Systems and environments Work done by a force
Kinetic energy Work KE Theorem
Potential energy
Conservative and Energy Diagrams
non-conservative forces
1
A variety of problems can be solved with Newton’s
Laws and associated principles.
Some problems that could theoretically be solved
with Newton’s Laws are very difficult in practice.
◦ These problems can be made easier with other techniques.
The concept of energy is one of the most important
topics in science and engineering.
Every physical process that occurs in the Universe
involves energy and energy transfers or
transformations.
Energy is not easily defined.
Introduction
2
1
, The new approach will involve changing from a
particle model to a system model.
model
In this chapter, systems are introduced along with
three ways to store energy in a system.
Introduction
3
A system is a small portion of the Universe.
◦ We will ignore the details of the rest of the Universe.
A critical skill is to identify the system.
◦ The first step to take in solving a problem
A valid system:
◦ May be a single object or particle
◦ May be a collection of objects or particles
◦ May be a region of space
◦ May vary in size and shape with time
Section 7.1
4
2
, The general problem solving approach may be used
with an addition to the categorize step.
Categorize step of general strategy
◦ Identify the need for a system approach
◦ Identify the particular system
◦ Also identify a system boundary
An imaginary surface that divides the Universe into the
system and the environment
Not necessarily coinciding with a real surface
The environment surrounds the system
Section 7.1
5
A force applied to an object in empty space
◦ System is the object
◦ Its surface is the system boundary
◦ The force is an influence on the system from its
environment that acts across the system boundary.
Ball and mass on incline
◦ System ball, block, cord.
◦ Influence – gravitational, normal, and frictional
forces, force exerted by pulley on cord.
◦ Force of cord on ball and block (tension) are
internal forces – not an influence.
Section 7.1
6
3
,
F F
F
7
work W, done on a
The work,
system by an agent exerting
a constant force on the
system
W = F ∆r cos θ
◦ The displacement is that of the point of application
of the force.
◦ A force does no work on the object if the force does
not move the system through a displacement.
◦ The work done by a force on a moving object
(system) is zero when the force applied is
perpendicular to the displacement of its point of
application.
◦ The work is a scalar quantity, although the force
and the displacement are both vectors
Section 7.2
8
4
Kinetic energy Work KE Theorem
Potential energy
Conservative and Energy Diagrams
non-conservative forces
1
A variety of problems can be solved with Newton’s
Laws and associated principles.
Some problems that could theoretically be solved
with Newton’s Laws are very difficult in practice.
◦ These problems can be made easier with other techniques.
The concept of energy is one of the most important
topics in science and engineering.
Every physical process that occurs in the Universe
involves energy and energy transfers or
transformations.
Energy is not easily defined.
Introduction
2
1
, The new approach will involve changing from a
particle model to a system model.
model
In this chapter, systems are introduced along with
three ways to store energy in a system.
Introduction
3
A system is a small portion of the Universe.
◦ We will ignore the details of the rest of the Universe.
A critical skill is to identify the system.
◦ The first step to take in solving a problem
A valid system:
◦ May be a single object or particle
◦ May be a collection of objects or particles
◦ May be a region of space
◦ May vary in size and shape with time
Section 7.1
4
2
, The general problem solving approach may be used
with an addition to the categorize step.
Categorize step of general strategy
◦ Identify the need for a system approach
◦ Identify the particular system
◦ Also identify a system boundary
An imaginary surface that divides the Universe into the
system and the environment
Not necessarily coinciding with a real surface
The environment surrounds the system
Section 7.1
5
A force applied to an object in empty space
◦ System is the object
◦ Its surface is the system boundary
◦ The force is an influence on the system from its
environment that acts across the system boundary.
Ball and mass on incline
◦ System ball, block, cord.
◦ Influence – gravitational, normal, and frictional
forces, force exerted by pulley on cord.
◦ Force of cord on ball and block (tension) are
internal forces – not an influence.
Section 7.1
6
3
,
F F
F
7
work W, done on a
The work,
system by an agent exerting
a constant force on the
system
W = F ∆r cos θ
◦ The displacement is that of the point of application
of the force.
◦ A force does no work on the object if the force does
not move the system through a displacement.
◦ The work done by a force on a moving object
(system) is zero when the force applied is
perpendicular to the displacement of its point of
application.
◦ The work is a scalar quantity, although the force
and the displacement are both vectors
Section 7.2
8
4
