PHY2048_chapter5

# PHY2048_chapter5 - Chapter 5 Force and Motion In chapters 2...

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Chapter 5 Force and Motion In chapters 2 and 4 we have studied “kinematics” i.e. described the motion of objects using parameters such as the position vector, velocity and acceleration without any insights as to what caused the motion. This is the task of chapters 5 and 6 in which the part of mechanics known as “dynamics” will be developed. In this chapter we will introduce Newton’s three laws of motion which is at the heart of classical mechanics. We must note that Newton’s laws describe physical phenomena of a vast range. For example Newton’s laws explain the motion of stars and planets. We must also note that Newton’s laws fail in the following two circumstances: 1. When the speed of objects approaches (1% or more) the speed of light in vacuum (c = 8×10 8 m/s). In this case we must use Einstein’s special theory of relativity (1905) 2. When the objects under study become very small (e.g. electrons, atoms etc) In this case we must use quantum mechanics (1926) (5-1)

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Newton’s First Law Scientists before Newton thought that a force (the word “ influence ” was used) was required in order to keep an object moving at constant velocity. An object was thought to be in its “natural state” when it was at rest. This mistake was made before friction was recognized to be a force. For example, if we slide an object on a floor with an initial speed v o very soon the object will come to rest. If on the other hand we slide the same object on a very slippery surface such as ice, the object will travel a much larger distance before it stops. Newton checked his ideas on the motion of the moon and the planets. In space there is no friction, therefore he was able to determine the correct form of what is since known as : Newton’s first law If no force acts on a body, the body’s velocity does not change; that is the body does not accelerate (or decelerate) (5-2)
Note: If several forces act on a body (say , , and ) the net force is defined as: i.e. is the vector sum of , , and A B C net ne net A B C C t A B F F F F F F F F F F F F = + + r r r r r r r r r r r r Force: The concept of force was tentatively defined as

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PHY2048_chapter5 - Chapter 5 Force and Motion In chapters 2...

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