TEKS 7.7A: Contrast situations where work is done with different amounts of force to situations where
no work is done such as moving a box with a ramp and without a ramp, or standing still.
When is work done?
• You do work any time you
exert a force on an object
that causes the object to
move in the same
direction as the force.
• In addition, the force you
exert must be in the same
direction as the object’s
motion.
Work is done on the
bin because force and
motion are in the
same direction.
No work is done on the
bin because force and
motion are in different
directions.
(contd.)
TEKS 7.7A: Contrast situations where work is done with different amounts of force to situations where
no work is done such as moving a box with a ramp and without a ramp, or standing still.
• The amount of work done depends on both the amount
of force you exert and the distance the object moves.
The amount of work done on an object can be
determined using this formula:
Work = Force × Distance
• The SI unit of work is the newton-meter (N•m).
1. Calculate How much work do rock climbers do on their backpacks if their
packs weigh 90 N and they climb to a height of 30 m?
TEKS 7.7A: Contrast situations where work is done with different amounts of force to situations where
no work is done such as moving a box with a ramp and without a ramp, or standing still.
How do machines affect work?
• A machine is a device that allows you to do work in an easier
way. Machines do not reduce the amount of work you do.
Instead, they just change the way you do work.
• A machine makes work easier by changing at least one of
three factors:
• the amount of force you exert,
• the distance over which you exert your force, or
• the direction in which you exert your force.
2. Apply It is your turn to help raise the school’s flag. As you pull down on the
rope, the flag raises to the top of the pole. Which factor did the pulley at the top
of the pole change in order to make work easier?
(contd.)
TEKS 7.7A: Contrast situations where work is done with different amounts of force to situations where
no work is done such as moving a box with a ramp and without a ramp, or standing still.
• When you do work, the force you exert is called the input force.
You exert your input force over the input distance.
• A machine exerts the output force over the output distance.
Output force times output distance equals output work. Output
work is never greater than input work.
Input force = output force
Input distance = output distance
Input force = force used to pull the box
Input distance = length of the ramp
Output force = weight of the box
Output distance = height of the ramp