CHAPTER 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?
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.
Mechanical  advantage (MA)  output force
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?