Chapter 7 - Work and Energy

1
Chapter 7 - Work and Energy

• Work
• Definition of Work units
• Work done by a constant force (e.g
  friction,weight)
• Work done by a varying force (e.g. a spring)
• Work in 3 dimensions General Definition
• Work and Kinetic Energy
• Definition of Kinetic Energy
• Work-Energy Principle

2
Definitions

• Work - The means of transferring energy by the
  application of a force.
• Work is the product of the magnitude of
  displacement times the component of that force in
  the direction of the displacement.
• Work is a scalar
• Energy - The state of one or more objects. A
  scalar quantity, it defines the ability to do
  work.

3
Units
Erg Dyne-cm g-cm2/s2
Joule (J) N-m kg-m2/s2
Energy
M-L2/T2
Ft-lb slug-ft2/s2
4
Problem 1

• A 1500 kg car accelerates uniformly from rest to
  a speed of 10 m/s in 3 s.
• Find the work done on the car in this time

5
How much work is done by this guy?
Walking at a constant speed
6
Problem 3

• m 50 kg
• displacement 40 m
• force applied 100 N
• 37o angle wrt floor
• mk 0.1
• Find net work done moving the crate

7
Vector Multiplication Scalar Product
8
A more elegant definition for work
9
Problem 4

• How much work is done pulling the wagon 100 m in
  the direction shown by the boy applying the force

10
Work done by a varying force
11
Work in three dimensions
12
Problem 5
Fx (N)
3
2
1
x (m)
10
15
5
How much work is done by this force?
13
Hookes Law and the work to compress/extend a
spring
14
Kinetic Energy and the Work-Energy Principle
15
And you can show this with calculus too!
16
Problem 6

• A 3 kg mass has an initial velocity, v (5i -
  3j) m/s.
• What is the kinetic energy at this time?
• The velocity changes to (8i 4j) m/s.
• What is the change in kinetic energy?
• How much work was done?

17
Problem 7

• A 2 kg block is attached to a light spring of
  force constant 500 N/m. The block is pulled 5 cm
  to the right and of equilibrium. How much work
  is required to move the block?
• If released from rest, find the speed of the
  block as it passes back through the equilibrium
  position if
• the horizontal surface is frictionless.
• the coefficient of friction is 0.35.