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Newton’s 1 st Law Inertia

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Force Anything capable of changing an object’s state of motion Any push or pull Causes object to speed up, slow down, or change direction (acceleration)

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Galileo’s Experiment

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Inertia Galileo reasoned without friction ball would keep going Also reasoned that ball initially at rest would stay that way unless something caused it to move Inertia - what Galileo called the tendency of an object to maintain its initial state

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Law of Inertia Object at rest will remain at rest or object in motion will remain in uniform motion in a straight line

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Newton Related mass to inertia More massive objects resist change more Newton’s 1 st Law – in the absence of an unbalanced force, a body at rest remains at rest and a body in motion with a constant velocity Force(s) acting must be external and must not total zero

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Summary

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Newton’s 2 nd Law

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Free Body Diagram Used to show all net forces acting on an object What can an object with a net force of zero be doing? Draw the free body diagram for a box being acted on by all the following forces: 5N (W), 3N(S), 8N (N), and 7N(E) Can it be at rest? Can it be at a constant speed? What is it doing?

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Force on Object Objects acted on by a net unbalanced force will accelerate in the direction of the force This means they will speed up, slow down, or change direction

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Newton’s 2 nd Law The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to the mass. The direction of acceleration is the same as the direction of the force a = ∑F / m ∑F = ma

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Force Redefined Force can be redefined as an action capable of accelerating an object Unit of force is the Newton (N) One Newton is the force needed to accelerate a 1 kg mass at 1 m/s 2 1 N = 1 kg m/s 2

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Examples What force is needed to accelerate a 1000kg car at ½ g? What is the net force required to bring a 1500 kg car to rest from 100 km/hr within a distance of 55 m?

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A 50 kg go-cart starts from rest and is accelerated by a 550 N force. If the cart experiences a 280 N friction force and 35 N of air resistance, find a) it’s acceleration b) it’s velocity after 3.0 s c) its displacement after 3.0 s

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What force must be exerted on a 500 kg crate in order to move it from rest to 3.0 m/s in 1.0 s?

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