Transcript Work Output

Work, Power, and Machines
Chapter 14
Chapter 14 Pretest
• According to Newton’s first law, if no net
force acts on an object, the object
continues in motion with constant ______.
a) Velocity
b) Force
c) acceleration
Chapter 14 Pretest
• A horizontal force on an object can be
broken down into these components: 5 N
north and 5 N east. If no other forces act
on the object, in what direction will the
object move?
– Northeast
• Newton’s second law states that the net
force acting on an object equals the
product of what two variables?
– Mass and acceleration
Chapter 14 Pretest
• A machine produces an output force of
12.3 N when an input of 8.6 N is applied.
What is the ratio of the machine’s output
force to its input force?
– 1.4
• A person exerts 22 N on a box. If a
frictional force of 3 N opposes this force,
what is the net force acting on the box?
– 19 N
Chapter 14 Pretest
• A machine has an output force of 57.3 N
when a force of 32.6 N is used to operate
the machine. What is the percentage
increase of the force?
– 176%
Chapter 14 Pretest
• A small wheel has a radius of 32 cm, and
a large wheel has a diameter of 128 cm.
What is the ratio of the diameters of the
large wheel to the small wheel?
a)
b)
c)
d)
4
2
0.25
0.5
Work
• Work is the product of force and distance. Work
is done when a force acts on an object in the
direction the object moves. If a force acts on an
object but the object does not move, no work is
done.
1. A man pushes a grocery cart at constant speed
from one end of an aisle to the other. Identify the
force, the distance, and the work.
• The man supplies the force by pushing on the cart. The
length of the aisle is the direction. The work done is the
product of the force and the distance.
2. Describe two examples of work you do on a typical
day.
• Lifting a heavy school bag, pushing or pulling a door open
14.1- Work
• In science, work is the product of force
and distance.
– Work is done when a force acts on an object
in the direction the object moves.
• Work requires motion.
– If there is no movement, no work is done.
– For a force to do work on an object, some of
the force must act in the same direction as the
object moves.
14.1 – Work
• Work depends on two things:
– The direction of the force
– The direction of the movement
• A force does not have to act entirely in the
direction of the movement to do work.
– Any part of a force that does not act in the
direction of motion does no work on an object.
14.1 – Calculating Work
• Work done is calculated by multiplying the
constant force acting in the direction of
motion by the distance the object moves.
• Work = Force X Distance
• SI unit for work is joule (J)
– Force = newtons (N), distance = meters (m)
14.1 - Power
• Power is the rate of doing work.
• Power is required when you want work
done fast.
– Doing work at a faster rate requires more
power.
– To increase power
• you can increase the amount of work done in a
given time
• or you can do a given amount of work in less time.
14.1 Calculating Power
• Power is calculated by dividing the amount of
work done by the time needed to do the work.
• Power = Work / Time
• SI unit for power is watt (W)
– Work = joules (J), time = seconds (s)
• Horsepower is another common unit for power.
– 1 horsepower (hp) is equal to ~746 watts
Chapter 14.1 Assessment
• What conditions must exist in order for a
force to do work on an object?
– Some of the force must act in the same
direction as the object moves.
• What formula relates work and power?
– Power = work/time
• How much work is done when a vertical
force acts on an object moving
horizontally?
– No work is done.
Chapter 14.1 Assessment
• A desk exerts an upward force to support a
computer resting on it. Does this force do work?
– The supporting force does no work because the
object it acts on (the computer) does not move.
• Two cars have the same weight, but one of the
cars has an engine that provides twice the
power of the other. Which car can make it to the
top of a mountain first?
– The car with the more powerful engine will reach the
top first.
• Which car does more work to reach the pass?
– Both cars do the same amount of work to reach the
top.
Chapter 14.1 Assessment
• You carry two heavy bags of groceries
upstairs to your kitchen. Will you do more
work on the bags if you carry them up one
at a time?
– Carrying one bag at a time uses only half the
force, but requires that the force be applied
through twice the distance. The work done is
the same in both cases.
14.2 - Machines
• A machine is a device that changes a
force.
• Machines make work easier to do.
