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发布时间:2013-12-02 12:24:05  

Mass, Weight and Density Mass and Weight

Mass is a measure of the amount of matter or substance in a body. It cannot be changed by its location,shape and speed.

SI unit: kg

Weight is a force and has direction. This direction is downward, towards the centre of the Earth. This force is also called gravitational force or gravity.

SI unit: N

Gravity is experienced in the region surrounding the Earth called the gravitational field. The force is the strongest on the surface of the Earth and gets weaker further away.

Gravitational field strength g is defined as the gravitational force acting per unit mass on an object. It is about 10 N kg-1.

On moon, it is 1.6 N kg.

The more mass an object has, the greater the gravitational force acting on it., their relationship is linear and directly proportional.


W=mg where


Common weighing instrument measures the weight of an object, but they are calibrated to give readings in grams (g) or kilograms (kg) using the g value of the Earth. Thus, they cannot be used on the moon or any other places with different gravitational field.

To avoid using different weighing scales for different gravitational field strengths, the mass o an object can be measured easily using the beam balance.

Refer Unit 5: Turning Effect of Forces.


Inertia of an object refers to the reluctance of the object to change its state of rest or motion. An object with more mass has greater inertia.

Strapping astronauts in the horizontal position will minimize the flow of blood away from the brain caused by the inertia of the blood due to the sudden large upward acceleration of the space shuttle. The blood that flows away from the head will deprive their brains of oxygen if the astronauts are strapped in a vertical position.


The density of a substance is defined as its mass per unit volume. For pure substances, it is a fixed property.

Density p (Greek alphabet ‘rho) p = m/V where

SI unit of density is kilogram per cubic metre (kg m-3).

Note: if mass is measured in grams and volume in cubic centimetres, the unit of density would be gram per cubic centimetre.

Thus, the common unit of density used is g cm-3 since the objects we handle daily have much smaller masses and volumes.

Average density = mass/ volume

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