Friction

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# Friction - PowerPoint PPT Presentation

Friction. And Forces in Two Dimensions. Review. A force is a push or pull. Newton’s 3 Laws of Motion: Objects resist changes in velocity unless acted upon by a net force. The magnitude of an object’s acceleration depends on the net force and inversely on the object’s mass. a = F net / m

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## Friction

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Presentation Transcript

### Friction

And Forces in Two Dimensions

Review
• A force is a push or pull.
• Newton’s 3 Laws of Motion:
• Objects resist changes in velocity unless acted upon by a net force.
• The magnitude of an object’s acceleration depends on the net force and inversely on the object’s mass.
• a = Fnet / m
• For every force there is an equal and opposite counter-force.
Friction
• A force that opposes motion.
• It was once thought that friction was caused by surface roughness.
• In this model, microscopic nooks and bumps on the surface of an object lock and slip against similar features on another surface.
• It is now known that this model is incorrect.
• Friction is caused by brief electromagnetic interactions between charged particles in two surfaces.
Friction
• Two important types of friction:
• Static friction
• Prevents motion from beginning
• Kinetic friction
• Prevents motion from continuing
• Static friction is usu. stronger than kinetic friction.
• This is why, when you’re pushing a heavy object across the floor, it’s harder to start it moving than it is to keep it moving.
Friction
• Ff =  FN
• Ff = Friction (N)
•  = Coefficient of friction (no units)
• FN = Normal force = mg cos
• Coefficient of friction
• Unique for every pair of substances and set of conditions.
• Different CoF for static and kinetic friction.
Coefficient of Friction
• CoF must be determined experimentally for any pair of surfaces.
•  = Ff / FN
Sample Problem
• The coefficient of static friction between two blocks of wood is 0.500. What applied force is necessary to start a 250.-kg wooden box sliding along a horizontal wooden surface?
• Fa must be equal to or greater than Ff,static.
• Ff =  FN
• FN = Fw = mg
• FN = (250. kg)(9.81 m/s2)
• FN = 2450 N
• Ff = (0.500)(2450 N)
• Ff = 1230 N
Sample Problem 2
• A box with a mass of 40.0 kg requires an applied force of 250. N to keep it moving at a constant speed along a horizontal surface. What is the CoF between the box and the floor?
• Ff =  FN
• Ff =  mg
• (250. N) =  (40.0 kg)(9.81 m/s2)
• (250. N) =  (392 N)
•  = 0.638
In lab today
• We will calculate the coefficient of friction between a shoe and the surface of a ramp.
• You will volunteer the use of your shoes!
• Place the shoes on a ramp.
• Incline the ramp until the shoe begins to slide.
• Measure the angle at which the shoe begins to slide.
• Calculate the coefficient of friction using some simplifying math.
• Repeat the procedure but first wet the shoe’s sole and the board.