BitWithBite
BitWithBite / Mechanics / Gravitation
📘 Lesson 17 of 19 · Mechanics

🌍 Gravitation

Gravity is the force that pulls every mass in the universe toward every other mass — from apples falling to the ground to entire galaxies staying bound together.

Course progress: 89%

01 Key Concepts

Newton's Law of Universal Gravitation

Every two masses attract each other with a force F = G*m1*m2/r^2, where G is the gravitational constant, and r is the distance between their centers.

The Gravitational Constant (G)

G ≈ 6.674 x 10^-11 N*m^2/kg^2, an extremely small number reflecting how weak gravity is compared to other fundamental forces at everyday scales.

Gravitational Field Strength (g)

The force per unit mass at a location; near Earth's surface, g ≈ 9.8 m/s^2, which is also the acceleration experienced by any freely falling object there.

Weight vs. Mass

Mass measures the amount of matter in an object (constant everywhere); weight = m*g measures the force of gravity on that mass (which changes depending on location, like the Moon vs. Earth).

Orbital Motion

A satellite in orbit is really in continuous free-fall, with gravity providing exactly the centripetal force needed to keep it moving in a circular (or elliptical) path.

02 Key Formulas

03 Solved Examples

Example 1 Find the weight of a 70 kg person on Earth (g=9.8 m/s^2).
  1. Apply Weight = m*g = 70*9.8.
Answer: 686 N
Example 2 The Moon's gravity is about 1/6th of Earth's. Find the weight of the same 70 kg person on the Moon.
  1. Moon's g ≈ 9.8/6 ≈ 1.63 m/s^2.
  2. Weight = 70*1.63.
Answer: Approximately 114.3 N (much less than on Earth, though mass stays 70 kg)
Example 3 If the distance between two masses doubles, how does the gravitational force between them change?
  1. Force is inversely proportional to r^2 (the square of distance).
  2. Doubling r means dividing force by 2^2 = 4.
Answer: The force becomes 1/4 as strong

04 Practice Questions

1Find the weight of a 50 kg person on Earth (g=9.8).
490 N
2If distance between two masses triples, how does force change?
Becomes 1/9 as strong (divided by 3^2)
3What is the difference between mass and weight?
Mass is the amount of matter (constant); weight is the force of gravity on that mass (varies by location)
4What provides the centripetal force keeping satellites in orbit?
Gravity
5Approximately what is g near Earth's surface?
9.8 m/s^2

📄 Gravitation — Downloadable Worksheet

10 questions with a full answer key. Grab the PDF to print, or try the interactive version in your browser.

🗒 Cheat Sheet ⬇ Download PDF 🖥 Interactive Worksheet