# Mass

We learnt that mass is a measure of the inertia of a body. If the mass of an object is greater, its inertia will also be greater. Mass of an object remains the same whether it is on the earth, the moon or anywhere in space. It is a constant.Â

We compare masses by comparing the gravitational forces acting on them. If the earth attracts two objects with equal force, can we say that their masses must be equal? Let us find out. Consider two objects of masses m_{1} and m_{2} and hold them at the same height above the surface of the earth. The force of attraction between mass m_{1} and the earth is given by

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The force of attraction between mass m_{2} and the earth is given by

So, by dividing the two equations we get

Now, if F_{1} = F_{2}, it follows that m_{1} = m_{2}.

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Therefore, if somehow we could check whether the forces F_{1} and F_{2} are equal, we would be able to compare the two masses. A beam balance enables us to do so. It consists of a beam supported at the centre on a knife-edge. Two identical pans are suspended at the two ends of the beam. You are advised to go to your school physics laboratory and learn the working of a beam balance. When the objects in the two pans have equal masses, the pans remain stationary and the beam is horizontal. When this happens, the forces of attraction F_{1} and F_{2} on the two masses must be equal. If one of mass (say m_{2}) is a standard mass, the other mass m_{1} is determined in terms of this standard. The mass of a body measured by this method is called its 'gravitational mass'.

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