Download - Newton’s Second Law Examined
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Newton’s Second Law Examined
“A change in motion is proportional to the motive force impressed and takes place
along the straight line in which that force is impressed”
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Another way to put it…
• Acceleration is proportional to the net force on an object and inversely proportional to its mass
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Sample Problem 1
Rocket Guy weighs 905 N and his jet pack provides 1250 N of thrust, straight up. What is his acceleration.
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Sample Problem 2
A helicopter of mass 3770 kg can create an upward lift force (F) when empty. It can accelerate straight upward at a maximum of 1.37 m/s2. A careless crewman overloads the helicopter so that it is just unable to lift off. What is the mass of the cargo?
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Sample problem 3
Len pushes toward the right on a 12 kg box with a force of 31 N. Martina applies a 11 N force on the box in the opposite direction. The magnitude of kinetic friction between the box and the floor is 4.5 N as the box slides to the right. What is the box’s acceleration?
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Newton’s Third law
• What is Newton’s Third Law of Motion? • What does it mean?
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Normal Force
• When two objects are in direct contact, the force one object exerts in response to the force exerted by the other. This force is perpendicular to the objects’ contact surface.
• Huh???????????
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Processing TimeWhat is the normal force of the desk on the boxes? What is theNormal force of the 9 kg box on the top twoBoxes?
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Tension
Force exerted by a string, cord, twine, rope, chain, cable, etc
Often times, tension “takes the place” of the normal force.
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Friction
• A force that resists the motion of one object sliding past another
• Varies by materials in contact• Proportional to normal force • What are the different types of friction?
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Static friction
• Force opposing sliding when no motion• Balances “pushing” force until object slides• Max static friction proportional to:– Coefficient of static friction– Normal force
Equation?
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Sample problem 1
• Anna is pushing a 7 kg box with 7 N of force to the left, but the box does not move. What is the force of static friction?
• What is the max static frictional force if the coefficient of static friction for these materials is 0.31?
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Kinetic Friction
• Friction opposing sliding in motion• Force constant as object slides • Proportional to:– Coefficient of kinetic friction– Normal force
Equation?
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Sample Problem 1
• What is the force of friction of a sliding box down a ramp if the box has a mass of 5 kg and the coefficient of kinetic friction is 0.67?
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Sample problem 2
• While rearranging your living room, you push your couch across the floor at a constant speed with a horizontal force of 69 N. You are using special pads on the couch legs to help it slide easier. If the couch has a mass of 59.5 kg, what is the coefficient of kinetic friction between the pads and the floor?
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Independent Practice