Physics
Work, Energy, and Simple Machines
Work and Energy
• Work is defined as the product of the force
exerted on an object and the distance the object
moves in the direction of the force.
• W = Fd
• Work is a scalar quantity; it has no direction.
• The SI unit of work is the joule (J). 1 J = 1N.m
• The joule is named for James Prescott Joule, a
19th century English physicist and brewer.
Work Done
• *work is done only if the object moves in the
direction of the force.
• Example: A student lifts a box of books that
weighs 185 N. The box is lifted 0.800 m. How
much work does the student do?
Solution
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Known:
Unknown:
F = mg = 185 N
Work = ?
d = 0.800 m
W = Fd
W = 185 N x 0.800 m
• Work = 148 J
Work and Direction of Force
• Work is done only if a force is exerted in the direction of motion.
• For work done at an angle, only the movement that is in the
direction of the force is considered work.
• The work you do when you exert a force at an angle to a motion
is equal to the component of the force in the direction of the
motion times the distance moved.
• For work done at an angle:
• W = F(cos q)d or Fd(cos q)
• Negative work indicates that work is being done on an object;
positive work indicates that you are doing work.
Energy
• The effect of doing work is that you give an object
energy.
• Energy is defined as the ability to do work or the ability
to produce a change in itself or its surroundings.
• By doing work on an object, you have transferred
energy from your body to the object.
• Therefore, Work is the transfer of energy by
mechanical means.
Power
• Power is the rate of doing work or the rate at
which energy is transferred.
• Power is measured in watts (W). One watt is
one Joule of energy transferred in one second.
• Because a watt is so small, power is often
measured in kilowatts (kW).
• P = W/t
Power = work divided by time
Example Problem
• An electric motor lifts an elevator that weighs
1.20 x 104 N a distance of 9.00 m in 15.0 s.
• A. What is the power of the motor in watts?
• B. What is power in kilowatts?
Solution
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Given:
Unknown:
F = 1.20 x 104 N
power = ?
d = 9.00 m
t = 15.0 s
P = (1.20 x 104 N)9.00 m / 15.0 s
P = 7.20 x 103 w
P = 7.20 x 103 w (1 kw/1000w) = 7.20 kW
Machines
• A machine eases the load either by changing the
magnitude or the direction of a force, but does not change
the amount of work done.
• The force you exert on a machine is called the effort force,
Fe.
• The force exerted by the machine is called the resistance
force, Fr.
• The ratio of resistance force to effort force is called the
mechanical advantage of the machine:
• MA = Fr/Fe
Simple Machines
• Simple machines include lever, pulley,
wheel-and-axle, inclined plane,
wedge, and screw.
Compound Machines
• A compound machine consists of 2 or more simple
machines linked so that the resistance force of 1
machine becomes the effort force of the second.
• For example, in the bicycle, the pedal and sprocket act
like a wheel-and-axle. The effort force is the force you
exert on the pedal; the resistance is the force the
sprocket exerts on the chain.
Examples of Compound Machines