– They do this by one of the following:
• change the size of a force needed,
• the direction of a force,
• or the distance over which a force acts.
14.2 – Machines Do Work
• Increasing Force
– A small force exerted over a large distance
becomes a large force exerted over a short
distance
– If a machine increases the distance over
which you exert a force, then it decreases the
amount of force you need to exert
• Example – jacking up a car
14.2 – Machines Do Work
• Increasing Distance
– Some machines decrease the applied force,
but increase the distance over which the force
is exerted
– A machine that decreases the distance
through which you exert a force increases the
amount of force required.
• Example: rowing with oars
14.2 – Machines Do Work
• Changing Direction
– Some machines change the direction of the
applied force
• Example: rowing with oars
14.2 – Work Input
• Because of friction, the work done by a
machine is always less than the work done
on the machine.
• The force you exert on a machine is called
the input force.
• The distance the input force acts through
is known as the input distance.
• The work done by the input force acting
through the input distance is called the
work input.
14.2 – Work Output
• The force that is exerted by the machine is
called the output force.
• The distance the output force is exerted
through is the output distance.
• The work output of a machine is the
output force multiplied by the output
distance.
14.2 - Assessment
• How can using a machine make a task
easier to perform?
– Machines make work easier to do by altering
the size of the force needed, the direction of a
force, or the distance over which a force acts.
• How does the work done on a machine
compare to the work done by a machine?
– Because of friction, the work done by a
machine is always less than the work done on
the machine.
14.2 - Assessment
• What changes can a machine make to a
force?
– Size of force; direction of force; distance
through which a force acts
• A machine produces a larger force than
you exert to operate a machine. How
does the input distance of the machine
compare to its output distance?
– Because the output force is greater than the
input force, the input distance must be greater
than the output distance.
14.2 - Assessment
• You do 200 J of work pulling the oars of a
rowboat. What can you say about the
amount of work the oars do to move the
boat?
– Less than 200 J of work are done to move the
boat because some of the input work is lost
due to friction.
• How can you increase the work output of a
machine?
– Increase the work input to the machine, or
reduce any friction that limits work output.
14.2 - Assessment
• When you swing a baseball bat, how does
the output distance the end of the bat
moves compare with the distance you
move your hands through? Why might
this difference be useful?
– The output distance is much greater than the
input distance. The high speed of the bat end
helps the batted ball to also acquire a high
speed (and travel far).
14.2 - Assessment
• An advertisement for a new type of wrench
claims it reduces the force needed to
tighten the bolt. If the advertisement is
correct, what do you know to be true about
the input distance?
– If the input force is decreased, then the input
distance must be increased.
14.3 – Mechanical Advantage
• The mechanical advantage of a machine is the
number of times that the machine increases an
input force.
• The mechanical advantage determined by
measuring the actual forces acting on a machine
is the actual mechanical advantage.
• Actual Mechanical Advantage = Output Force
(AMA)
Input Force
14.3 – Ideal Mechanical Advantage
• The ideal mechanical advantage of a machine is
the mechanical advantage in the absence of
friction.
– If any machine were frictionless, its AMA would be the
maximum possible value.
• Because friction is always present, the actual
mechanical advantage of a machine is always
less than the ideal mechanical advantage.
– AMA is less than IMA
– Engineers design machines that use lowfriction materials and lubricants.
14.3 – Calculating Mechanical Advantage
• Ideal mechanical advantage is easier to
measure than actual mechanical advantage
because it depends only on the locations of the
forces and the distances over which they act.
• Ideal Mechanical Advantage = Input Distance
(IMA)
Output Distance
• *Remember that the effects of friction are
neglected when calculating ideal mechanical
advantage!
14.3 – Calculating Efficiency
• Because some of the work input to a machine is
always used to overcome friction, the work
output of a machine is always less than the work
input.
• The percentage of the work input that becomes
work output is the efficiency of a machine.
• Because there is always some friction, the
efficiency of any machine is always less than
100 percent.
• Efficiency = work output x 100%
work input
14.3 – Assessment
• Why is the actual mechanical advantage
of a machine always less that its ideal
mechanical advantage?
