Physics 151 Week 9 Day 2

1
Physics 151 Week 9 Day 2

• Topics Forces, Apparent Weight, Friction
• Applying Newtons 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 Newtons 2nd Law
equations in component form Visualizations Forc
e 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
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Brainstorm What do we know about Friction Force?

1. Force created by the surface against the motion
2. It is parallel to the surface

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Static Kinetic Friction - Part I Demonstration
Explain the graph, i.e. why does it look this way?
Slide 4-19
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• 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
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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