Physics 151 Week 9 Day 2

Topics: Forces, Apparent Weight, & Friction Applying Newton’s 2nd Law Apparent Weight Friction

Static FrictionKinetic FrictionCoefficient of FrictionNormal forces and FrictionApplications

General Force Model

Newton 0th LawObjects 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 LawAn 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 LawRecall 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

Net Force Model

Slide 4-19

Newton's 2nd Lawacceleration 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 DiagramSystem SchemaMotion visualizations as needed

Remainder of week:Friction ModelApparent Weight

Example ProblemA 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

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

Example Problem

Slide 5-27

Brainstorm: What do we know about Friction Force?1. Force created by the surface – against the motion2. It is parallel to the surface

Static & Kinetic Friction - Part I Demonstration

Slide 4-19

Explain the graph, i.e. why does it look this way?

Static & Kinetic Friction - Part III

Below is graph of the friction force exerted by the table on the box.

A. Label times a-f that match the free-body diagrams in the previous problem.

B. 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

Coefficients of Friction – Do as warm-up for Wed.

Slide 4-19

What can you deduce/generalize about friction forces from this table?

Describe 3-4 real world situations that can be explained by this table