Mass, weight and gravity

Introduction

Gravity is a force that attracts objects to one another. Gravity is dependent on the mass of the objects involved, so while two peas are gravitationally attracted to one another, the force is insignificant. For large masses such as planets, however, gravity is a much stronger force that pulls objects towards their centres.

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The Law of Universal Gravitation

According to Newton's Law of Universal Gravitation, each particle of matter attracts every other particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. In other words, the larger a planet's mass and the closer to a planet an object is, the stronger the force of gravity on that object.

Mass vs. weight

The terms mass and weight are seemingly interchangeable in everyday conversation. In science, however, they have very different meanings. Mass refers to the amount of matter that an object contains. Weight refers to the force with which the gravity of a planet (such as the Earth) pulls on an object.

Mass and weight are measured differently. In order to measure an object's mass, a balance is used. Balances are devices that compare the mass of an object with other objects, called masses that are known to have a certain mass (such as 5 grams or 1 kilogram). Weight, on the other hand, is measured using a spring scale. Spring scales measure the force with which gravity pulls down on an object. Most bathroom scales are spring scales that are calibrated (adjusted to read measurements in certain units) to show mass because they assume you will be using it at Earth gravity. If you were to stand on a bathroom scale placed in a swimming pool or on the moon, it would show your mass as less than it actually is.

Mass is measured in units such as grams and kilograms (g, kg). Weight is measured in newtons (N), as it is a type of force. Your weight can be determined by the following formula:

W = mg

where W is weight, m is mass and g is the acceleration due to gravity. On Earth, the acceleration due to gravity is about 9.8 metres per second per second (m/s2). A person with a mass of 50 kg would have a weight of 50 x 9.8 = 490 N.

Since different celestial objects like planets, moons and asteroids are different sizes, your weight would change if you went to a different planet. On the moon, for example, your weight is about one-sixth of your weight on Earth. On Jupiter, however, you weigh 2.6 times more than you weigh on Earth.

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Gravity in space

Gravity causes celestial objects to orbit one another. Planets orbit a sun because a sun has a lot of mass and therefore a lot of gravity. Smaller moons orbit planets in turn because the planets have significantly more mass than the moons. Why don't planets simply fall into the gravity of a sun, or moons fall into the gravity of the planets they are orbiting? Massive objects orbit one another because they are in a constant state of free fall.

Consider throwing a ball. The ball travels parallel to the surface of the Earth for a while, then curves downwards and hits the ground. Now imagine firing cannon at high speed. The cannonball would follow the curve of the Earth for longer before falling. For part of that time, the ball falls at the same rate as the curve of the Earth. If you were able to fire a cannon off a tall mountain on the moon (so we don't have to worry about air resistance or hitting other mountains), the cannonball would fall at the same rate of the curve of the moon. Therefore, the cannonball would travel around the moon in an elliptical (oval-shaped) orbit without stopping. This state is called free fall. The same principle keeps the planets in orbit around the sun, and moons in orbit around planets. It also allows artificial satellites such as the International Space Station and communications satellites to orbit the Earth.

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