NEWTON’S THIRD LAW AND Tension FORCES
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To develop an understanding and consequences of Newton's Third Law. •
To consider the characteristics of three different types of passive forces: tension (in strings, ropes, springs, and chains), normal forces (which support objects affected by gravity), and friction. Overview
You may have learned this statement of Newton’s third law: “To every action there is an equal and opposite reaction.” What does this sentence mean? Unlike Newton’s first two laws of motion, which concern only individual objects, the third law describes an interaction between two bodies. For example, what if you pull on your partner’s hand with your hand? Can you pull on your partner without your partner pulling back? The answer turns out to be a definite No! This is because of an essential characteristic of force itself; A force always acts between things, in this case your hand and your partner’s hand. The action referred to in the phrase above is the force applied by your hand, and the reaction is the force that is applied by your partner’s hand. Together, they are known as a force pair In the mechanistic world view established by Newton, we view the universe as being composed of many separate objects interacting with each other via the exchange of one or more forces between them. To make sense of the massive complexity of the universe, we often focus our attention on a single object at a time, and mentally isolate the object from its environment. Each and every interaction with the environment is then replaced by some type of force. This allows us to deconstruct and understand the behavior of otherwise complex systems.1 To study this interaction, you can use two Force Sensors. As one object (your hand) pushes or pulls on another object (your partner’s hand) the Force Sensors will record those pushes and pulls. They will be related in a very simple way as predicted by Newton’s third law. The action referred to in the phrase above is the force applied by your hand, and the reaction is the force that is applied by your partner’s hand. Together, they are known as a force pair. NEWTON’S THIRD LAW and tension forces
An Introduction to Newton’s Third Law
In order to apply Newton’s laws to complex situations with strings, pulleys, inclined planes, and so forth, we need to consider a third force law formulated by Newton having to do with the forces of interaction between two objects. To “discover” some simple aspects of the third law, you should make some straightforward observations using the following equipment:
2 spring scales (Yellow-50N)
1 rod and table clamp
Fig. 5.1. Ways to test Newton’s third law.
5.1.1. Activity: Newton’s Third Law–Forces of Interaction
Set up the situations shown in the diagram above and see if there are any circumstances in which the object that is pulling and the object that is being pulled exert different forces on each other. Be sure to do the second case, in which the person on the right pulls so that it accelerates to the right. Does it matter if the right hand is pulling on the system with a net force to the right, causing acceleration? Describe your conclusions below. You will now repeat the same experiment, but this time use electronic force sensors and the computer based acquisition system
Obtain two force sensors, a short loop of string, and a rubber band. You may have to wait for another group to finish if there are not enough force sensors. Log into your workstation (username Physics120, no password). Plug one sensor into Port A and the other...
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