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WORK!
WORK is done on an object when a force is
exerted on the object that causes the object to
move some distance.
There is no work done if
the object doesn’t move.
Work is only done if the force
is exerted in the same
direction as the motion.
Work
Force, Motion, and Work
The amount of work done on an
object depends on the direction
of the force and the object’s
motion.
Work
Amount of Work
The amount of work done on an object can be determined by multiplying force
times distance. When April lifts a trumpet or a tuba up the stairs, she does work.
The amount of work depends on the
amount of force exerted and the distance
the object moves.
To calculate the amount of work done on
an object:
Work = Force X Distance
Work(J) = Force(N) X Distance(m)
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Force is measured in Newtons (N)
Distance is measured in meters (m)
Work is measured in Newton-meters (N . m)
One Newton-meter is a Joule (J)
One Joule is the amount of work you do when you
exert a force of 1 N to move an object a distance
of 1 m.
Work = Force X Distance
Calculate Work
Work = Force X Distance
While mowing your yard, you push the lawnmower with a force of
50N for a distance of 20 meters. How much work did you do on
the lawnmower?
Calculate:
Work = Force X Distance
Work = 50 N x 20 m
Work = 1000 Nm
Work = 1000 J
Understanding Simple Machines
A simple machine is a device that makes work easier by changing
the amount of force exerted, the distance the force is exerted, or
the direction the force is exerted.
A machine does not change the amount of work that is done, it
only makes the work easier.
Understanding Simple Machines
Using Machines
In the picture below, using a ramp makes it easier for April to move the
speaker onto the stage. When you use a machine to do work, you exert a
force over some distance. The force you exert is called the input force. The
input force is exerted over a certain distance, called the input distance. The
machine does work by exerting a force over another distance called the
output distance. The force the machine exerts is called the output force. The
output force is the force you would have to use without the machine.
Without a machine
With a machine
Understanding Simple Machines
Most machines increase the force that is exerted on them. A machine’s
mechanical advantage is the number of times a machine increases a force
exerted on it. The ratio of output force to input force is the mechanical
advantage of a machine.
Mechanical Advantage = Output force
Input force
Drums are tuned
by tightening
and loosening
bolts. Drum keys
make the bolts
easier to turn.
Understanding Simple Machines
There are 6 types of simple machines:
• Inclined
plane
• Wedge
• Screw
• Lever
• Pulley
• Wheel and Axle
Inclined Plane
Inclined Plane:
An inclined plane is a flat, slanted surface. An inclined plane allows you to
exert a smaller input force over a longer distance. A ramp is an inclined
plane. The ramp exerts a larger output force over a shorter distance.
Inclined Plane
The man is lifting the boxes into a
truck bed that is 1 meter high. The
ramp is 3 m long. It requires a force
of 30 N for the man to push the stack
of boxes up the ramp. The boxes
weigh 90 N.
• Compare the amount of work that
would be done with and without
the ramp.
• What is the mechanical advantage
of the ramp?
Wedge
Wedge
A wedge is two inclined planes placed back-to-back. With a wedge, the
inclined plane itself moves. A wedge allows you to exert a smaller input force
over a longer input distance. It also changes the direction of the force.
Screw
Screw
A screw is an inclined
plane wrapped around a
cylinder. The threads of
a screw act like an
inclined plane to increase
the distance over which
input force is exerted.
Lever
A lever is a rigid bar that is free to pivot, or rotate, about a fixed point.
The fixed point that a lever rotates around is called the fulcrum. A lever
has three parts: input force, output force, and fulcrum. Depending on
where these three parts are, levers can increase force, increase distance,
or simply change the direction of a force.
Lever
Three
Classes of
Levers
The three
classes of
levers differ
in the
positions of
the fulcrum,
input force,
and output
force.
Lever
Lever
Levers in the Body
Levers can be found throughout your body. In the last two panels of the
diagram, where would the output force be? What is the class of lever
for each part of the body?
Pulley
A pulley is a simple machine made of a grooved wheel with a
rope or cable wrapped around it.
Pulley
Pulley
Types of Pulleys
Pulley systems are classified by the number and position
of the wheels they contain.
Wheel and Axle
A simple machine made of two connected objects that rotate about a
common axis is called a wheel and axle. The object with the larger
diameter is called the wheel and the object with the smaller diameter is
called the axle. Because the wheel is larger than the axle, a small input
force on the wheel is exerted over a long distance. In return, the axle
rotates and exerts a large output force over a short distance.
Wheel and Axle
Compound Machines
A compound machine
is a machine that
combines two or more
simple machines.
Piano movers use
compound machines to
get their job done.
Rube Goldberg Machine
Rube Goldberg was
a famous
cartoonist who
drew cartoons
depicting machines
that made simple
tasks
extraordinarily
complicated.