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How Do You Find the Mass of an Object

Uttara Manohar Sep 30, 2018
Mass can be defined as the amount of matter in an object to which its weight is proportional, though it is not the same as weight, which is a different concept. Read on to know how to calculate the mass of an object.
Mass is an important concept in physics and can be explained using Newton's second law of motion. It is applied in so many areas of our lives that we never take notice of this very evident occurrence. Let us look into what Newton discovered as a part of the laws of motion.

Mass vs. Weight

In everyday usage, weight and mass are used interchangeably, though they are different concepts. Mass is a property of a physical body which determines the body's resistance to being accelerated by a force and the strength of its mutual gravitational attraction with other bodies. Thus, the mass of an object refers to the total matter of the object.
On the other hand, weight is the force created when the mass is acted upon by a gravitational field, and hence is dependent upon the magnitude of the gravitational pull. Thus, the weight of an object will be different on different planets due to their individual gravitational accelerations, whereas its mass will remain constant.
For example, if the weight of an object is 50 kg on the Earth, the same object will weigh 8.3 kg on moon. The reason for this phenomenon will be explained in the following paragraphs.

Different Definitions of Mass

Inertial mass is defined as the measure of an object's resistance to changing its state of motion when an external force is applied to it. This definition implies that an object with small inertial mass will change its motion more readily, and an object with large inertial mass will change the motion less readily.
Passive gravitational mass can be defines as the measure of the strength of an object's interaction with a gravitational field. Thus, it means that within the same gravitational field, an object with a smaller passive gravitational mass will experience a smaller force as compared to an object with a larger passive gravitational mass.
Active gravitational mass can be defined as the magnitude of the gravitational field due to a particular object. For instance, it is observed that the gravitational field on the Moon is weaker than that of the earth owing to the fact that the moon has less active gravitational mass as compared to the earth.

How to Calculate Mass

Newton's second law of motion states that, "the vector sum of the forces on an object is equal to the mass of that object multiplied by the acceleration vector of the object".
Thus, the equation for Newton's second law can be represented as F = m.a, where:
  • F is the net force in Newton (N)
  • m is the mass in kilograms (kg)
  • a is the acceleration in meters per square second (m/s2)
Now, to find mass of an object, we could write the same equation as m = F/a, where:
  • F = W, which is the weight of the object
  • a = g, which is the acceleration due to gravity (g = 9.81 m/s2 for the Earth)
In this way, Newton's second law can be used to measure the mass of an object.

Units Used to Measure Mass

The primary instrument which is used to measure the weight of an object, is the scale or balance scale. The mass is then calculated by the formula, m = F/a. There are different units of measurement used to measure the mass. In the SI system of units, mass can be measured in kilograms.
Other units that can be employed to measure mass are grams, tones, or eV, which is an energy unit that is typically used to measure the mass of subatomic particles. Other mass units which can be used in several other contexts are pound, solar mass, Planck mass, and atomic mass units as well.