Sct. 1 Work and Power
Work

Is done when a force causes an object to
move.
 Carrying books
 Pushing grocery cart
 Lifting weights
 Dodge truck pulling Ford out of mud.
Applying force to do work.
Work is only done in direction of applied
force and when object is moving.


Lifting books off of desk = work by arms
Carrying books to class = not work by
arms but by legs
Calculating Work
Work = Force x distance
W=Fxd
Joules = N x meters

What is the total amount of work done
when a football player carries the ball with
a force of 25 N for 50 meters?
HW: Jot down a note about work that you did between now and
tomorrow in class. Explain how having more power would have
affected the work.
W
W
W
W
=
=
=
=
Fxd
25 N x 50 m
1250 N x m
1250 J
Power
How quickly work is accomplished.
Who is more powerful?


A man who can lift 50 N of weight in 10
seconds or a woman who can lift 50 N of
weight in 8 seconds?
Woman. Same weight and work
accomplished, less time.
Calculating Power
Power = work/time
P = w/t
Watts = Joules/second

What is your power if you do 200 Joules of
work in 20 seconds?
P
P
P
P
=
=
=
=
W/t
200 Joules/20 seconds
10 Joules/sec.
10 Watts
Work and Energy
When you do work on an object, because
it has moved, you increase it’s potential
or kinetic energy.

Potential Energy = energy of position/stored
energy
 Height above earth
 Stretched rubber band

Kinetic Energy = energy of motion
 All moving objects have kinetic energy
 Work = objects in motion
Work and Power HomeWORK.
How much work must you do to move 100, 10 N rocks a distance
of 50 meters?
How far did you push the grocery cart if it required 500 Joules of
work and needed 25 N of force?
How much power is needed for a Dodge truck to pull a Chevy
truck from the mud in 10 seconds if it requires 4000 Joules of
work?
Who has more power? Mr. Replogle lifts 15 pieces of cake (1
piece = 1 J of work) to his mouth in 60 seconds and Mr. Brode
lifts 21 pieces of cake to his big mouth in 80 seconds.
What kind of energy does the cake have as it is being swallowed
to the depths of Mr. Brode’s belly?
What kind of energy did the cake have when it was being held
high above his mouth ready to plummet to its dark grave?
w=f x d
w=1000N x 50 m
w= 50,000 J
w=f x d
500J = 25 N x d
d= 20 m
P=w/t
P= 4000J/10 sec
P= 40 Watts
Barton Power = 15 J/1500 sec. = .o1 Watts
Brode Power = 21 J/1740 sec. = .012 Watts
Brode more powerful!
Kinetic Energy
Potential Energy
Section 2 Using Machines
Machine

A device that makes doing work easier
 Pencil
 Lawn mower
 Screw driver
 Car
 Computer
 pulley
Machines contd.
Change the way a person does work, not how
much work is done.
Have:


Input work- effort put in
Output work- the work done by the machine
No machine can create work


Input work always greater than output work.
Due to friction
Mechanical Advantage
The number of times a machine
multiplies the input force.
MA = output force/input force
EX: A car jack can lift a 2000 N car
when a person applies 50 N of force.
MA = 2000 N / 50 N = 40
Work in = Work out
In an ideal machine. Not a real
machine
Fin x din = Fout x dout
If a machine produces 50 N of force
when 10 N are put in and the distance
out is 2 m, how much distance must the
machine have put in?
Fin x din > Fout x dout
10 N x din > 50 N x 2 m
din > 10 m
Ways that machines make
work easier.
Exert a smaller force over a greater distance
to increase output force.

Car jack
Exert a larger force over a smaller distance to
increase output distance.

Rake
Allow you to change the direction of the input
force.

Pulley
Efficiency
Ability of a machine to use all of the
work in.
E = output work/input work x 100%
Efficiency is reduced by friction.
Therefore always less than 100%
Efficiency Calculation
How efficient is a car that produces 100
Joules of work for every 300 Joules of
fuel consumed?
E = work out/work in
E = 100 J / 300 J
E = .33 or 33%
Group Work
What is the mechanical advantage (ma = force
out/force in) of a wrench that you put 50 N of force
into and it loosens a bolt with a force of 250 N?
What is the mechanical advantage of a rake that you
must use 100 N of force to rake leaves with and the
leaves require 20 N of force to move?
What is the efficiency of a car jack that lifts a car
using 5000 J of work when you put 10000 J of work
into the jack?
What is the efficiency of a machine that produces
500 J of work when 400 J of work is put in? Does
this machine exist, and if so, what is it?
Homework
Fin x din = Fout x dout
At home pick up a hammer and 2 nails.
Start nails in a board and have the same amount of
nail sticking out of the board.
First nail, hold the hammer as close to the head as
possible.
Second nail, hold the hammer as close to the end of
the handle as possible.
Compare the force out.