Answer:
“Newton’s Law of Gravity” gives the gravity force between two objects with masses m
and M separated by distance r:
F G
m M
r
2
G is called the Gravitational Constant, and has the value
6.671011N m
2
kg2
(N is for
Newton, the physicists’ unit of force) or
1.51011 lbm
2
kg2
.
Let’s go back to the example given in the text: two 150 lb people separated by 1 meter.
The mass of each person is
150lb
2.2
lb
kg
68kg
. Putting these into the formula gives
F 1.5101 1 lb m
2
kg2
(68kg)
2
(1m)
2
107
lb.
Newton’s Law of Gravity actually gives the force only between two small objects. If one
of the objects is a sphere (such as the Earth) then it turns out that you can still use the
formula, but you must use the distance to the center of the sphere as the value for r. As
an example, let’s put in numbers for a 1 kg object sitting on the surface of the Earth.
Then the force of attraction is given by the gravity equation with m = 1 kg, M = the mass
of the Earth =
24 610
kg, and r = radius of the Earth (that’s the distance to the center of
the sphere). This distance is r = 6371 km ≈
6
610
meters. Without plugging in the
numbers, can you guess what the answer will turn out to be? Guess, and then check this
footnote2
to see if you guessed correctly.
Suppose you weigh 150 lbs on the Earth. Then your mass is
150lb
2.2
lb
kg
68kg
. What will
you weigh on the Moon? We can calculate that by using Newton’s Law of Gravity, and
putting in the M = the mass of the Moon =
22 7.310
kg, r = the radius of the moon =
6
1.710
meters. The answer is F = 25 lb. That means you will weigh 25 lb. on the
surface of the Moon.
Explanation:
hope this helps :)
