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Physics 151 Week 9 Day 2

Physics 151 Week 9 Day 2. Topics: Forces, Apparent Weight, & Friction Applying Newton ’ s 2nd Law Apparent Weight Friction Static Friction Kinetic Friction Coefficient of Friction Normal forces and Friction Applications . General Force Model. Newton 0th Law

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Physics 151 Week 9 Day 2

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  1. Physics 151 Week 9 Day 2 Topics: Forces, Apparent Weight, & Friction Applying Newton’s 2nd Law Apparent Weight Friction Static Friction Kinetic Friction Coefficient of Friction Normal forces and Friction Applications

  2. General Force Model • Newton 0th Law • Objects are dumb - They have no memory of the past and cannot predict the future. Objects only know what is acting directly on them right now • Newton's 1st Law • An object that is at rest will remain at rest and an object that is moving will continue to move in a straight line with constant speed, if and only if the sum of the forces acting on that object is zero. • Newton's 3rd Law • Recall that a force is an interaction between two objects. If object A exerts a force on object B then object B exerts a force on object that is in the opposite direction, equal in magnitude, and of the same type. • Visualizations: • Force Diagrams • System Schema

  3. Net Force Model Newton's 2nd Law acceleration of an object = sum of forces acting on that object / the mass of the object Note: Solve Newton’s 2nd Law equations in component form Visualizations: Force Diagram System Schema Motion visualizations as needed Remainder of week: Friction Model Apparent Weight Slide 4-19

  4. Example Problem A sled with a mass of 20 kg slides along frictionless ice at 4.5 m/s. It then crosses a rough patch of snow which exerts a friction force of 12 N. How far does it slide on the snow before coming to rest? Slide 5-21

  5. Example Problem A 75 kg skier starts down a 50-m-high, 10° slope on frictionless skis. What is his speed at the bottom? Slide 5-27

  6. Brainstorm: What do we know about Friction Force? Force created by the surface – against the motion It is parallel to the surface

  7. Static & Kinetic Friction - Part IDemonstration Explain the graph, i.e. why does it look this way? Slide 4-19

  8. Static & Kinetic Friction - Part III Below is graph of the friction force exerted by the table on the box. 
 Label times a-f that match the free-body diagrams in the previous problem. If the mass of the box is 3.0 kg, the maximum Ffs is 10 N, and Ffk has an average of 6.0 N, find the coefficients of static and kinetic friction. Slide 4-19

  9. Coefficients of Friction – Do as warm-up for Wed. What can you deduce/generalize about friction forces from this table? Describe 3-4 real world situations that can be explained by this table Slide 4-19

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