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Explanation of the Force of Gravity - Succeed in Understanding Physics. Also refer to physical science, Isaac Newton, Universal Gravitation, acceleration, mass, weight, Earth, Moon, planets, Sun, Ron Kurtus, School for Champions. Copyright © Restrictions
Force of Gravity
by Ron Kurtus (revised 23 August 2009)
Gravity is a term used for gravitation for objects relatively close to Earth. Gravitation is the force that attracts bodies of matter toward each other, often at great distances. Gravity is the force that pulls objects toward the Earth.
The equation for the force of gravity is F = mg. The major result of this force is that all objects fall at the same rate, regardless of their mass. Gravity on the Moon and on other planets have different values of the acceleration due to gravity, but the effects of the force are similar.
Questions you may have include:
- What is the gravity equation?
- What is the most outstanding characteristic of gravity?
- What is gravity elsewhere?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
Useful tools: Metric-English Conversion | Scientific Calculator.
Gravity equation
According to Newton's Law of Universal Gravitation, gravitation is the force that attracts objects toward each other. The equation for that force is:
F = GMm/R2
For objects relatively close to the Earth, the equation reduces to:
F = mg
where
- F is the force pulling objects toward the Earth
- m is the mass of the object
- g is the acceleration due to gravity; this number is a constant for all masses of matter
- mg is the product of m times g
(To verify that F = GMm/R2 = mg for objects close to Earth, see Gravity Equation Comes From Universal Gravitation Equation.)
The gravitation for objects close to the Earth is called gravity. Once an object gets into outer space, the gravitation equation takes over.
Acceleration due to gravity
The acceleration due to the force of gravity on Earth is g:
g = 9.8 m/s2 in the metric or SI system of measurement
g = 32 ft/s2 in the English system of measurement
In the equation F = mg, you must use the same measurement system for mass, m, as you do for g.
Note: g is often incorrectly called the acceleration of gravity. That is misleading, since gravity does not accelerate. The expression should be the acceleration due to gravity, which is a more accurate definition for g.
Weight
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:
w = mg
where w is the weight in newtons (N) or pounds (lb).
Note: There is often confusion concerning the designation of weight and mass. Although a kilogram is supposed to be a unit of mass, it is often used to designate weight.
The weight of 1 kg of mass is w = 9.8 newtons.
Objects fall at the same rate
The most outstanding characteristic of gravity is the fact that all objects fall at the same rate—assuming the effect of air resistance is negligible. This is because the acceleration due to gravity, g, is a constant for all objects, no matter what their mass.
This seems counterintuitive, since you would expect a heavy object to fall faster than an object that weighed less. But it is a fact. Try dropping two objects at the same time, from the same height, making sure they are heavy enough not to be affected by air resistance. You will see they hit the ground at the same time.
(See Gravity Equations for Falling Objects for more information.)
Gravity elsewhere
When you talk about gravity, you mean gravitation near the Earth. However, the same gravity equation holds for objects near the Moon or other planets, except that the value of g is different. In those cases, you typically tell where the gravity is, such as "gravity on the Moon" or "gravity on Mars."
Gravity on the Moon
The force of gravity on the Moon is approximately 1/6 of that on the Earth for a given mass. Thus:
Fm = mgm
where
- Fm is the force or weight on the Moon
- m is the mass of an object
- gm is the acceleration due to gravity on the Moon
The value for gm is 1.6 m/s2 or 5.3 ft/s2. That is approximately 1/6 of the value for g on Earth. Thus, an object on the Moon would weigh about 1/6 of its weight on Earth.
Weight on the Moon
If you weight 60 kg (132 pounds) on the Earth, you would weight only 10 kg (22 lbs) on the Moon.
Dropped objects
If you dropped two objects of different weights on the Moon, they would fall to the ground at the same rate. You wouldn't have to worry about the effect of air resistance, since there is no air on the Moon.
Since gm = g/6, the objects would fall at a slower rate.
(See Gravity Equations for Falling Objects and then apply gm to get the different values.)
Summary
Gravity is the force that pulls objects toward the Earth. It is a special case of gravitation. The equation for the force due to gravity is F = mg, resulting in the fact that all objects fall at the same rate, regardless of their mass. Gravity on the Moon and gravity on other planets have different values of the acceleration due to gravity, but the effects of the force are similar.
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Resources
The following resources provide information on this subject:
Websites
Gravitation and Gravity Resources
Books
Top-rated
books on Simple Gravity Science
Top-rated
books on Advanced Gravity Physics
Mini-quiz to check your understanding
If you got all three correct, you are on your way to becoming a Champion in Physics. If you had problems, you had better look over the material again.
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