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Lecture 2: More Forces

Lecture 2: More Forces. Springs. Similar to tension, normal Reactive Changes length according to tension F = -k Δ x. k=50 N/m. 10kg. Normal. friction. hand. Gravity. y. x. Free Body Diagrams. Choose Object (book) Label coordinate axis Identify All Forces Hand (to right)

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Lecture 2: More Forces

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  1. Lecture 2: More Forces

  2. Springs • Similar to tension, normal • Reactive • Changes length according to tension • F = -k Δx k=50 N/m 10kg

  3. Normal friction hand Gravity y x Free Body Diagrams • Choose Object (book) • Label coordinate axis • Identify All Forces • Hand (to right) • Gravity (down) • Normal (table, up) • Friction (table, left) Physics 40

  4. Clicker Question:A box is hanging in an elevator by a rope.The elevator is accelerating up at 2 m/s2 a= 2 m/s2 Compare the magnitude of the Tension to the magnitude of the Force of gravity A |Fg| < |T| B |Fg| = |T| C |Fg| > |T| • 10kg

  5. 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

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

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

  8. 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

  9. Example: pulling a box What is the acceleration of the box? 40N m=10kg • µk=0.3

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

  11. Clicker QuestionTwo blocks are being pulled along a frictionless floor by a 30N force 30N T 10kg 5kg frictionless What is the magnitude of the Tension, T, on the rope between the blocks? A T = 30N B T < 30N C T > 30N

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

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

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