Transcript What is work? (cont.) - Riverdale Middle School

Chapter Introduction
Lesson 1
Work and Power
Lesson 2
Using Simple Machines
Chapter Wrap-Up
How do machines make
doing work easier?
Work and Power
Essential Questions
• What is the relationship between
work, power and efficiency?
Work and Power
Work
Power
Efficiency
What is work?
• In science, work is what is necessary
for a force to move an object through a
distance.
• Work is calculated by multiplying the
force applied to an object by the
distance the object moves.
What is work? (cont.)
• In order for you to do work, two things
must occur:
– You must apply a force to an object.
– The object must move in the same
direction as your applied force.
What is work? (cont.)
• Is work being done?
– Pushing a grocery cart in a store?
• Yes, because the cart is moving in the same
direction as the force (push).
– Standing and holding a bag of groceries?
• No, Although you are applying a force to the
grocery bag by holding it, the grocery bag is
not moving so no work is done.
What is work? (cont.)
• Work is important in science because it
is related to energy.
• Work done when you lift an object which
also increases the object’s energy.
– Moving objects have kinetic energy
– Gravitational Potential Energy (GPE) of an
object increases as its height above the
ground increases.
What is work? (cont.)
• Doing work on a tray transfers energy to
the tray. The added energy can be either
kinetic energy or potential energy.
How is work measured? (cont.)
• Work is equal to the force of a push or a
pull multiplied by the distance the object is
moved.
• The product of force and distance has the
unit newton·meter. The newton·
meter is also known as the joule (J).
How is work measured? (cont.)
How is work measured? (cont.)
• The work done on an object depends on
the direction of the force applied and the
direction of the motion.
Motion of
bed
Applied
Force
How is work measured? (cont.)
• Now think of a box being pulled by a
rope.
– In this scenario, the applied force is at an
angle (from your arm to the rope). The
applied force has a horizontal part and a
vertical part.
Applied
force
Vertical
force
Motion of
the box
Horizontal
force
How is work measured? (cont.)
• When the applied force and the motion of the
object are NOT in the same direction, only the
part of the force that is in the same direction
as the motion of
the object is used in
Vertical
the work
force
equation.
Motion of
the box
• The vertical part
Horizontal
force
of the applied force
does no work on the box because it is not in
the same direction as the motion of the box.
How is work measured? (cont.)
• The work done to lift an object equals
the weight of the object multiplied by the
distance it is lifted.
– Work = weight x
distance
What is power?
•
Power is the amount of work done per
unit of time.
•
You can also think of power as how
fast energy is transferred to an object.
What is power? (cont.)
• You can calculate power by dividing the
work done by the time needed to do the
work.
• Power is expressed in joules per
second(J/s). One Joule per second is
also know as a watt (W).
What is power? (cont.)
What is efficiency?
• A machine is any device that makes
doing something easier.
• Some machines are simple and other
machines are more complex.
What is efficiency? (cont.)
• The force you apply to a machine is the
input force.
• The machine changes the input force to
an output force.
Output
force
Input
force
What is efficiency? (cont.)
• The amount of input force multiplied by
the distance over which the input force is
applied is the input work.
• Winput = Finput x dinput
Distance
applied
Input
force
What is efficiency? (cont.)
• Machines convert input work to output
work by applying an output force on
something and making it move.
– Woutput = Foutput x distanceoutput
Output
distance
Output
force
What is efficiency? (cont.)
• The output work done by a machine
never exceeds the input work because
of friction.
– Friction converts some of the input work to
thermal energy.
What is efficiency? (cont.)
• Efficiency is the ratio between the work done
by a machine and the work put into it.
• Because output work is always less than
input work, a machine’s efficiency is always
less than 100 percent.
– Ex. Elevators are 85% efficient, car motors are
17% efficient
What is efficiency? (cont.)
When you lift an object, what else
are you doing?
A. decreasing the object’s energy
B. increasing the object’s energy
C. making the object do work
D. receiving the object’s energy
Which is the rate at which work is
done?
A. energy
B. force
C. power
D. work
To calculate the work done lifting
an object, which is multiplied by
the weight of the object?
A. distance the object is lifted
B. energy used to lift the object
C. force applied to the object
D. power needed to lift the object
Using Simple Machines
Essential Question
• What is the relationship between
work input and work output in a
simple machine?
