When trying to lose weight, everyone thinks about going to the gym or going on a diet. But nobody thinks about going to a different planet with a lower gravitational field strength.

So what exactly is weight? How is it different from mass? And why do things fall?

In this lesson we'll learn about gravitational force or gravity. We're most familiar with gravity as the force that causes objects to fall. But gravity also causes tides to go in and out, keeps the moon orbiting around the earth, and keeps the earth (and all of the planets) orbiting around the sun.

We'll learn about weight (which is just the gravitational force on object) as well as apparent weight (which is just the normal force supporting you from below).

So where does this gravitational force come from?

If that's true, why don't we experience that with most objects? Why doesn't a pen start flying towards a cup? Even though many objects are close together, their masses are so small that the gravitational force is too weak to overcome the other forces that prevent them from moving (usually friction). It takes an object with an enormous amount of mass, like the earth, for you to experience the force of gravity in everyday life.

Albert Einstein has given us a brand new theory of gravity to work with: general relativity. But this course is only going to be using Newton's Law of Universal Gravitation (aka Newtonian gravity). Just like ignoring air resistance still gives us really accurate predictions for projectile motion, Newton's equation gives us extremely accurate predictions for most things affected by gravity.

Multiple-Choice Questions

Answers

• Khan Academy - Introduction to gravity
• Professor Dave - Newton's Law of Universal Gravitation
• Khan Academy - Mass and weight clarification

• Organic Chemistry Tutor - Gravity, Universal Gravitation Constant
• Organic Chemistry Tutor - Gravitational Acceleration

• Neil deGrasse Tyson: Einstein vs Newton - Who Was Right?
• Einstein's Gravity
• Steve Mould: Cavendish experiment

• Gravitational force between objects
• Gravitational interaction, 2 objects
• Gravitational interaction, 4 objects

Other Resources

When trying to lose weight, everyone thinks about going to the gym or going on a diet. But nobody thinks about going to a different planet with a lower gravitational field strength.

So what exactly is weight? How is it different from mass? And why do things fall?

In this lesson we'll learn about gravitational force or gravity. We're most familiar with gravity as the force that causes objects to fall. But gravity also causes tides to go in and out, keeps the moon orbiting around the earth, and keeps the earth (and all of the planets) orbiting around the sun.

We'll learn about weight (which is just the gravitational force on object) as well as apparent weight (which is just the normal force supporting you from below).

So where does this gravitational force come from?

If that's true, why don't we experience that with most objects? Why doesn't a pen start flying towards a cup? Even though many objects are close together, their masses are so small that the gravitational force is too weak to overcome the other forces that prevent them from moving (usually friction). It takes an object with an enormous amount of mass, like the earth, for you to experience the force of gravity in everyday life.

Albert Einstein has given us a brand new theory of gravity to work with: general relativity. But this course is only going to be using Newton's Law of Universal Gravitation (aka Newtonian gravity). Just like ignoring air resistance still gives us really accurate predictions for projectile motion, Newton's equation gives us extremely accurate predictions for most things affected by gravity.