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Gravitational Attraction Between Boy and Girl

by Ron Kurtus (15 September 2016)

You can find the gravitational force of attraction between a boy and a girl by applying the Universal Gravitation Equation, provided you know the mass of each person and their separation.

For example, suppose a boy who weighed 165 lb (74.8 kg-force) sat near a 50 kg-force (110 lb) girl, what would be the gravitational attraction between them, assuming the separation from their centers was 0.5 meters (19.7 in)?

Note that weights are in kilograms-force or newtons (N). To use in the gravitation equation, newtons must be converted to kilogram-mass by dividing by 9.8 m/s2.

(See Confusion about Mass and Weight Units for more information.)

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

Universal Gravitation Equation

The solution requires an application of the Universal Gravitation Equation:

F = GMm/R2


Determine mass of each person

To calculate the force of attraction, you must first convert the weight on Earth of each person to his or her mass, using the relationship:

W = mg or m = W/g


Thus, the mass of the boy is:

M = (74.8 kg-force)/(9.8 m/s2) = 7.6 kg-mass

Th mass of the girl is:

m = (50 kg-force)/(9.8 m/s2) = 5.1 kg-mass

Substitute values

Next, substitute the values into the equation:

F = GMm/R2


Resulting force

The result is:

F = (6.674*10−11 N-m2/kg2)(7.6 kg)(5.1 kg)/(0.5 m)2

F = 258.7*10−11/0.25

F = 1035*10−11 N

F = 1.035*10−8 N

Approximate answer

The force of attraction is approximately:

F = 10−8 N

That is a very small gravitational attraction, but it can be measured on a sensitive instrument, such as one using piezoelectric sensors.


You can find the gravitational force between a boy and a girl by applying the Universal Gravitation Equation, provided you know the mass of each person and their separation.

The result is a very small gravitational attraction.

Think clearly and logically

Resources and references

Ron Kurtus' Credentials


Measure small forces with high initial load

Converting units of mass to equivalent forces on Earth - Wikipedia

Weight - Wikipedia

Mass - Wikipedia

Kilogram - Wikipedia

Mass and Weight: the Gravity Force - Engineering Toolbox

Gravitation Resources


Top-rated books on Gravity

Top-rated books on Gravitation

Questions and comments

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