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FRICTION

FRICTION. Sliding Friction. Often called kinetic friction A force opposite to direction of motion Due to bumps in surfaces and electric forces. Surface under microscope. F f. Kinetic Friction. Depends on nature of the two surfaces

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FRICTION

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  1. FRICTION

  2. Sliding Friction • Often called kinetic friction • A force opposite to direction of motion • Due to bumps in surfaces and electric forces Surface under microscope Ff

  3. Kinetic Friction • Depends on nature of the two surfaces • Directly proportional to the normal force between the surfaces • Ff = mkFN

  4. Coefficient of friction mk • Usually between zero and one • Calculate from Ff / FN • Very low for ice and some other materials

  5. Friction: Good or Bad • Mostly undesirable since reduces useful force and wastes energy • Friction produces heat • Necessary for walking • Necessary for braking

  6. Static Friction • Force to start something moving • Usually larger than kinetic friction for same surfaces • Requires force to be exerted • Before sliding begins, is equal and opposite to applied force • = msFN at breakaway only(otherwise less)

  7. Friction vs Applied Force • Starting with body at rest FA Ff msFN here only Ff = mkFN here 450 FA

  8. Kinetic Friction for Block • Normal force equals weight FN mg Ff = mkFN = mk mg

  9. What is the Frictional Force? • Block on an inclined plane mg

  10. LHP • Sketch large diagrams of the forces involved in the following situations: • A block dragged across the floor • A block sitting on a ramp with no movement • A block being dragged up a ramp

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