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Download Newton`s Laws of Motion
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Newton’s Laws of Motion Newton’s First Law • An object at rest remains at rest and an object in motion remains in motion at constant speed and in a straight line unless acted upon by an unbalanced force. – An object at rest means it is not moving – An unbalanced force is a push or pull. Inertia • Newton’s first law is sometimes called the law of inertia. • Inertia is the tendency of all objects to resist any change in motion. – More mass= more inertia Newton’s Second Law • The acceleration of an object depends on the mass of the object and the amount of force applied. • Force= mass x acceleration F M A Units • • • • Force measured in Newtons (N) Acceleration measured in m/s/s Mass is measured in kg A Newton can be described as the amount of force required to give a 1 kg mass an acceleration of 1 m/s/s Try it out… • Find the force needed to accelerate a 800-kg car at a rate of 5m/s/s – F=MA – F=(800kg) X (5 m/s/s) – F= 4000 N Newton’s Third Law • If one object exerts a force on another object, then the second object exerts a force of an equal strength in the opposite direction. “For every action there is an equal and opposite reaction.” “Action…reaction” • Forces always occur in pairs. Single, isolated forces never happen! – However, you cannot always detect the motion. • You cannot see the Earth’s equal and opposite reaction when the Earth’s gravity pulls on something. Ex: When you drop your pencil gravity pulls it downward. At the same time, the pencil pulls the earth upward. You do not see the earth accelerate. • These given names are confusing for two reasons: – Either force in an interaction can be the ‘action’ force or the ‘reaction’ force. – Unfortunately we associate ‘action’ and ‘reaction’ with ‘first an action, then a reaction’. • This is NOT what occurs in the third law. The action force and the reaction force exist at the SAME time. “Equal” • Equal means two things: 1. Both forces are exactly the same size. They are equal in magnitude. 2. Both forces exist exactly at the same time. They are equal in time. – So why don’t they always cancel out? • They are not always acting on different objects. – Volleyball- setting vs. blocking “Opposite” • Opposite means that the two forces always act in opposite direction. • Exactly 180 degrees apart Momentum • The momentum an object has depends on mass and velocity. – Formula: Momentum= Mass x Velocity – Unit: kgm/s (“kilogram-meters per second”) – The more momentum an object has the harder it is to stop. – If a car and a Mack truck are traveling at the same speed the truck has more mass, therefore it has more momentum and is harder to stop. Try it out…. • What is the momentum of a bird with the mass of 0.018 kg flying at 15 m/s? Momentum= M x V Momentum= 0.018 kg x 15 m/s Momentum= .27 kgm/s Try it out… • Which has more momentum? A 3 kg sledge hammer swung at 1.5m/s or a 4kg sledgehammer swung at .9m/s. Momentum= M x V Momentum= M x V Momentum= 3 kg x 1.5 m/s Momentum= 4 kg x 0.9 m/s Momentum= 4.5 kgm/s Momentum= 3.6 kgm/s The Law of Conservation of Momentum • The total momentum of any group of objects remains the same, or is conserved unless outside forces act on the object. – The total momentum of objects that interact does not change, in the absence of outside forces. – Momentum may be transferred from one to another, but none is lost. Collisions With Two Moving Objects • Some of the momentum is transferred. • The momentum of one object decreases while the momentum of the other increases. Collisions With One Moving Object • All of the momentum is transferred Collision With Connected Objects • The momentum is transferred so that both objects are moving at the same momentum.