third Law of Motion

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