Work, Machines &
Mechanical Advantage
Machines
• A machine is a device that changes a
force.
• Machines make work easier to do.
• They change the size of a force
needed, the direction of a force, or
the distance over which a force acts.
How do machines work?
1. Work is inputted into a machine by
you.
2. The machine converts it and the
machine outputs work
• Machines CANNOT change the
amount of work done!
Machines Make Work Easier
• They do this in one of three ways
1. Increase force
2. Increase distance
3. Change direction
Increasing Force
A small force exerted over a
large distance becomes a
large force exerted over a
short distance.
Increasing Force
•Output force is bigger than input force
•Output distance is smaller than input distance
Increasing Distance
A machine that decreases the
distance through which you
exert a force increases the
amount of force required.
Increasing Distance
•Input distance is smaller than output distance
•Input force is bigger than output force
Changing Direction
Machines can change the
amount of force and the
distance the force acts through
as well as the direction of the
force.
Changing Direction
• If only the direction changes, input force will be
the same as the output force.
Work Input and Work Output
Because of friction, the work done
by a machine is always less than
the work done on the machine.
Work Input and Work Output
• Some of the energy used on the machine is
converted to non-useful forms (heat, light,
sound)
• Most of the time its converted to heat created
by friction
• http://phet.colorado.edu/sims/friction/friction_en.html
• Some machines are better at ‘saving’ energy
than others.
• This relationship is called efficiency
Work Input to a Machine
• The force exerted on a machine is the
input force.
• The distance the input force acts through
is the input distance.
• The work input equals the input force
multiplied by the input distance.
Input vs. Output
Work Output of a Machine
• The force exerted by a 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.
Actual Mechanical Advantage
(AMA)
• equals the ratio of the output force to the input
force.
• The mechanical advantage of a ramp with a rough
surface is less than that of a similar smooth ramp
because a greater force is needed to overcome
friction.
**No units!**
Ideal Mechanical Advantage (IMA)
• machine is the mechanical advantage in the
absence of friction.
• Because friction reduces mechanical
advantage, engineers often design machines
that use low-friction materials and lubricants.
**No units!**
Mechanical Advantage Lever
Input
Output
Resistance Arm
Mechanical Advantage for Inclined Plane
(Ramp)