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**A 100kg wooden box is standing on a wooden floor**

A 100kg wooden box is standing on a wooden floor. You push the box with a force of 100N to the right. The box does not move. Draw a complete free body diagram of the box and label all forces.

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FN=1000N F=100N FW=1000N

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FN=1000N F=100N Ffr.=100N FW=1000N

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FN=1000N F=200N FW=1000N

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FN=1000N Ffr.=200N F=200N FW=1000N

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FN=1000N F=300N FW=1000N

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FN=1000N Ffr.=300N F=300N FW=1000N

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FN=1000N Maximum Ffr.= ? Which force will make the box accelerate? FW=1000N

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Static Friction, ƒs Static friction acts to keep the object from moving If F increases, so does ƒs If F decreases, so does ƒs Static friction has a maximum: ƒs µs FN µs….coefficient of static friction

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**More About Friction Friction is proportional to the normal force**

The coefficient of friction (µ) depends on the surfaces in contact The direction of the frictional force is opposite the direction of the applied force The coefficients of friction are nearly independent of the area of contact

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FN=1000N Which force will make the box accelerate? Maximum Ffr.= μ·FN FW=1000N

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FN=1000N Which force will make the box accelerate? Maximum Ffr.= μ·FN Maximum Ffr.= 0.42·1000N FW=1000N

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FN=1000N Which force will make the box accelerate? Maximum Ffr.= μ·FN Maximum Ffr.= 0.42·1000N Maximum Ffr.= 420N FW=1000N

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FN=1000N Maximum Ffr.= 420N Which force will make the box accelerate? – Any force that is larger than 420N. FW=1000N

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FN=1000N Fnet= 80N a=0.8m/s2 Maximum Ffr.= 420N F=500N FW=1000N

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When a 12-newton horizontal force is applied to a box on a horizontal tabletop, the box remains at rest. What is the force of static friction acting on the box? According to your reference table, Approximate Coefficents of Friction, what is the minimum horizontal force needed to start a 300. N steel block on a steel table in motion? According to your reference table, Approximate Coefficents of Friction, what is the minimum horizontal force needed to start a 300. kilogram steel block on a steel table in motion? Jim wishes to push a 100. N wood crate across a wood floor. According to your table, Approximate Coefficents of Friction, what is the minimum horizontal force that would be required to start the crate moving? According to your table, Approximate Coefficents of Friction, which road surface would offer the greatest traction for the rubber tires of a 1000kg car? Which force would be required to make the car accelerate on each surface? dry concrete wet concrete dry asphalt wet asphalt A 100kg wooden box is at rest on a wooden floor. How much force is necessary to make the box accelerate? A 100kg wooden box is at rest on a wooden floor. Your friend is pressing the box straight downwards with a force of 500N. How much force is necessary to make the box accelerate? A 100kg wooden box is at rest on a wooden floor. Your friend is lifting the box straight upwards with a force of 500N. How much force is necessary to make the box accelerate?

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PHYS16 – Lecture 10 & 11 Force and Newton’s Laws September 29 and October 1, 2010

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