How much work is done in lifting a 20kg suitcase off the ground to a height of 0.75m?

Answer 1

Look at it this way for just a second: The gravitational potential energy of the

suitcase when it's lifted to a higher place is (weight) x (change in height).

The weight of the suitcase is (mass ) x (acceleration of gravity), so the change

in potential energy is

(mass) x (gravity) x (change in height) = (20) x (9.8) x (0.75) = 147 joules.

The suitcase has 147 more joules of potential energy after it rises 0.75 m,

so that's the minimum amount of work it takes to lift it that high.

Now, the reason we wanted to do it this way is that human muscular effort

is pretty inefficient. If you lift that 44-pound suitcase by hand and put it on

that ledge that's about 21/2 feet off the ground, you'll wind up doing quite

a bit more than 147 joules of work in the process. But the 147 is the portion

of all your effort that goes into actually raising the suitcase. All the rest of the

energy you use to do the job goes to fatigue the muscles in your arm, raise

your core temperature and make you perspire, and increase your heart rate

and respiration rate. None of that moves the suitcase at all, but you can't

produce the 147 joules without burning that additional energy. It's how the

body operates.