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one way to find the amount of work done is by using the formula FD cosine theta but this number for the amount of work done represents the amount of energy transferred to an object for instance if you solve for the work done and you get positive 200 joules it means that the force gave something 200 joules of energy so if you have a way of determining the amount of energy that something gains or loses then you have an alternate way of finding the work done since the work done on an object is the amount of energy it gains or losses for instance imagine a 50 kilogram skateboarder that starts at rest if a force starts the skateboarder moving at 10 meters per second that force did work on the skateboarder since it gave the skateboarder energy the amount of kinetic energy gained by the skateboarder is 2500 joules that means that the work done by the force on the skateboarder was positive 2500 joules it's positive because the force on the skateboarder gave the skateboarder 2,500 joules if a force gives energy to an object then the force is doing positive work on that object and if a force takes away energy from an object the force is doing negative work on that object now imagine that the skateboarder who's moving with 10 m/s gets stopped because he crashes into a stack of bricks the stack of bricks does negative work on the skateboarder because it takes away energy from the skateboarder to find the work done by the stack of bricks we just need to figure out how much energy it took away from the skateboarder since the skateboarder started with 2,500 joules of kinetic energy and ends with zero joules of kinetic energy it means that the work done by the bricks on the skateboarder was negative twenty-five hundred joules it's negative because the bricks took away energy from the skateboarder let's say we instead lift the bricks which are 500 kilograms upwards a distance of 4 meters to find the work that we've done on the bricks we could use FD cosine theta but we don't have to we could just figure out the amount of energy that we've given to the bricks the bricks gain energy here and there gaining gravitational potential energy which is given by the formula MGH if we solve we get the bricks gained nineteen thousand six hundred joules of gravitational potential energy that means that the work we did on the bricks was positive nineteen thousand six hundred joules it's positive because our force gave the bricks energy this idea doesn't just work with gravitational potential energy and kinetic energy it works for every kind of energy you can always find the work done by a force on an object if you could determine the energy that that force gives or takes away from that object [Music]