Practice Applying Gravity Formulas

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  • 0:03 The Force of Gravity
  • 1:47 Sample Problems
  • 5:46 Lesson Summary
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Lesson Transcript
Instructor: Michael Blosser

Michael has a Masters in Physics and a Masters in International Development. He has over 5 years of teaching experience, teaching Physics, Math, and English classes.

In this lesson the reader will be introduced to the force of gravity and the formulas used for calculating the force of gravity. This lesson will use real-world examples about how we can use gravity formulas to calculate for unknown forces and variables.

The Force of Gravity

Did you know that there is an invisible force constantly working on us, continually pulling us toward the center of the earth? This is called the force of gravity. Gravity is constantly acting upon any object with a mass, pulling us toward Earth's surface. But what is the force of gravity, and how can we account for it in different situations?

The force of gravity is when two objects with mass are attracted and pull on each other with an unknown force. This unknown force is called the force of gravity. The more massive an object, the stronger its pull on another object that has mass. The earth exhibits a force a gravity on all of us, pulling us to Earth's surface. For this reason, spaceships need to attain a certain velocity in order to escape the force of gravity from the earth and leave our atmosphere. The force of gravity is what keeps the earth and the other planets in our solar system orbiting around the sun.

Isaac Newton developed his theory of universal gravitation in the 17th century, which states the gravitational force between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between then, giving us this equation:

Image 1


with Fg = gravitational force between two objects


m1 and m2 being the mass of the two objects


G = the gravitational constant = 6.67 x 10-11 N*m2/kg2


and r being the distance between the objects.

Sample Problems

Example One (Mars and Jupiter)

How much would a person who has a mass of 65 kg weigh on Mars and Jupiter? The mass of Jupiter is 1.898 x 1027 kg, the mass of Mars is 6.39 x 1023 kg, the radius of Jupiter is 6.9911 x 107 m, and the radius of Mars is 3.39 x 106 m. Hint: A 65 kg person would weigh 143.3 lbs on Earth.

Answer

Since we are being asked for the weight of an object, we know we will be using Newton's universal gravitational formula given above. Now, to solve for the person's weight on Mars and Jupiter, we just plug in the values given and solve.


Mars:

Image 1

Image 2


And using the conversion factor that 1 N = 4.45 lbs, we determine that a person who has a mass of 65 kg would weigh 54 lbs on Mars.


Jupiter:

Image 1

Image 3


And using the conversion factor that 1 N = 4.45 lbs, we determine that a person who has a mass of 65 kg would weigh 362.2 lbs on Jupiter.

Example Two

Teeter-Totter
Teeter-Totter

A small child of 15 kg is on a teeter-totter a length L1 of 5 meters away from the pivot point of the teeter-totter. What distance, L, would a boy of 20 kg need to sit (in distance away from the pivot point of the teeter-totter) to balance the small child?

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