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Physics
Force Test
Mr. Lew
In “The Elegant Universe”, it was said that one of the forefathers of Physics, Sir Isaac Newton,
developed a law of gravity that he called the “Universal Law of Gravitation.” This law described the
force of gravity that acted “in the heavens and on earth”; from the force acting on an apple falling from
a tree to the forces acting between planets in the universe. His “Universal Law of Gravitation”
equation is stated below:
2
" Nm %
$ 2 '[kg ][kg ]
Gm 1m2 # kg &
F=
=
= [N ]
2
2
r
m
where:
F = the force acting between the two objects m1 and m2 [N]
G = Newton’s Universal Gravitational constant = 6.67 x 10-11 [Nm2/kg2]
(note how small this number is…it gives you an idea why the force of gravity is so weak!!!)
m1 = the mass of object 1 [kg]
m2 = the mass of object 2 [kg]
r = the distance between the centers of the objects [m]
!
For reference, complete the following calculations:
I. What is the weight in Newton of a person of mass = 90.72 [kg] on the earth using W = mg?
II. If the coefficient of friction between a 1kg hockey puck and ice is 0.2, what force is needed to
move the puck across the ice at a constant velocity on earth (g = 9.8 [m/s2]?
Now, use “Newton’s Universal Law of Gravitation” to find the force of gravity for the following cases:
Use the following values for your calculations (source: Wikipedia)
Radius of earth = 6371 [km]
Mass of earth = 5.97 x 1024 [kg]
a.
b.
c.
d.
e.
f.
g.
The force of gravity (weight) on a 200 lb person on the surface of the earth (90.72 [kg])
The force of gravity on the same person on an airplane at a cruising altitude of 35,000 [ft]
(10,668 [m])
The force of gravity on the same person at a distance of 330 [km] above the surface of the earth
(this distance is the cruising altitude of the International Space Station)
The force of gravity on the same person at a distance of 6371 [km] above the surface of the earth
(this distance is equal to one earth radius)
The force of gravity on the same person at a distance of TWO times 6371 [km] above the surface
of the earth (two earth radii)
The force of gravity on the same person at a distance of THIRTY times 6371 [km] above the
surface of the earth (the distance from the surface of the earth to our moon)
If the coefficient of friction between a 1kg hockey puck and ice is 0.2, what force is needed to
move the puck across the ice at a constant velocity AT AN ALITITUDE OF TWO times 6371
[km] above the earth?