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Exploring Forces and Friction in Physics Lectures

Learn about forces, springs, friction types, and relationships between tension, gravity, and acceleration with practical examples and clicker questions in this comprehensive physics lecture. Understand static and kinetic friction, Newton's laws, and solve block-pulling problems.

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Exploring Forces and Friction in Physics Lectures

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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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