SPH3U: Energy, Work, Power
Work
Recall
** Energy is the ability to do work **
Recall
** Energy is the ability to do work **
But what is WORK anyway?
Work

In English we often use the word ‘work’
to mean something that requires a lot of
effort.

In Physics, work has a specific definition
Work

We will define work (mechanical work)
to be when a force is applied to an
object, and the object is displaced in the
same direction as the force (or at least a
component of the force).
Work

W = FΔd
(when the force and displacement are in the same
direction)
Work is a scalar quantity (not a vector)
 Work is measured in Joules (J)
 1 Joule = (1N)(1m)

Examples / Non-Examples

In each situation, is work being done on the book?
A person pushes a book across the floor
A person holds a book for 2 minutes.
A person holds a book and walks across
the room
A person drags the book with a string at
an angle of 60º
A book is floating in space and drifting
along at 10 m/s
Examples / Non-Examples

In each situation, is work being done on the book?
Yes. Force and Displacement
are in the same direction
A person pushes a book across the floor
No. Force but no
displacement.
A person holds a book for 2 minutes.
No. Displacement but no force
applied to book.
A person holds a book and walks across
the room
Yes. Part of the force is in the
direction of the displacement.
A person drags the book with a string at
an angle of 60º
No. There is displacement but
no force.
A book is floating in space and drifting
along at 10 m/s
Work and Direction

Example: A woman pulls a wagon with
an applied force of 50N [60º up from
horizontal]. The wagon moves 2 meters
forward. What is the work done on the
wagon?
Positive and Negative Work

It is possible to have both positive and
negative work.
Positive and Negative Work

Positive work – the displacement and force
are in the same direction


Ex: I push the book across the desk
Negative work – the displacement and force
are in exact opposite directions.

Ex. The force of friction acts against the book as
I am pushing it. Friction is doing negative work.
Homework
1)
2)
Read section 5.1 in your text
Questions: #1 – 4, 7-8