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Work and Simple Machines

Unit 8 Notes

Aj Corso

on 14 May 2018

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Transcript of Work and Simple Machines

A transfer of energy that occurs when a force makes an object move; measured in joules.
Work = force x distance
W = F x d
Requirement of Work
the object must move
Arnold lifts a 1250 N barbell 2 m. How much work did he do?
Work = Force x Distance
W = 1250 N x 2m
W = 2500 J
If there is no movement, there is no work!
ex. pushing against a wall
For work to be done, the direction of the applied force must be in the SAME direction of motion!!!
ex. pushing against the front of a moving car and the car keeps moving opposite the direction you are pushing = NO work!
Joule = N x m
If Jane weighs 700 N and does 1400 J of work climbing a ladder. How tall is the ladder?
D = W/F
D = 1400 J / 700 N
D = 2 m
The amount of work done in a certain amount of time.
Power = work / time
P = W/t
Unit of Power = Watt
1 Watt = 1 J/s
People performing the same task do the same amount of WORK, but the person who does it fastest uses the most POWER.
A RATE is anything divided by time.
Arnold lifts a 1250 N barbell 2 m in 3 s. How much power was used?
Power = Work / Time
Work = Force x Distance
W = 1250 N x 2 m
W = 2500 J

P = 2500 J / 3 s
P = 833 Watts
Jane weighs 700 N and generates 800 watts of power climbing a 6 m ladder, how long would it take her to reach the top?
W = F x d
P = W/t
W = 700 N x 6 m
W = 4200 J

t = W/P
t = 4200/800
t = 5.25 s
Energy, Work, and Simple Machines
Chapter 10

Mechanical Advantage
Wheel & Axle
Inclined Plane
A device that makes doing work easier.
3 Ways
increasing the force applied to an object
changing the direction of the applied force
increasing the distance over which a force can be applied
Compound Machine
a combination of 2 or more simple machines
number of times a machine multiplies the effort force applied to it
measure of how much work put into a machine is changed into work done by a machine
makes machines less efficient
energy is transferred from mechanical energy into thermal energy
no machine is 100% efficient... why???
Oil and other lubricants are used to reduce friction.
but where does the energy go?? is it lost?
If a machine has a mechanical advantage of 1 does it help you do any work?
No. If the effort you put in equals the resistance you feel, then it is not really a machine because it isn't really helping you do work.
Air Resistance
friction due to air
As a pendulum swings, it gradualy comes to a stop because of air resistance.
ex. Newton's Cradle stops swinging.
Formula for Mechanical Advantage
Mechanical Advantage = resistance force / effort force
MA = Fr / Fe
There is no unit for MA, it is just a number.
decreases the amount of effort force needed because it increases the distance over which the effort force acts
the surface of the inclined plane makes a difference in efficiency
ex. sandpaper vs. oiled wood
to increase the mechanical advantage of an inclined plane, increase the length of the plane
As the length of the inclined plane increases, the effort force required to go up the ramp decreases.
wheelchair ramp
winding mountian road
MA = Length of ramp / Height of ramp
loading ramp
wheelchair ramp
winding mountain road
a rigid bar that pivots about a fixed point
3 parts of every lever
Effort Force - where you push or pull on the lever
Resistance Force - what you are trying to move or what is pushing back on you. (also called the load)
Fulcrum - the pivot point (not always in the middle)
3 classes of levers
each class of lever has one of these parts in the middle
1st Class
F - Fulcrum is in the middle
screwdriver opening a paint can
sea-saw (teeter totter)
2nd Class
R - Resistance is in the middle
rolling a trash can to the curb
3rd Class
E - Effort is in the middle
"that spells free, Credit report.com bay bee!"
My trick to remember the classes of levers...
MA of Levers
MA = Effort arm length
Resistance arm length
the LONGER the EFFORT arm,the easier it is to do work because it INCREASES the MA
for example, opening a paint can is way easier with a longer screwdriver than with a shorter one.
You always want to be pushing or pulling
on the side with the longest arm!
A lever is used to lift your mom's car to change a flat tire. The length of the effort arm is 4 feet. The length of the resistance arm is 2 feet. What is the MA of the lever?
MA = Effort arm / Resistance arm
MA = 4 feet / 2 feet
MA = 2
Spongebob needs a lever to raise a giant 10,000 N crabby patty off the ground and he only has 200 N of force. What is the mechanical advantage or ratio of the resistance force to the effort force?
MA = resistance / effort
MA = 10,000N / 200 N
MA = 50
The ratio is 50 to 1
swinging a baseball bat, hockey stick, broom, golf club
lifting weights
an inclined plane that wraps around a metal rod
2 Purposes
holds things together
raise heavy objects
MA is just the same as for an inclined plane
MA = Length of Incline
The longer the incline, the greater the MA
an inclined plane with one or two sloping sides
door stop
If you sharpen a knife what happens to the MA and the effort required to cut something?
MA (or how helpful the machine is) increases.
The wedge becomes more useful.
The effort required to cut through something decreases because the machine is more useful.
a simple machine that consists of a grooved wheel with a rope, chain, or cable that runs along a groove, changes the direction of the effort force, and can be fixed or movable.
2 Types
Fixed Pulleys
are attached to something that doesn't move, such as a ceiling or wall
only change the direction of the force
effort is not multiplied
MA is only 1
Movable Pulleys
one end of the rope is fixed
the wheel is free to move
does multiply force
MA = number of supporting ropes
do not count the rope if you pull down on it, only the ones holding up the weight or that you are pulling up on
Block & Tackle Pulley
is a system of pulleys consisting of fixed and movable pulleys
200 N
How much force does the man have to pull with to raise the 200N block?
60 N
How much force does the hand need to pull down with?
MA = count the rope segments,
doesn't count if the person is pulling down on it
simple machine that consists of two different-sized wheels that rotate together
MA = radius of wheel
radius of axle
What is the MA of this car wheel and axle?
Radius = 5 in
Radius = 30 in
MA = Radius of wheel / radius of axle
MA = 30 in / 5 in
MA = 6
Radius of Wheel = 50 cm
Radius of Axle = 10 cm
Force of Bucket = 400 N
MA = radius of wheel / radius of axle
MA = 50 cm / 10 cm
MA = 5

Resistance / MA = Effort
400 N / 5 = 80 N


Kinetic Energy-

Work-Energy Theorem-




Effort Force-

Resistance Force-

Mechanical Advantage-

Ideal Mechanical Advantage--


Compound Machine-
product of force and displacement in the direction of the force.

non-material property capable of causing changes in matter.

energy of object due to its motion

work done on object is equal to the change in its kinetic energy

SI unit of energy equal to one Newton-meter

rate of doing work

unit of power, one joule per second
device that changes force needed to do work

force extended on a machine

force exerted by a machine

ratio of resistance force to effort force in a machine

the ratio of effort distance to resistance distance

ratio of output work to input work

machine consisting of two or more simple machines
Kinetic Energy
energy of object due to its motion
Kinetic Energy = one half the mass x velocity squared

KE = 1/2*m*v^2

measured in Joules
Work-Energy Theorem
Work is equal to the change in KE!
W = KE
W = KE final - KE initial
How can we calculate Work if we apply a force at an angle?
Work at an Angle
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