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Isaac Newton discovered that gravity is
a force that acts at a distance and attracts bodies of
matter toward each other. The force of gravity from the
Earth on an object is the acceleration of gravity times
the mass of the object. That equals the object's weight.
The law of gravity determines how fast objects will
fall.
Questions you may have include:
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How did Newton discover gravity?
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What is the difference between mass
and weight?
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What are the laws of gravity?
Discovery of gravity
Building on the work of Galileo and
Kepler, Isaac Newton formulated the theory of
gravitation in the 1680s. The story goes that Newton was
sitting under a tree when an apple fell and hit him on
the head. This made him curious and inspired him to
determine that there was a force called gravity that
pulled the apple down from the tree.
It wasn't until the early 1900s when
Albert Einstein gave another interpretation of the force
gravity in his General Theory of Relativity. Recently
there have been new theories that the force of gravity
is caused by particles or by waves.
Gravity determines weight
Gravity is a force that attracts bodies
of matter toward each other. Therefore, it is a force
that is everywhere there is matter.
We are attracted toward the Earth, and
the Earth is attracted to us. The Moon has gravity, and
it affects the water in the Earth's oceans, causing the
tides. In the study of atomic particles, there is even a
weak force of gravity between all particles.
The amount of matter in an object is
called its mass. The force of gravity is dependent on
the amount of mass a body has. That means that the
gravity on the Earth is greater than the gravity on the
Moon, since the Earth has much more matter or mass than
the Moon.
The force of gravity on an object caused
by the mass of the Earth equals the mass of the object
(m) times the acceleration caused by gravity (g). The
equation is:
F = m*g
This acceleration due to the force of
gravity on Earth equals 9.8 m/s2 in the metric system
and 32 ft/s2 in the English system.
Note: g is often called the acceleration
of gravity. That can be misleading, since gravity does
not accelerate. The acceleration due to the force of
gravity is a more accurate definition for g.
The weight of an object is the
measurement of the force of gravity on that object. You
weigh something on a scale, according to the force that
the Earth pulls it down. Thus the weight is actually the
force of gravity on that object:
Weight =
m*g
The acceleration of gravity on the Moon
(gm) is 1/6 of the value on the Earth (g). Thus, if you
put the same object on the Moon and weighed it, its
weight would be 1/6 the weight on Earth. In other words,
a 180-pound man would only weigh 30 pounds on the Moon.
The difference between mass and weight
sometimes causes confusion, especially when dealing with
units of measurement. In the metric system, the unit of
mass is the gram. To get the weight of an object in the
metric system, you multiply the mass in kilograms by the
acceleration of gravity (9.8 m/s2), resulting in the
units of Newton.
On the other hand, the unit of weight in
the English system of measurement is pounds. You divide
the pounds by 32 ft/s2 to get the mass in slugs. I don't
know of anyone who uses slugs, thus a reason for using
metric units.
Laws of gravity
The common laws of gravity are
approximations that concern objects close to Earth. The
standard gravitational law concerns objects at greater
distances. For objects close to Earth (or any other
large body), the laws concern how bodies freely fall.
They state that freely falling objects accelerate and
that the rate of acceleration is independent of their
mass.
The universal or standard gravitational
law states that the force of gravity between two objects
is proportional to the product of the masses of the
objects and inversely proportional to the square of the
distance between them.
One thing this means is that as objects
get further apart, the effect of gravity drops
dramatically. For example, the force of gravity from the
Earth on an object 3 km away is only 1/9 the force on an
object 1 km away.
In most general applications, we study
objects close to Earth, so this effect is negligible.
If you drop an object relatively near
the Earth, it will speed up according to the
acceleration due to gravity (g).
Object speeds up
When you let go of the object, its
velocity is zero.
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Since g = 32 ft/s2 = 9.8
m/s2, the velocity will be 32 ft/s (9.8 m/s) after
one second.
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Because the object is accelerating,
the velocity after 2 seconds will be 2 x 32 ft/s =
64 ft/s (19.6 m/s).
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After 10 seconds, the velocity will
be 10 x 32 ft/s = 320 ft/s or 98 m/s.
You can see how the velocity of the
object gets faster and faster.
Although a falling object will continue
to accelerate until it is made to stop--like when it
hits the ground--air resistance will slow down that
acceleration. Air resistance is approximately
proportional to the square of the velocity, so as the
object falls faster, the air resistance increases until
it equals the force of gravity. The object has reached
what is called its terminal velocity.
There have been many calculations on
what the terminal velocity would be for a penny dropped
from a high building or airplane. Because a penny would
probably tumble, the calculations can become highly
complex. One estimate is that a penny dropped from a
high building will accelerate until it reaches around
230 mph.
Some dispute such a high terminal
velocity. A better example of terminal velocity is that
of dropping a baseball. Once a falling baseball reaches
94 miles per hour or 42 meters/second, it would remain
at the velocity and no longer accelerate.
One surprising characteristic of the
force of gravity is that the acceleration it causes in
falling bodies it independent of the mass of the object.
In other words, a 5-pound weight would
fall at the same rate as a 10-pound weight. If dropped
from the same height, they would take the same time to
hit the ground. Of course, in dropping a lightweight
object, air resistance often will slow the object down
more than a heavier object.
Not only does is the acceleration of
gravity independent of the mass of an object, but it is
also independent of the velocity of the object parallel
to the ground.
In other words, it an object is
traveling at some velocity parallel to the ground, it
will fall at the same rate as a stationary object. Thus
a bullet shot from a gun will hit the ground at the same
time as one that was simply dropped from the same
height.
All objects attract each other through
the force of gravity. The acceleration caused by gravity
is independent of the mass or weight of an object, as
well as any motion perpendicular to the ground. |
