Warm-Up November 1, 2011

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# Warm-Up November 1, 2011 - PowerPoint PPT Presentation

Warm-Up November 1, 2011. “Super Sliders” and other similar products claim to allow you to “effortlessly slide heavy furniture.” How do they work?. Friction. Section 5.2. Two Types of Friction. Kinetic Friction : When an object is moving

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Warm-UpNovember 1, 2011

“Super Sliders” and other similar products claim to allow you to “effortlessly slide heavy furniture.” How do they work?

### Friction

Section 5.2

Two Types of Friction
• Kinetic Friction: When an object is moving
• Static Friction: When an object is not moving (stationary)
Kinetic Friction
• Kinetic friction is always in the opposite direction of the object’s motion.
• Depends on the Normal Force (FN) and the types of surfaces that are sliding against each other
• The Coefficient of Kinetic Friction (µk) is a numerical property of the surfaces.
• µ = Greek letter “mu”
Example 1
• You are pushing a 20.0 kg box at a constant speed of 2.0 m/s with µk = 0.45. How much force do you exert on the box?
You-Try #1
• You are pushing a 25.0 kg box with a force of 125 N. The coefficient of kinetic friction is 0.35.
• What is the force of friction?
• What is the resulting acceleration?
Static Friction
• Static friction is always in the opposite direction of the (largest otherwise unbalanced) applied force.
• Depends on the Normal Force (FN), the types of surfaces that are sliding against each other, and the magnitude of the applied force
• The Coefficient of Static Friction (µS) is a numerical property of the surfaces.
• µ = Greek letter “mu”
Static Friction Force
• If there is a force trying to cause motion, the static friction force will increase up to a maximum value (µSFN) before it is overcome and motion starts.
Example 2
• A 30.0 kg box is at rest on the floor. If the coefficient of static friction is 0.55, what is the maximum push force you could apply to the box before it starts moving?
You-Try #2
• A 15.0 kg box is at rest on the floor. If you have to push with a force of 95 N before the box starts to move, what is the coefficient of static friction?
Warm-Up: November 2, 2011
• A 70.0 kg couch is at rest on the floor. If the coefficient of static friction is 0.65, what is the maximum push force you could apply to the couch before it starts moving?
• (Look at yesterday’s example 2 if you don’t remember how to solve this)
Friction Coefficients
• For a given surface combination, the coefficient of static friction will always be larger than the coefficient of kinetic friction.
• It’s harder to get something to start moving than to keep it moving.
Worksheet