The strength of this force field is approximately 10 N/kg on Earth. This means that gravity places a 10 N force on every kilogram of matter on Earth. It is also known as ‘acceleration due to gravity’.
Calculating Gravity
Using our knowledge of force fields, we can calculate the weight of an object by multiplying the mass of that object (kg) by the gravity in that location (g).
Weight (N) = mass (kg) x gravity (N/kg)
For example, if your mass was 60 kg, then your weight on Earth would be 600 N:
W = 60 x 10 = 600 N
On the Moon, the only thing that would change is your weight (the size of the force pulling you towards the Moon).
Your mass will be the same no matter where you are in the universe.
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Spending a long period of time in outer space can have strange effects on the body. Away from the large gravitational (‘g’) force on Earth, the spine begins to straighten out. Your internal organs shift upwards inside your torso, reducing your waist measurements by several inches. The forces you apply with your leg muscles shifts your blood upwards, creating a condition that NASA calls ‘chicken legs’. All these effects wear off once you return to Earth.
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Fig. 21.3.2 Earth’s gravitational force field
School in space
Use the table below to calculate the weight of your schoolbag on the following planets:
