Ch0115 - Chapter 15 Newtons Laws#2 Kinds of Forces Creating...

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Chapter 15 Newton’s Laws #2: Kinds of Forces, Creating Free Body Diagrams 86 15 Newton’s Laws #2: Kinds of Forces, Creating Free Body Diagrams There is no “force of motion” acting on an object. Once you have the force or forces exerted on the object by everything (including any force-per-mass field at the location of the object) that is touching the object, you have all the forces. Do not add a bogus “force of motion” to your free body diagram. It is especially tempting to add a bogus force when there are no actual forces in the direction in which an object is going. Keep in mind, however, that an object does not need a force on it to keep going in the direction in which it is going; moving along at a constant velocity is what an object does when there is no net force on it . Now that you’ve had some practice using free body diagrams it is time to discuss how to create them. As you draw a free body diagram, there are a couple of things you need to keep in mind: (1) Include only those forces acting ON the object whose free body diagram you are drawing. Any force exerted BY the object on some other object belongs on the free body diagram of the other object. (2) All forces are contact forces and every force has an agent. The agent is “that which is exerting the force.” In other words, the agent is the life form or thing that is doing the pushing or pulling on the object. No agent can exert a force on an object without being in contact with the object. (We are using the field point of view, rather than the action-at-a- distance point of view for the fundamental forces of nature. Thus, for instance, it is the earth’s gravitational field at the location of the object, rather than the material earth itself, that exerts the gravitational force on an object.) We are going to introduce the various kinds of forces by means of examples. Here is the first example: Example 15-1 A rock is thrown up into the air by a person. Draw the free body diagram of the rock while it is up in the air. (Your free body diagram is applicable for any time after the rock leaves the thrower’s hand, until the last instant before the rock makes contact with whatever it is destined to hit.) Neglect any forces that might be exerted on the rock by the air. If you see the rock flying through the air, it may very well look to you like there is nothing touching the rock. But the earth’s gravitational field is everywhere in the vicinity of the earth. It can’t be blocked. It can’t be shielded. It is in the air, in the water, even in the dirt. It is in direct contact with everything in the vicinity of the earth. It exerts a force on every object near the surface of the earth. We call that force the gravitational force. You have already studied the gravitational force. We give a brief synopsis of it here.
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Chapter 15 Newton’s Laws #2: Kinds of Forces, Creating Free Body Diagrams 87 The Gravitational Force Exerted on Objects Near the Surface of the Earth. Because it has mass, the earth has a gravitational field.
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This note was uploaded on 02/29/2012 for the course PHYS 227 taught by Professor Rabe during the Fall '08 term at Rutgers.

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Ch0115 - Chapter 15 Newtons Laws#2 Kinds of Forces Creating...

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