Bellringer
Compare and explain in complete
sentences what is work
Homework
CALCULATE KINETIC ENERGY OF AN OBJECT
DROPPED FROM A BUILDING 10 M HIGH, MASS 50 kg
JUST BEFORE IT HITS THE GROUND
Objectives
 Define work and identify the units
 Describe the conditions that must exist for a force to
do work on an object
 Calculate the work done on an object
 Describe and calculate power
 Compare the units of watts and horsepower as they
relate to power
Work
 Definition
- quantity of energy transferred by a force when
it is applied to a body and causes that body to move
in the direction of force
 Formula
-W=FxD
 Units
- Newton meter (N/m) or a joule (J)
Work Continued
 Two factors
1. size of force, and direction it is applied
ex: pulling a suitcase
* any part of a force that does not act in the
direction of motion does no work on an
object
2. movement of something by that force
Work Cont.
- A weight lifter who holds a barbell weighing 1000 N
does NO work
- Why?
- he may get very tired, but if the barbell is not
moved by the force he exerts, he does no work on
the barbell
- work is done on his muscles when he raises the
barbell
Work Problems
Q: A crane uses an average force of 5200 N to lift a
girder 25 m. How much work does the crane do on
the girder?
A: W = (5200N)(25m) = 1.3 x105 J
Q: While rowing in a race, John uses his arms to exert
a force of 165 N per stroke while pulling the oar
0.800 m. How much work does he do in 30 strokes?
A: W = (30)(165N)(0.800m) = 3960 J 4.0 x 103
Work Problems Cont.
Q: Jake, a 235 N track athlete completes his race,
which totals 1575 J, what is the total distance Jake
ran?
A: 1575 J = 6.70 m
235 N
Q: Joey, performed 900 J of work, while lifting a box
12 meters. What force did Joey exert on the box?
A:75 N
Power
 Definition
- a quantity that measures the rate at which work
is done
 Formula
- P = W/t
 Unit
- Joule per second (J/s) or Watts (W)
- Horsepower (Hp) = 746 W
Power Problems
Q: Using a jack, a mechanic does 5350 J of work to
lift a car 0.500 m in 50.0 s. What is the mechanic’s
power output?
A: P = 5350J/ 50.0s = 107 W
Q: Anna walks up the stairs on her way to class, She
weighs 565 N and the stairs go up 3.25 m vertically.
Calculate the power output if she climbs the stairs in
12.6 s. What is her power output if she climbs the
stairs in 10.5 s?
A: P = (565N)(3.25m)/12.6s =146 W
P = (565N)(3.25m)/10.5s = 175 W
Objectives
 Describe what a machine is and how it makes work
easier
 Relate the work input to a machine to the work
output of the machine
 Compare a machines actual mechanical advantage to
its ideal mechanical advantage
 Calculate the ideal and actual mechanical and actual
mechanical advantages of various machines
 Explain why efficiency of a machine is always less
than 100%
 Calculate the machines efficiency
Machines
 Definition
- a device that changes force
ex. jack, nutcracker, an oar
 How does a machine change force?
- 3 ways a machine makes work easier to perform
- change the size of force needed
- the direction of a force
- the distance over which a force acts
Changing the Force
 Increasing the force you applied
ex. jack applied to a car
- small force exerted over a large distance becomes a
large force exerted over a short distance
Changing the Distance
 Increasing distance
ex. rowing a boat with oars
- small movement of oar at the hands makes a large
distance the oar in the water will move.
*remember the trade off: small distance large force*
Changing the Direction
 Changing the direction of the applied force
ex. rowing a boat
- pulling back on the handle of the oars causes its
other end to move in the opposite direction.
Work Input
 Work Input
- the work done on a machine as the input force acts
through the input distance
- input force: force exerted on a machine
- input distance: distance the input force acts
through
- equals the input force multiplied by the input
distance
ex. rowing a boat
- input distance < output distance
- input force > output force
Work Output
 Work Output
- the work done by a machine as the output force
acts through the output distance
- output force: force exerted by a machine
- output distance: distance the output force is
exerted through
*due to friction the work done by a machine is always
less than the work done on the machine
Mechanical Advantage
 Definition
- a quantity that measures how much a machine multiplies
force or distance
 Two types
 Actual: measures the actual forces action on a machine
- AMA = output force
input force
 Ideal: measures the mechanical advantage in the absence
of friction
- IMA = input distance
output distance
Mechanical Advantage Problems
Q: Alex pulls on the handle of a claw hammer with a
force of 15 N. If the hammer has a actual mechanical
advantage of 5.2, how much force is exerted on a nail
in the claw?
