what causes friction? - pbworks
TRANSCRIPT
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Friction a force that opposes motion. Friction always points opposite of the direction of motion
What causes friction?
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Types of Friction
==> Static friction between 2 stationary surfaces==> Sliding or Kinetic friction between 2 surfaces which are sliding past each other==> Rolling friction between a circular/spherical object which rolls over a surface==> Fluid friction/retarding force created when an object passes through a fluid (such as gas or liquid)
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Static friction is larger than kinetic friction, i.e. it is harder to get the object moving than to keep it moving.
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The formula you will need to calculate kinetic friction:
Ff = μFwhereFf force of kinetic frictionFN Normal forceμ coefficient of kinetic friction, which describes surfaces in contact
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Important: word problems will often describe the types of surfaces in contact, but will not give you the value of the coefficient of kinetic or static friction.
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E.g.: A 150 N force is needed to move a 22 kg box at a constant speed along a leveled surface. (a) What is the coefficient of friction between the surfaces? (b) What is the possible material of these two surfaces?
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Example 2: While decorating his apartment, Terry slowly pushes a 82 kg china cabinet across the wooden floor, which resists the motion with a force of friction of 320 N. What is the coefficient of sliding friction between the cabinet and the floor?
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Example 3: Skye is trying to make her 70 kg Saint Bernard go out the back door but the dog refuses to walk. (a) If the coefficient of static friction between the dog and the floor is 0.75, how hard must she push to get him moving?(b) If the coefficient of sliding friction between the dog and the floor is 0.50, how hard must Skye push in order to move the dog with the constant speed?
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Example 4: Bobby is pulling a red wagon along a flat leveled street. In the wagon sits a nicely wrapped present he is planing to give to his friend Doris. If the coefficient of static friction between the present and the floor of the wagon is 0.6, how much can Bobby accelerate while still keeping the box from sliding in the wagon?
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Example 5: A 200 kg car is moving with the velocity of 25 m/s along a dry asphalt road when the driver hits the brakes. How many meters will the car travel before it comes to a complete stop?