Chapter%204 - COLLEGE PHYSICS, Part I Chapter 4: FORCES AND...

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Force Here we will be dealing with dynamics , i.e., the relationship between motion and force. The principles of dynamics are described by three laws, known as Newton’s laws of motion . While these are fundamental laws of Nature, they cannot be deduced from or proved by any other principles. Force can be defined in different ways, like “Force is a push or a pull on an object,” or “ Force is whatever can cause an object with mass to accelerate .”
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When a force involves direct contact between objects, it is called a contact force . When an object rests on a surface, there is a component of force perpendicular (normal) to the surface, which is called a normal force . There may be a component of force parallel to the surface, called friction force . When a rope is attached to an object and pulled, the force is called a tension . Finally, the gravitational pull exerted by the earth on an object is called the object’s weight . Force is always a vector quantity .
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In the SI system , the unit of force is the newton , abbreviated N. 1 N = 0.2248 lb The effect of any number of forces exerted on an object is the same as the effect of the resultant of all forces, i.e., their vector sum. This principle is called superposition of forces . 1 2 R F F = + r r r F 1 and F 2 are the components . R is the resultant , or the net force . 1 N = 1 kg × m/s 2
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In this example, the component vectors of F are F x and F y , and the corresponding components are F x and F y . The effect of simultaneous actions of F x and F y is the same as the effect of F . Any force can be presented by 3 ( x , y , and z ) components, but here we will consider only 2 dimensions. The x and y component do not have to be horizontal and vertical, but they should be perpendicular to each other. 1 2 3 ... R F F F F = + + + = r r r r r The same is true for the components: y y R F = , x x R F = 2 2 , x y R R R = + tan / y x R R θ= Any force can be replaced by its components, acting at the same point. As for other vector quantities, the magnitude and the direction of the force can be found from: (4.1) (4.2) (4.3)
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Force and Mass Any force can act on an object. For example, the weight of the object is not constant, it depends on where the object is. Mass is an intrinsic property of an object wherever the object is. Force is a vector, it has a certain direction Mass is a scalar, it does not have a direction Often there is a confusion between force and mass. For example, on a grocery product you can see: Net Wt. 1 ½ lb (680 g) This is wrong, because lb is a unit of force and g (or kg) is a unit of mass. Another example: In US they say “I weigh 125 lbs” and in Europe they say “I weigh
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This note was uploaded on 09/21/2011 for the course PHY 2053 taught by Professor Vellaris during the Spring '08 term at University of Central Florida.

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Chapter%204 - COLLEGE PHYSICS, Part I Chapter 4: FORCES AND...

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