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ORELA General Science: Practice & Study Guide63 chapters | 626 lessons | 49 flashcard sets

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Lesson Transcript

Instructor:
*David Wood*

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

You probably have an idea of what gravity is, but did you know that you, right now, are actually pulling on every other object in the universe? Find out more about the gravitational force and learn an equation to calculate its pull on other objects.

The universe has a lot of forces, a lot of pushes and pulls. We're always pushing or pulling something, even if only the ground. But it turns out that in physics, there are really only four fundamental forces from which everything else is derived: the strong force, the weak force, the electromagnetic force, and the gravitational force.

The **gravitational force** is a force that attracts any two objects with mass. We call the gravitational force *attractive* because it always tries to pull masses together, it never pushes them apart. In fact, every object, including you, is pulling on every other object in the entire universe! This is called Newton's **Universal Law of Gravitation**. Admittedly, you don't have a very large mass and so, you're not pulling on those other objects much. And objects that are really far apart from each other don't pull on each other noticeably either. But the force is there and we can calculate it.

This equation describes the force between any two objects in the universe:

In the equation:

*F*is the force of gravity (measured in Newtons, N)*G*is the gravitational constant of the universe and is always the same number*M*is the mass of one object (measured in kilograms, kg)*m*is the mass of the other object (measured in kilograms, kg)*r*is the distance those objects are apart (measured in meters, m)

So if you know how massive two objects are and how far they are apart, you can figure out the force between them.

Notice that the distance (*r*) on the bottom of the equation is squared. This makes it an inverse square law. Because of this, if you double the distance between two objects, you reduce the gravitational force between them to a quarter of what it was. Or if you triple the distance between them, you reduce the force to a ninth of what it was. Or if we go the other way, halving the distance between two objects multiplies the force by a factor of four. This can be used to make rough comparisons between situations.

Let's go through an example of how to use the equation. Let's try to calculate the force between an adorable baby and the planet Mars!

Let's say the baby weighs 4.5 kg and the planet Mars has a mass of 6.4x10^23. The distance between Earth and Mars changes, but it's an average of 2.25x10^11 meters. And as usual, G is 6.67 x 10^-11. All we have to do is plug those numbers in and solve for *F*:

And that gives us 3.8 x 10^-9 Newtons. This is a tiny force, partly because a baby has such a small mass and partly because they're so far apart.

The **gravitational force** is a force that attracts any objects with mass. You, right now, are pulling on every other object in the entire universe! This is called Newton's **Universal Law of Gravitation**. We can use the equation shared in this lesson to calculate the force between any two objects, as long as we know their masses and how far apart they are. We can also compare two situations by realizing that gravity is an inverse-square law, meaning that if you double the distance between two objects, you reduce the force to a quarter. And if you triple the distance between two objects, you reduce the force to a ninth.

When you are finished, you should be able to:

- State the Universal Law of Gravitation
- Write the equation to calculate gravitational force and state the meaning of its variables
- Explain how gravity is an inverse-square law

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ORELA General Science: Practice & Study Guide63 chapters | 626 lessons | 49 flashcard sets

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- Free-Body Diagrams 4:34
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- Newton's Laws and Weight, Mass & Gravity 8:14
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