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One way to find the
amount of work done

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is by using the formula
Fd cosine theta.

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But this number for
the amount of work done

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represents the amount of energy
transferred to an object.

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For instance, if you
solve for the work done

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and you get positive
200 joules, it

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means that the force gave
something 200 joules of energy.

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So if you have a
way of determining

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the amount of energy that
something gains or loses,

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then you have an alternate
way of finding the work done,

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since the work done on an
object is the amount of energy

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it gains or loses.

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For instance, imagine a
50-kilogram skateboarder

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that starts at rest.

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If a force starts the
skateboarder moving

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at 10 meters per
second, that force

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did work on the skateboarder
since it gave the skateboarder

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energy.

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The amount of kinetic energy
gained by the skateboarder

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is 2,500 joules.

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That means that the work done
by the force on the skateboarder

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was positive 2,500 joules.

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It's positive because the
force on the skateboarder

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gave the skateboarder
2,500 joules.

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If a force gives
energy to an object,

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then the force is doing
positive work on that object.

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And if a force takes away
energy from an object,

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the force is doing negative
work on that object.

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Now imagine that the
skateboarder, who's

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moving with 10
meters per second,

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gets stopped because he
crashes into a stack of bricks.

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The stack of bricks
does negative work

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on the skateboarder because
it takes away energy

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from the skateboarder.

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To find the work done
by the stack of bricks,

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we just need to figure out
how much energy it took away

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from the skateboarder.

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Since the skateboarder
started with 2,500 joules

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of kinetic energy and ends with
zero joules of kinetic energy,

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it means that the work done by
the bricks on the skateboarder

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was negative 2,500 joules.

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It's negative because the
bricks took away energy

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from the skateboarder.

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Let's say we instead
lift the bricks, which

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are 500 kilograms, upwards
a distance of four meters.

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To find the work that
we've done on the bricks,

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we could use Fd cosine theta.

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But we don't have to.

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We could just figure
out the amount of energy

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that we've given to the bricks.

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The bricks gain energy here.

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And they're gaining
gravitational potential energy,

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which is given by
the formula mgh.

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If we solve, we
get that the bricks

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gained 19,600 joules of
gravitational potential energy.

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That means that the work
we did on the bricks

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was positive 19,600 joules.

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It's positive because our
force gave the bricks energy.

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This idea doesn't just work with
gravitational potential energy

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and kinetic energy.

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It works for every
kind of energy.

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You can always find the work
done by a force on an object

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if you could determine the
energy that that force gives

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or takes away from that object.

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