The first of the fundamental forces we will be discussing is gravity. Gravity, as you
know, is what keeps the planets in orbit, what keeps us and our possessions from flying
off into space, and basically what helps keep the universe together.
This is an appropriate time to discuss the differences between weight and mass. Mass
is the amount of stuff a person or thing is made up of. Weight on the other hand, is the
measurement of the force pulling an object to the ground. For example, a person's mass
would not change if they moved from the Earth to Mars, but their weight would.
The formula to find the force of gravity is:
(g * m1 * m2) / r2
In this equation, g is the gravitational constant, m1 and
m2 are the masses of the two objects, and r is the distance
between the center of one to the center of the other.
The constant "g" was developed by Newton as the universal gravitational constant.
This constant actually stands for 6.673e-11 Nm2/kg2, but for purposes of brevity, I'll
just use "g" in my formulas.
In the constant above, you probably noticed a new unit symbol,
Before we do a sample problem, I'm going to give you some more constants. You don't have
to memorize these, but I highly recommend it. These are just a few, but we will add on
to the list as we need them.
mass of the earth = 5.98e24kg
mass of the moon = 1.99e30kg
mass of the sun = 7.36e22kg
radius of the earth = 6.37e6m
radius of the moon = 1.74e6m
radius of the sun = 6.96e8m
distance, including radii, of the earth to the moon = 3.845e8m
distance, including radii, of the earth to the sun = 1.50e11m
If nothing else, at least memorize the mass and radius of the earth, because in the
introductory problems, those will be seen most frequently. Let's go ahead and to a
sample problem.
What is the force of gravity between the earth and a 75kg man standing on its surface?
FG = (g * (5.98e24kg) * (75kg)) / (6.37e6m)2
FG = 737.572N
There might be some problems where a man is standing on a 500 m tall mountain. You
could add that to the radius to get 6.3705e6m, but it really isn't necessary. In general,
when working a problem involving gravity and something standing on a planet, you don't
need to add the extra distance unless it gets into the thousands.
The next lesson discusses the electric force.
