Gravity & Weight
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.
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What is gravity and how did we figure it out?
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Newton's law of universal gravitation equation
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Proportional reasoning with the gravitational force equation
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Example problem 1: the sun and the Earth
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Example problem 2: the moon and the Earth
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Example problem 3: the Earth and a ball
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Gravity on Earth
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Gravitational force between two small objects
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Newton's vs Einstein's theories of gravity
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Why would we use a gravitational field?
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Gravitational fields and field strength
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Gravitational force on a mass in a field
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Example problem: Earth's gravitational field strength at a point
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Example problem: gravitational force on the moon
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Proportional reasoning with the gravitational field strength equation
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Gravitational field near the surface of the Earth
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Weight equation
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Value of g at different heights and planets
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Recap: gravitational fields and gravitational forces
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Weight
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Mass vs weight
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Your weight at different heights, on the moon and Mars
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Weight in free body diagrams
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Example problem 1: weight of a ball
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Example problem 2: normal force on a box
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Example problem 3: weight of a ball on a different planet
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Example question intro
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Acceleration and weight on an incline
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Rotated x-y coordinate system
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Forces and components
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Angles and components of the weight force
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x direction: finding the acceleration
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y direction: finding the normal force
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Equations for the weight force components on an incline
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Weight and normal force at different angles
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Recap of weight
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Apparent weight
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Weight vs apparent weight
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Examples of apparent weight in an elevator
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Apparent weight in free fall (weightlessness)
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Why does apparent weight change on a roller coaster?
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Why do astronauts feel weightless?
Answers
Free-Response Questions
Law of gravitation
- 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
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.
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