Physics 151 Week 9 Day 1
Topics: Work, Energy, & Newton’s 2nd Law
 Energy and Work
 Force and Motion Graphs
Questions from last time
 Net force vs. acceleration
 Mass vs. Acceleration
 Applying Newton’s 2nd Law
 Apparent Weight
 Brainstorm: What do we know about friction?
Wednesday’s Class: All about Forces & friction
Slide 10-3
Slide 10-4
Forms of Energy
Mechanical Energy
Ug
K
Thermal
Energy
Us
Other forms include
E th
Echem
Enuclear
Slide 10-12
The Basic Energy Model
Slide 10-13
Energy Transformations
Kinetic energy K = energy of motion
Potential energy U = energy of position
Thermal energy Eth = energy associated with
temperature
System energy E = K + U + Eth + Echem + ...
Energy can be transformed within the
system without loss.
Energy is a property of a system.
Slide 10-14
Some Energy Transformations
Echem  Ug
K  Eth
Echem  Ug
Us  K  Ug
Slide 10-15
Energy Transfers
These change the energy of the system.
Interactions with the environment.
Work is the mechanical transfer of energy to
or from a system via pushes and pulls.
Slide 10-20
Energy Transfers: Work
W K
W Eth
W Us
Slide 10-21
The Work-Energy Equation
Slide 10-22
Work
Slide 10-28
Work Done by Force at an Angle to Displacement
Slide 10-29
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
Constant Force Model
Newton's 2nd Law
acceleration of an object = sum of forces acting on that object / the mass of the
object
Remainder of week:
Friction Model
Apparent Weight
Slide 4-19
Net Force and Motion Graphs
Net Force vs. Acceleration Graphs
Net Force vs. Mass Graphs
Reading Quiz
1. Which of the following statements about mass and weight is
correct?
A.
Your mass is a measure of the force gravity exerts on
you.
B. Your mass is the same everywhere in the universe.
C. Your weight is the same everywhere in the universe.
D. Your weight is a measure of your resistance of being
accelerated.
Slide 5-5
Answer
1. Which of the following statements about mass and weight is
correct?
A.
Your mass is a measure of the force gravity exerts on
you.
B. Your mass is the same everywhere in the universe.
C. Your weight is the same everywhere in the universe.
D. Your weight is a measure of your resistance of being
accelerated.
Slide 5-6
Example Problem
A 100 kg block with a weight of 980 N hangs on a rope. Find
the tension in the rope if
A. the block is stationary.
B. it’s moving upward at a steady speed of 5 m/s.
C. it’s accelerating upward at 5 m/s2.
Slide 5-15
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
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
Friction Brainstorm
One person in each team takes out a sheet of paper and
records their group brainstorming everything they know
about friction.
Example Problem
Burglars are trying to haul a 1000 kg safe up a frictionless ramp
to their getaway truck. The ramp is tilted at angle θ. What is the
tension in the rope if the safe is at rest? If the safe is moving up
the ramp at a steady 1 m/s? If the safe is accelerating up the
ramp at 1 m/s2? Do these answers have the expected behavior
in the limit θ → 0° and θ → 90°?
Slide 5-28
Example Problem
Macie pulls a 40 kg rolling trunk by a strap angled at 30° from
the horizontal. She pulls with a force of 40 N, and there is a 30 N
rolling friction force acting on trunk. What is the trunk’s
acceleration?
Slide 5-22