– The presence of friction results in the actual
mechanical advantage of a machine always
being less than its ideal mechanical
advantage.
• Why can no machine be 100% efficient?
– Because there is always some friction, the
efficiency of any machine is always less than
100%.
14.3 - Assessment
• You test a machine and find that it exerts a force
of 5N for each 1N of force you exert operating
the machine. What is the actual mechanical
advantage of the machine?
AMA = output force = 5N = 5
input force 1N
• How can two machines appear identical and yet
not have the same actual mechanical
advantage?
– If the amount of friction acting on the machine differs,
then the actual mechanical advantage of the
machines will also differ.
14.3 - Assessment
• What information would you use to
calculate the efficiency of a machine?
– Efficiency is calculated from work input and
work output.
• When is the ideal mechanical advantage
of a machine greater than 1?
– The IMA of a machine is greater than 1
whenever the output force is greater than the
input force.
14.3 - Assessment
• Suppose you are an inventor in 1900. You
are constructing a bicycle of your own
design. What could you do to ensure your
bicycle efficiently changes work input into
forward motion?
– Main goal is to keep friction to a minimum.
14.3 - Assessment
• You have just designed a machine that
uses 1000 J of work from a motor for
every 800 J of useful work the machine
supplies. What is the efficiency of your
machine?
Efficiency = (work output / work input) x100%
= (800J/1000J) x 100% = 80%
14.3 - Assessment
• If a 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?
Efficiency = (work output / work input) x 100%
Work output = (efficiency x work input) / 100%
= (40% x 1000 J) / 100% = 400 J
14.4 – Simple Machines
• The six types of simple machines are:
– Lever
– Wheel and axle
– Inclined plane
– Wedge
– Screw
– Pulley
14.4 - Levers
• A lever is a rigid bar that is free to move around a fixed
point, called a fulcrum.
• There are three classes of levers based on locations of
input force, output force, and fulcrum.
• The input arm of a lever is the distance between the
input force and the fulcrum.
• The output arm of a lever is the distance between the
output force and the fulcrum.
• Ideal Mechanical Advantage = input arm / output arm
14.4 – First-Class Levers
• First-Class Levers – the fulcrum is always
located between the input force and the output
force.
• Ideal Mechanical Advantage - can be greater
than, less than, or equal to 1
• Examples: seesaw, scissors, tongs
14.4 – Second-Class Levers
• Second-Class Levers – the output force is
located between the input force and the fulcrum.
• Ideal Mechanical Advantage – always greater
than 1
• Example: wheelbarrow
14.4 – Third Class Levers
• Third-Class Levers – the input force is located
between the fulcrum and the output force.
• Ideal Mechanical Advantage – always less
than1.
• Examples: baseball bats, hockey sticks, golf
clubs, broom
14.4 – Wheel and Axle
• A wheel and axle is a simple machine that
consists of two disks or cylinders, each one with
a different radius.
• IMA = input force radius / output force radius
– Can use diameter rather than radius in formula
– IMA can be greater than or less than 1.
• Examples: car steering mechanism, turning a
screw with a screwdriver
14.4 – Inclined Planes
• A inclined plane is a slanted surface along
with a force moves an object to a different
elevation.
• IMA = distance along the incline plane
change in height
• Examples: switchback roads, ramp
14.4 – Wedges and Screws
• Wedges – a V-shaped object whose sides are two
inclined planes sloped toward each other.
• IMA greater than 1.
– A thin wedge of a given length has a greater IMA than a
thick wedge of the same length.
• Examples: knife blade, zipper, axe
• Screws – is an inclined plane wrapped around a cylinder.
– Screws with threads that are closer together have a
greater IMA.
14.4 - Pulleys
• Fixed Pulleys – a wheel attached in a fixed
location
– IMA is always equal to 1
– Examples: flagpole, window blinds
• Movable Pulley – is attached to the object being
moved rather than to a fixed location
– Examples: pull in sails, platforms for skyscraper
window washers
• Pulley System – combined fixed and movable
pulleys
14.4 – Compound Machines
• A compound machine is a combination of
two or more simple machines that operate
together.
• Examples: scissors, car, washing
machine, clock