Using Simple Machines
Simple machine
Wheel and axle
Mechanical
advantage
Pulley
Lever
Fulcrum
Inclined plane
Screw
Wedge
What is a simple machine?
• Simple machine is a device with few, if
any, moving parts that makes it easier to
do work.
– Ex. screwdriver
• A machine makes work easier by
changing the size of the force, the
distance the force acts, or the direction
of a force.
What is a simple machine? (cont.)
• The two main classes of simple
machines are the lever and the inclined
plane.
• Lever class includes:
– Wheel and axle and the pulley
What is a simple machine? (cont.)
• Inclined plane includes:
– Wedge and screw
What is a simple machine? (cont.)
• Mechanical advantage is the number of
times a simple machine multiplies an
effort force.
What is a simple machine? (cont.)
• Mechanical advantage can be less than
1, equal to 1, or greater than 1.
• A mechanical advantage greater than
1 means the output force is greater than
the input force.
What is a simple machine? (cont.)
What are the three kinds of levers?
• Lever is a simple machine consisting of
a bar and a pivot point.
– Fulcrum is the pivot point in a lever.
• The part of the bar on which a person
applies an effort force is called the effort
arm.
What are the three kinds of levers?
(cont.)
• The part of the bar on which the lever
produces an output force is called the
resistance arm.
• The position of the fulcrum, the effort arm and
the resistance arm vary among levers.
What are the three kinds of levers?
(cont.)
• In a first-class lever,
the fulcrum is
between the input
force and the output
force.
• The direction of the input force is opposite the
direction of the output force.
• When the effort arm is longer than the
resistance arm, the output force is greater than
the effort force
What are the three kinds of levers?
(cont.)
• Your neck is an example of a first-class
lever.
What are the three kinds of levers?
(cont.)
• A second-class lever
has the output force
between the input force
and the fulcrum.
• The output force and the input force act
in the same direction.
• A second-class lever makes the output
force greater than the input force.
What are the three kinds of levers?
(cont.)
• A wheel barrow is an example of a
second-class lever.
What are the three kinds of levers?
(cont.)
• A third-class lever
has the input between
the output force and
the fulcrum.
• The output force is less than the input
force.
• Both the input force and the output force
act in the same direction.
What are the three kinds of levers?
(cont.)
A fishing rod is an
example of a thirdclass lever.
What are the three kinds of levers?
(cont.)
• The ideal mechanical advantage of a
lever equals the length of the input arm
divided by the length of the output arm.
What are the three kinds of levers?
(cont.)
What other machines are like
levers?
• Wheel and axle is a simple machine that
consists of a wheel that applies an effort
force and a smaller axle that produces
an output force.
• Mechanical advantage of a wheel and
axle is calculated by dividing the length
of the effort arm by the length of the
resistance arm.
What other machines are like
levers? (cont.)
• For a wheel and axle, the length of the input
arm is the radius of the wheel and the length
of the output arm is the radius of the axle.
• A screwdriver is a wheel and axle.
The handle is the wheel and the shaft is the
axle.
What other machines are like
levers? (cont.)
• A wheel and axle can also make work
easier in another way.
– If the radius of the wheel is smaller than the
radius of the axle, the output distance is
increased and the output force is
decreased.
– Ex. Helicopters and ceiling fans.
What other machines are like
levers? (cont.)
• Pulley is a grooved wheel that turns by
the action of a rope in the groove.
• In a pulley, the rope forms
the arms and the wheel
serves as the fulcrum.
What other machines are like
levers? (cont.)
• A pulley can be fixed or movable.
• A fixed pulley makes work
easier by changing the
direction of the effort force.
What other machines are like
levers? (cont.)
• Movable pulleys are attached to
the object being lifted and
decrease the force needed to lift
the object.
• A single movable pulley
multiplies the effort force by 2,
so it has a mechanical
advantage of 2.
• A single movable pulley does
not change the direction of the
effort.
What other machines are like
levers? (cont.)
• The ideal mechanical
advantage of a pulley
or a pulley system is
equal to the number
of sections of rope
supporting the
object.
What are inclined planes?
• Inclined plane is a straight, slanted
surface that can multiply an effort force.
• It takes less force to move an object
upward along an inclined plane than it
does to lift the object straight up.
