Astudent lifts a 50 pound (lb) ball 4 feet (ft) in 5 seconds (s). how many joules of work has the student completed?

           Work = (weight) x (distance)

  Work = (50 lb) x (1 kg / 2.20462 lb) x (9.81 newton/kg)

                           x (4 feet) x (1 meter / 3.28084 feet)

           = (50 x 9.81 x 4) / (2.20462 x 3.28084)  newton-meter

           =        271.3 joules .

We don't need to know how long the lift took, unless we
want to know how much power he was able to deliver.

                   Power = (work) / (time)    

                               = (271.3 joule) / (5 sec)  =  54.3 watts .


The easy way:

         Work = (weight) x (distance)

                   = (50 pounds) x (4 feet)  =  200 foot-pounds

Look up (online) how many joules there are in 1 foot-pound.

There are  1.356 joules in 1 foot-pound.

So  200 foot-pounds = (200 x 1.356) = 271.2 joules.

That's the easy way.

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