CHAPTER 4/8
Work & Machines
WHAT IS WORK?
Work is done when a force causes an object
to move in the direction of the force.
Transfer of kinetic energy
Applying a force does NOT always result
in work being done.
If you apply a force to an object and it doesn’t
move, no work was done.
If the applied force is not in the same
direction as the movement of the object NO
work is done.
IS WORK BEING DONE?
http://www.cbsnews.com/video/
watch/?id=50138922n
HOW MUCH WORK?
Work depends on force as well as distance.
 Would you do more work by hiking up the hill or climbing up the
side of the cliff?
d1
d2
F2
F1
 Because the climber ends up the same place, the same
amount of work is done. The hiker goes a shorter distance
climbing the cliff but uses more force. The hiker goes a longer
distance but uses less force hiking up the hill.
CALCULATING WORK
Work
Joule
(J)
Force
(N)
Newton
(N)
Distance
Meters
(m) (m)
Joule – the unit used to express energy;
equivalent to the amount of work done by a
force of 1 N acting through a distance of 1 m
in the direction of the force
WORK PRACTICE PROBLEMS
 Using a force of 10 N, you push a shopping cart 10m.
How much work did you do?
 You use 75 N of force to push a box 3 m across the floor.
How much work has been done?
 Lifting a dog 2 m off the ground requires 12 J of work.
How much force was required?
POWER: HOW FAST WORK IS BEING DONE
 Power- the rate at which work is done or energy is
transformed

Watt- the unit used to express power;
equivalent to joules per second
 Increasing Power
 Do more work in a given time
 Do the same work in less time
POWER PRACTICE PROBLEMS
A stage manager at a play raises a curtain by doing
5,976 J of work on the curtain in 12 s. What is the
power output of the stage manager?
If it takes you 10 s to do 150 J of work on a box to
move it up a ramp, what is your power output?
POWER PRACTICE PROBLEMS
 A light bulb is on for 12 s, and during that time, it uses
1,200 J of electrical energy. What is the power
(wattage) of the light bulb?
 You and a friend together apply a force of 1,000 N to a
car, which makes the car roll 10 m in 90 s. How much
work did you and your friend do together? What was
the power output?
8.2 WHAT IS A MACHINE
Machine- A device that makes work easier by
changing the size or direction of a force.
Work in, work out
 Work Input- the work that you do on a machine. You apply
a force, called the input force, to the machine through a
distance.
 Work Output- the work a machine applies through a force,
called the output force, through a distance.
 Work output can NEVER be greater than work input!!!
HOW MACHINES HELP
Look at the picture of the paint can and screw driver:
 Work done by screwdriver on lid = work you do on screwdriver
 (Work output can never be greater than work input.)
 Machines allow force to be applied over a greater distance, which
means that less force will be needed for the same amount of work.
FORCE DISTANCE TRADE OFF
When force or distance decreases, the other
must increase.
 Greater force, less distance
 Less force, greater distance
MECHANICAL ADVANTAGE
Mechanical Advantage - The number of
times the machine multiplies force.
Calculating Mechanical Advantage
 Mechanical advantage equals the output force divided by
the input force.
output
force
Mechanical  advantage (MA) 
input force
MECHANICAL ADVANTAGE PROBLEMS
 A grocer uses a handcart to lift a heavy stack of canned food.
Suppose he applies an input force of 40 N to the handcart. The
cart applies an output force of 320 N to the stack of canned
food. What is the mechanical advantage of the handcart?
 A pulley is being used to lift a dresser to the 2nd floor of a
building. Suppose a input force of 30 N is applied to the pulley.
The pulley applies an output force of 150 N to the dresser.
What is the mechanical advantage of the pulley?
MECHANICAL EFFICIENCY
The output of a machine is always less than the
work input. Why??
 Some of the work done by the machine is used to
overcome the friction created by the use of the
machine. The less work a machine has to do to
overcome friction, the more efficient the machine is.
 Calculating Efficiency Mechanical
 Efficiency equals work output divided by work input multiplied by
100.
Efficiency =
workoutput
 100
workinput
MECHANICAL EFFICIENCY PROBLEMS
What is the mechanical efficiency of a machine
whose work input is 100 J and work output is 30 J?
What is a machines mechanical efficiency who
work input is 150 J and work output is 100 J?
SIMPLE MACHINES
Levers
Pulleys
Wheel and Axles
Inclined Planes
Wedges
Screws
Compound Machines
LEVERS
A simple machine that consist of a bar that
pivots at a fixed point called a fulcrum.
There are 3 classes of levers – depend on the
location of the fulcrum, the load, and the input
force.
LEVERS
First-Class Levers
 The fulcrum is between the input force and the load.
LEVERS
 Second-Class Levers
The load of a second-class lever is between
the fulcrum and the input force.
LEVERS
Third-Class Levers
 The input force in a third-class lever is between the
fulcrum and the load.
PULLEY
Pulley- a simple machine that consists of a
wheel over which a rope, chain, or wire
passes
PULLEY
• Fixed Pulley - Attached to something that does not
move.
 You pull down on the rope to lift the load upward.
 The pulley changes the direction of the load.
 Ex
• Moveable Pulley- Attached to the object being
moved.
Does not change the force’s direction.
Increase input force
Ex
PULLEY
 Block & Tackle Pulley - A fixed and moveable pulley
are used together.
 Mechanical advantage of the block and tackle
depend on the number of wheels.
WHEEL & AXEL
A wheel and axle is a simple machine consisting of
two circular objects of different sizes.
Mechanical Advantage = radius of wheel/radius of
axel
MECHANICAL ADVANTAGE OF A WHEEL
& AXEL
 What is the mechanical advantage of a wheel and axel
when the radius of the wheel is 100 cm and the radius
of the axel is 10 cm?
 What is the mechanical advantage of a wheel and axel
when the radius of the wheel is 75 cm and the radius
of the axel is 15 cm?
INCLINED PLANES
Simplest of all the machines.
 Common inclined planes:
Ramp
Wedge
screw
Compound Machines
Compound machines are machines that
are made of two or more simple machines.
Mechanical Efficiency
Low because compound machines have more
moving parts than simple machines do, thus there
is more friction to overcome
What simple machines are found in a bike?