Download Work and Machines

Work and Machines
Unit 8-4 & 8-5
Work and Machines
I. Work
a. In science, the word work has a different
meaning than you may be familiar with.
b. The scientific definition of work is: using
a force to move an object a distance
when both the force and the motion of
the object are in the same direction.
What does it mean?
c. There is no work without motion.
d. No work without forces going in the
same direction
Work or Not?
• According to the
scientific definition,
what is work and what
is not?
– a teacher lecturing to her
class
– a mouse pushing a piece
of cheese with its nose
across the floor
5
II. Direction and force
a. Horse does not work
on the rider – force not
in same direction
b. Work is NOT being
done.
Force on
person
c. Horse does work on
the cart - force and
motion are going in
the same direction
Force on
cart
motion
motion
III. What is work?
A. A scientist delivers a speech to an audience of his
peers.
No
A. A body builder lifts 350 pounds above his head.
Yes
A. A mother carries her baby from room to room.
No
A. A father pushes a baby in a carriage.
Yes
A. A woman carries a 20 kg grocery bag to her car?
No
IV. Calculating Work
Formula:
Work = Force x Distance
A.
B.
C.
D.
E.
F.
Formula: W = F x D
The unit of force is newtons
The unit of distance is meters
The unit of work is newton-meters
One newton-meter is equal to one joule
So, the unit of work is a joule
W=FD
Work = Force x
Distance
Calculate: If a man
pushes a concrete
block 10 meters
with a force of 20 N,
how much work has
he done?
W=FD
Work = Force x
Distance
Calculate: If a man
pushes a concrete
block 10 meters
with a force of 20 N,
how much work has
he done? 200 joules
(W = 20N x 10m)
V. How Machines Make
Work Easier
a. A machine is a device that helps make work
easier to perform by accomplishing one or
more of the following functions:
i.
ii.
iii.
iv.
transferring a force from one place to another,
changing the direction of a force,
increasing the magnitude (amount) of a force, or
increasing the distance or speed of a force.
b. A machine does not change the amount of
work done. It can simply make the work easier
for you.
VI. Reasons Why Machines are
NOT 100% Efficient
a. Machines have moving parts so there
is friction
b. Some of the work done is used to
overcome friction