11. Newton’s third law
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11. Newton’s Third Law
Key Points
Newton’s third law is often stated in the form below,
For every action, there is an equal but opposite reaction.
The examples below illustrate how this law can be applied.
Example 11.1 If you stand on the ground, your feet push down on the ground. In response to this the ground exerts an upward force on each of your feet. Both of these forces are normal reaction forces.
The diagrams show these normal reaction forces. There is one normal reaction for each foot.
In this example it is important to note that this example does not involve the weight of the person, although \displaystyle W={{R}_{1}}+{{R}_{2}} .
Example 11.2 A second example concerns an object that is falling vertically towards the earth. The earth exerts a downward force on the object. This is known as the weight of the object. The object also exerts an upward force of equal magnitude on the earth. These forces are shown in the exaggerated diagram shown below.
Example 11.3 As a final example consider a motorboat towing a dinghy. The motorboat exerts a forward force on the dinghy through the tow rope. Te dinghy in turn exerts a backwards force on the boat through the tow rope. The forces are equal in magnitude but opposite in direction. The diagram shows these forces.