What are inclined planes? (cont.)
• The mechanical advantage of a ramp is
equal to the output force divided by the
effort force.
What are inclined planes? (cont.)
• The ratio of output force to effort force is
the same as that of effort distance to
output distance.
• For this reason, a ramp’s mechanical
advantage can also be found by dividing
the length of the incline by its height.
What are inclined planes? (cont.)
• Calculate the mechanical advantage of each
inclined plane.
Length = 2.0m, height = 1.0m
IMA = 2 ÷ 1 = 2
Length = 5.0m, height = 3.0m
IMA = 5.0 ÷ 3.0 = 1.7
• The longer and shallower the ramp, the greater
the mechanical advantage.
What are inclined planes? (cont.)
• Screw is a simple machine made of an
inclined plane wrapped around a central bar
that can multiply an effort force.
• Spiral ridges called
threads move into an
object as the head of
the screw turns.
• The space between
the threads is called
the pitch.
What are inclined planes? (cont.)
• The mechanical advantage of a screw and
ramp are calculated in a similar way.
• IMA = effort distance
output distance
• The effort distance is
the distance around the
head and the output
distance is the pitch of
the screw.
What are inclined planes? (cont.)
• Wedge is an inclined plane
that changes the direction
of an applied force.
• A wedge can be a single
inclined plane or two
inclined planes joined back
to back.
• The thinner the wedge, the
greater the mechanical
advantage.
What are compound machines?
• When two or more simple machines are
combined, they form a compound
machine.
• Levers, screws,
wheels and axles
and gears
combine to make
a bicycle, a
compound
machine.
What are compound machines? (cont.)
• The efficiency of a compound machine is
calculated by multiplying the efficiencies
of each simple machine together.
• Each simple machine decreases the
overall efficiency of the compound
machine.
• Lubricants, such as oil, reduce the
amount of energy that is wasted as heat.
What are levers, wheels and
axles, inclined planes, wedges,
screws, and pulleys examples of?
A. complex machines
B. compound machines
C. idea machines
D. simple machines
Which uses less force to raise an
object compared to lifting the
object straight up?
A. fulcrum
B. inclined plane
C. screw
D. wheel
Which is a simple machine
consisting of a grooved wheel with
a rope or cable wrapped around it?
A. gear
B. pulley
C. screw
D. wedge
Visual Summary
Chapter Review
Standardized Test Practice
A machine makes work
easier by changing the
force that is needed or
the direction or the
distance through
which a force is
applied.
Lesson 1: Work and Power
• For work to be done on an object, an
applied force must move the object.
• When work is done on an object, the energy
of the object increases.
• Power is the rate at which work is done.
• Because of friction, the output work done by
a machine is always less than the input work
to the machine.
• Friction between moving parts converts
some of the input work into thermal energy
and decreases the efficiency of the
machine.
Lesson 2: Using Simple Machines
• Simple machines make it easier for people
to do work. They have few, if any, moving
parts.
• Machines make work easier by changing the
size of the force required, the distance over
which the object moves or the direction of the
required force.
• The mechanical advantage of a machine is
the ratio of the output force to the input force.
• A compound machine is made of two or more
simple machines that operate together.
What two things must you know
to calculate work?
A. distance and length
B. force and distance
C. force and power
D. height and weight
What do you divide work by to
calculate power?
A. distance
B. force
C. time
D. weight
Which refers to the force you
apply to a machine to make it
work?
A. energy
B. input force
C. output force
D. power
What does a machine apply to an
object?
A. output power
B. output force
C. mechanical advantage
D. input force
What does a lever rotate around?
A. axle
B. fulcrum
C. screw
D. wedge
What is transferred when work is
done?
A. distance
B. energy
C. force
D. power
The work done on an object
depends on the direction of the
force applied and which of these?
A. power
B. force of the motion
C. distance of the motion
D. direction of the motion
What is the ratio of a machine’s
output force to its input force?
A. efficiency
B. equal output
C. mechanical advantage
D. output work
What is a simple machine made
of a bar that rotates about a fixed
point?
A. fulcrum
B. lever
C. wedge
D. wheel and axle
What term describes an inclined
plane wrapped around a cylinder?
A. wedge
B. screw
C. fulcrum
D. axle