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Newton’s 2 nd Law

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Enduring Understanding: Studying dynamics (causes of motion) has had a profound effect on the way humans view their world. Essential Question: What may be accomplished by understanding dynamics? Essential Question: How may we symbolically represent dynamics information? Essential Question: How are dynamics a predictor of kinematics?

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It Takes An Unbalanced Force To Cause An Acceleration A Change In Speed Or Direction Is Called Acceleration Deals With Force And Acceleration Newton’s 2nd Law

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The acceleration of an object is directly related to the net force and inversely related to its mass. The acceleration of an object is directly related to the net force and inversely related to its mass. F = ma F = ma F = net force (N) F = net force (N) m = mass (kg) m = mass (kg) a = acceleration (m/s 2 ) a = acceleration (m/s 2 )

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How much net force would be needed to cause a 20 kg mass to accelerate at 3 m/s 2 ? m = 20 kg a = 3 m/s 2 F = ma F = (20 kg) x (3 m/s 2 ) = 60 N

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A 50 N force is applied to an object and a 20 N force of friction works against it. If the object has a mass of 5 kg, what is its acceleration? +++++++++++++++++++++++++++++++++++++++++++ Net Force = 50 N – 20 N =30 N F = ma (30 N) = (5 kg) x a a = 30/5 = 6 m/s 2

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Newton’s 3 rd Law For every action there is an equal and opposite reaction.

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For motion to occur, there must be an action/reaction pair.

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The Tao of Physics

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Determining Friction Friction is an interaction between two surfaces. Friction is an interaction between two surfaces. Friction is directly related to the normal force. Friction is directly related to the normal force.

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Friction can be static or kinetic. Static friction must be overcome to get an object moving. Static friction must be overcome to get an object moving. Kinetic friction is the friction acting on an object in motion. Kinetic friction is the friction acting on an object in motion.

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Which do you think is larger, static or kinetic friction? Static friction is always larger than kinetic friction. Static friction is always larger than kinetic friction.

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A coefficient of friction is used to determine the friction between two surfaces. u is the symbol for the coefficient of friction. Friction = F n x u F n = The normal force

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What is the force of friction if u =.2 and the normal force is 40 N ? = 40 N u =.2 F n = 40 N u =.2 Friction = F n Friction = F n x u = (40 N) x (.2) Friction = 8 N

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What is the force of friction if u =.3 and the mass is 10 kg? The normal force is equal to the weight. So, the normal force is (10kg) x (9.8 m/s 2 ) = 98 N

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= 98 N u =.3 F n = 98 N u =.3 Friction = F n Friction = F n x u = (98 N) x (.3) Friction = 29.4 N

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