3force_FA10 - Newton’s First Law Sir Isaac Newton Every...

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1 Sir Isaac Newton Born: 4 Jan 1643 in Woolsthorpe, Lincolnshire, England Died: 31 March 1727 in London, England 1 The key to understanding Newton's ideas about motion is to investigate the description of "force" and "mass." The main idea is that "forces cause accelerations," and, that the amount of acceleration depends on the "mass" of the object. Simply, a force is the reason why an object might accelerate. Newton’s First Law Every object continues in its state of rest or of uniform speed in a straight line unless it is compelled to change that state by a "net force" acting on the object . •T h i s doesn't say that every moving object has a "force" acting on it. h i s doesn't say that a stationary object has no forces acting on it Newton’s First law is really just a special case of Newton’s Second Law (for a=0) 2 Inertia All objects resist changes in their state of motion. Mass (m) as a Measure of the Amount of Inertia. 3 Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The direction of the acceleration is in the direction of the applied net force. y y x x ma F ma F = = , a m F r r = 4 Example: Newton's 2nd Law enables us to compare the results of the same force exerted on objects of different mass.
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2 5 Example : To stop a 2000 kg car moving at 14m/s in 2.0s requires an average breaking force, F = ma = (2000kg)(-14m/s)/(2s) = -14000 N. Example : Two forces, F 1 =45.0N and F 2 =25.0N act on a 5.00kg block sitting on a table as shown. What is the horizontal acceleration (magnitude and direction) of the block? 6 F 1x = F 1 cos(65.0) = 19.0 N Σ F x = m a x Σ F x = F 1x -F 2x = 19.0 N - 25.0 N = (5.00kg)a x a x = -1.2 m/s 2 BA AB F F r r = Newton’s Third Law Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
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This note was uploaded on 11/22/2010 for the course PHYS 1146 taught by Professor Balasubramamaheswaran during the Spring '10 term at Northeastern.

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3force_FA10 - Newton’s First Law Sir Isaac Newton Every...

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