Philip P. answered • 12/24/17

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Gravitational potential at the surface of the planet is:

**U = mah**

- m = mass of an object
- a = gravitational acceleration at the planet's surface
- h = object's height above the surface

The potential per unit mass is:

U/m = a·h

20 joule/kg = a·(20 m)

**1 joule/kg-m = a**

Work to lift a mass m to a height h is:

**W = F·d = ma·h**

- m = 5 kg
- a = 1 m/s
^{2} - h = 4 m

Plug in the numbers and compute the answer. The units are joules.