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Explanation of the Overview 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
Overview of the Force of Gravity
by Ron Kurtus (revised 21 July 2010)
Gravity is the gravitational force on or near the surface of the Earth. While gravitation is the force that attracts bodies of matter toward each other—often at great distances—gravity is the force that pulls nearby 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
Note: For verification 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 designated by g. Its value is:
g = 9.807 meters per second-squared (m/s2) in the metric or SI system of measurement
g = 32.2 feet per second-squared (ft/s2) in the English system of measurement
Note: Since most textbooks use g = 9.8 m/s2 and 32 ft/s2, we will also use the rounded-off version in these lessons.
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 correct description of g.
Weight and mass
The weight of an object of a given mass is the force of gravity on that object:
w = mg
where
- w is the weight in newtons (N) or pound-force (lb)
- m is the mass in kilograms or pound-mass (lb-mass)
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. You must be aware that weight of 1 kg of mass is w = 9.8 newtons.
Also, a pound is supposed to be a force, but is often called a mass. The mass of 1 pound-force is 1/32 pound-mass.
Weighing an object
You can find the weight of an object on a calibrated scale—usually with a spring resisting the force of the weight.
The mass of an object can be measured with a balance scale, comparing with an object of a given mass.
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 Equivalence Principle of Gravity 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
Using a spring scale, if you weigh 60 kg (132 pounds) on the Earth, you would weight only 10 kg (22 lbs) on the Moon. However, using a balance scale on both Earth and the Moon, your mass would be the same.
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
Force of Gravity - Universe Today Magazine
Earth's gravity - Wikipedia
How does gravity work? - How Stuff Works
Standard gravity - Average value, as compared to variation due to position on Earth - Wikipedia
International Gravity Formula - Variation of gravity with distance from equator - Geophysics dept. University of Oklahoma
I feel 'lighter' when up a mountain but am I? - National Physics Laboratory FAQ
Gravity and Gravitation 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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Overview of the Force of Gravity