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Today

Today. Finish Newton’s Laws More Forces. Friction(Static). No motion between surfaces (can be rolling) Magnitude of frictional force (parallel to surfaces) is proportional to the normal force. f static <= m s N m s coefficient of Static friction Be Careful!

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Today

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  1. Today • Finish Newton’s Laws • More Forces

  2. Friction(Static) • No motion between surfaces (can be rolling) • Magnitude of frictional force (parallel to surfaces) is proportional to the normal force. • fstatic <= ms N ms coefficient of Static friction • Be Careful! • Static friction <=, can be any value up to msN • It is reactive just like Normal, Tension 42

  3. Example: speeding truck What is the magnitude of the force of friction on box? m=10kg µs = 0.4 a= 2m/s2

  4. Example: speeding truck What is the maximum acceleration of the truck? m=10kg µs = 0.4 a= 2m/s2

  5. Friction (kinetic) • Similar name, but behaves very differently! • fkinetic = mk N mk coefficient of Kinetic friction • The equals sign means we always know the magnitude • Direction is always opposite of motion

  6. Example: pulling a box 40N m=10kg • µk=0.3

  7. Newton’s 3rd law • Forces come in pairs • Same type (gravity, normal, friction) • Different objects • Always same magnitude, opposite direction

  8. Example: pulling 2 blocks 30N T 10kg 5kg frictionless • Solve for acceleration • Solve for tension

  9. Example: pulling 2 blocks 30N T 10kg 5kg µk= 0.2 • Solve for acceleration • Solve for tension

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