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Physics 151 Week 9 Day 3
Topics: Forces, Apparent Weight, & Friction
 Apparent Weight
 Friction
Static Friction
Kinetic Friction
Coefficient of Friction
Normal forces and Friction
Applications
Energy Model
Types of Energy
Kinetic Energy KE = 1/2 mv2
Potential Energy:
Gravitational Potential Energy
Spring Potential Energy
PEg = mgy
Pes = 1/2 kL2
Conservation of Energy (Closed System)
Before
After
KEi  PEgi  PEsi  Esys  KE f  PEgf  PEsf  Eth
Visualizations:
• Energy Bar Charts
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
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
Demonstrations: What do we know about Friction Force?
2 - Increases with more contact surface
- Not dependent on surface area, but surface type
Does our model of friction depend on area?
Does real-world friction depend on area?
Parking on a Hill
A. If you park on a hill with a 10 degree slope with the car
held by the parking brake, what is the magnitude of the
frictional force that holds your car in place?
B. The coefficient of static friction between your car's
wheels and the road when wet is 0.30. What is the
largest angle slope on which you can park your car in the
rain so that it will not slide down the hill?
C. The coefficient of kinetic friction between your wheels
and the wet road surface is 0.25. If someone gave your
your car a push on the wet hill and it started sliding
down, what would its acceleration be?
Slide 4-19
Penguin in a box
A loaded penguin in a box, together weighing 60 N, rests on a plane
inclined at 20° to the horizontal. Between the box and the plane, the
coefficient of static friction is 0.26 and the coefficient of kinetic friction
is 0.15.
1. What is the minimum magnitude of the applied force F,
parallel to the plane, that will prevent the sled from
slipping down the plane?
2.
2.
Haul the Crate
A 10 kg wooden crate is placed on wood slats in the back of
a pick-up truck with no tail gate.
How fast can the truck accelerate before the crate falls off?
Assume the coefficient for static friction for the crate on the
wood slats is 0.40 and the coefficient for kinetic friction is
0.20.
Scales and Elevators (Apparent Weight)
Consider a person with a mass of 60 kg is in an elevator
standing on a scale. The elevator is accelerating upward.
• Draw a system schema and 2 force diagrams:
One for the person and one for the scale
• What does the scale read?
• Use Newton’s 2nd law to determine what the scale
reads (This is apparent weight)
Slide 4-19
Scales and Elevators (Apparent Weight)
Suppose a person with a mass of 60 kg is in an elevator
standing on a scale.
Use the system schema and force diagrams of the scale and
the person to determine what the scale would read for the
following situations:
A. If the elevator is descending at 4.9 m/s.
B. If the elevator has a downward acceleration of
4.9 m/s/s.
C. If the elevator has an upward acceleration of
4.9 m/s/s.
Slide 4-19
Apparent Weight
Slide 5-24
Example Problem
A 50 kg student gets in a 1000 kg elevator at rest. As the elevator
begins to move, she has an apparent weight of 600 N for the first 3
s. How far has the elevator moved, and in which direction, at the
end of 3 s?
Slide 5-25
Clicker Question
The apparent weight of an object is
A. the pull of gravity on the object.
B. the object’s mass times the acceleration of gravity.
C. the magnitude of the contact force that supports the
object.
D. the pull of gravity on an object that is accelerating
upward.
Slide 5-7
Answer
2. The apparent weight of an object is
A. the pull of gravity on the object.
B. the object’s mass times the acceleration of gravity.
C. the magnitude of the contact force that supports the
object.
D. the pull of gravity on an object that is accelerating
upward.
Slide 5-8