Transcript Chapter 4 – Work and Machines

Chapter 8 – Work and
Energy
8. 1 Work
pp. 188-192
TERMS TO LEARN
Work – the action that results when a
force causes an object to move in the
direction of the force being applied
Joule – the SI unit of work
JAMES PRESCOTT JOULE
An English
physicist
The SI unit of
work (the joule)
is named after
him
8.1 Work
In science, work is a
form of energy you either
use or get when a force
is applied over a
distance.
You do 1 joule of work if
you push with a force of
1 newton for a distance
of 1 meter.
WORK
The scientific meaning of work is that
work is done on an object:
 only when a force makes an object
move in the direction of the force
AND
 only when the force is being applied
to the object.
CALCULATING WORK
The equation you use to calculate
work is:
work = force × distance
W=F×d
The SI unit of work is the joule (J)
1 J=1N·m
8.1 Work
When thinking about work, remember that work
is done by forces that cause movement.
If nothing moves (distance is zero), then no work
is done.
8.1 Work
Force (N)
Work (joules)
W=Fxd
Distance (m)
8.1 Work and energy
Doing work always means transferring energy.
The energy may be transferred to the object you
apply the force to, or it may go somewhere else.
8.1 Work and energy
You can do work to
increase an object’s
potential energy.
Then the potential
energy can be
converted to kinetic
energy.
8.1 Work
If force is equivalent to the weight
of the object in newtons, and
height (h) is equivalent to
distance (d),
Then multiplying the weight by
height gives you the amount of
work the object can accomplish
as it moves down (as well as its
potential energy).
8.1 Work
Force A does no work
because it does not cause
the block to move.
Force B is applied at an
angle to the direction of
motion, so only part of force
B does work.
The most effective force to
move the block is force C.
Solving Problems
How much work is done by a person who
pulls a cart with a force of 50 newtons if
the cart moves 20 meters in the direction
of the force?
Solving Problems
1. Looking for:

…work done by person
2. Given:


…force = 50 N (forward);
…distance = 20 m
3. Relationships:

Work = force x distance
4. Solution

50 N × 20 m = 1,000 joules.
8.1 Work
Complete Skill Sheet 8.1
Work