A: output force = (5.2)(15N) = 78 N
Q: If you exert 100 N on a jack to lift a 10,000 N car,
what would be the jack’s actual mechanical advantage
(AMA)
A: AMA= 10,000 N / 100 N = 100
Mechanical Advantage Problems
Q: Calculate the ideal mechanical advantage (IMA) of
a ramp that is 6.0 m long and 1.5 m high?
A: IMA = 6.0m / 1.5m = 4.0
Q: The IMA of a simple machine is 2.5. If the output
distance of the machine is 1.0 m, what is the input
distance?
A: Input distance = (2.5)(1.0m) = 2.5 m
Efficiency of Machines
 Definition
- a quantity, usually expressed as a percentage,
that measures the ratio of useful work input
 Formula
- Efficiency = useful work output
work input
- % of work input that becomes work output
- due to friction, efficiency of any machine is always
less than 100%
Efficiency Problems
Q: Alice and Jim calculate that they must do 1800 J of
work to push a piano up a ramp. However, because
they must also overcome friction, they must actually
do 2400 J of work. What is the efficiency of the
ramp?
A: 1800 J/ 2400 J x 100 = 75%
Q: If the machine has an efficiency of 40%, and you do
1000 J of work on the machine, what will be the work
output of the machine?
A: Work Output = (Efficiency x work input) / 100%
Work Output = (40% x 1000 J) / 100% = 4.0 x 102 J
Objectives
 Name, describe and give an example of each of the
six types of simple machines
 Describe how to determine the ideal mechanical
advantage of different types of simple machines
 Define and identify compound machines
 Recognize simple machines within compound
machines
Simple Machines
 Definition
- one of the six basic types of machines
 2 types or families
1. lever
2. inclined planes
Levers
 Definition
- a rigid bar that is free to move around a fixed
point
ex. screwdriver
- all levers have a rigid arm that turns around a
point called the fulcrum
- force is transferred from one part of the arm to
another
- original input force can be multiplied or
redirected into output force
- levers are divided into 3 classes, based on the
locations of the input force, output force, and
the fulcrum
Lever Family Cont.
 3 classes
1. First class
- fulcrum is in the middle of an arm
- the input force acts on one end
- the other end applies an output force
- MA can be: <1, >1, =1
ex. teeter totter, scissors, tongs
2. Second class
- fulcrum is at one end of the arm
- input force is applied to the other end
- output force is in the middle
- MA will always > 1
ex. wheel barrow
Lever Family Cont.
3. Third class
- input force is in the middle
- output force is one one end
- fulcrum is on the other end
- multiplies distance rather than force
MA always < 1
ex. baseball bats, hockey sticks,
golf clubs, human body
Wheel and Axis
 Definition
- simple machine that consists of two disks or
cylinders, each one with a different radius
ex. steering wheel, screwdriver
- made of a level or a pulley (wheel) connected to a
shaft (axle)
- small input force, multiplied to become a large
output force
Inclined Planes
 Definition
- slanted surface along which a force moves an
object to a different elevation
ex. knife, ax, zipper, wedge, screw
- ramp redirects the force applied to lift object upward
- turns a small input force into a large output force by
spreading the work out over a large distance
- wedge: functions as two inclined planes back to
back, turning a downward force into two forces
directed out to the sides
-screw: an inclined plane wrapped around a cylinder
Pulleys
 Definition
- a simple machine that consists of a rope that fits
into a groove in a wheel.
- very similar to a lever
- point in the middle of the pulley is like a fulcrum
- rest of the pulley acts like the rigid arm
 3 Types of Pulleys
- fixed pulleys
- moveable pulleys
- pulley system
Types of Pulleys
- fixed pulleys: changes the direction of the input
force
- moveable pulleys: changes both the direction and
the size of the input force
- pulley system: made of both fixed and moveable
pulleys
Compound Machines
 Definition
- a machine that is made of more than one simple
machine
ex. scissors, jacks, bicycle, washing machine,
car, clock