A beam balance measures mass as opposed to weight, so the mass you weigh will be the same on the moon as it is on earth. Gravity is taken out of the equation, unlike a spring balance that measures weight and would measure an article to be 1/6 of the weight on the moon as it would be on earth using the same spring balance that relied on gravity.

The principle is that of moment, or turning force/torque), calculated by force x distance. Fundamentally, in the case of a balance beam, the force is gravity acting on each side of the fulcrum of the balance, and distance is the distance from that fulcrum.

Since gravity will be constant wherever you are, only moment or torque will be relevant. A spring balance is not a comparison technique, so gravity changes will be relevant to the result - hence only weight can be measured.

Basically, if the two forces each side of the balance point (fulcrum) are equal, the balance will be horizontal. The pointer on the balance indicates this condition. The sample being weighed has a specific mass generating a fixed moment at its fixed position. The moment exerted by the mass on the other side of the fulcrum can be varied according to the position of the sliding weights on the beam, or lever.

These positions have been calibrated to correspond to specific relative masses (popularly known as weights), so when each side is balanced you can read the weight that is balanced against the sample.

To be completely scientific, we are measuring torque when we use a balance beam, and moment and toque each side will be equivalent when the beam is balanced. Torque is a function of arm length and applied force.

However, the point is that a balance beam measures true mass, and not just weight that changes with changing gravitational force. A beam balance measures mass as opposed to weight, so the mass you weigh will be the same on the moon as it is on earth. Gravity is taken out of the equation, unlike a spring balance that measures weight and would measure an article to be 1/6 of the weight on the moon as it would be on earth using the same spring balance that relied on gravity.

The principle is that of moment, or turning force/torque), calculated by force x distance. Fundamentally, in the case of a balance beam, the force is gravity acting on each side of the fulcrum of the balance, and distance is the distance from that fulcrum.

Since gravity will be constant wherever you are, only moment or torque will be relevant. A spring balance is not a comparison technique, so gravity changes will be relevant to the result - hence only weight can be measured.

Basically, if the two forces each side of the balance point (fulcrum) are equal, the balance will be horizontal. The pointer on the balance indicates this condition. The sample being weighed has a specific mass generating a fixed moment at its fixed position. The moment exerted by the mass on the other side of the fulcrum can be varied according to the position of the sliding weights on the beam, or lever.

These positions have been calibrated to correspond to specific relative masses (popularly known as weights), so when each side is balanced you can read the weight that is balanced against the sample.

To be completely scientific, we are measuring torque when we use a balance beam, and moment and toque each side will be equivalent when the beam is balanced. Torque is a function of arm length and applied force.

However, the point is that a balance beam measures true mass and not just weight that changes with changing gravitational force. http://wiki.answers.com/Q/What_is_the_principle_of_beam_balance Sources of error in Weighing Scales

1) Mis-calibratation

Any type of weighting scale should go through routine calibration. The calibration of electronic circuits may drift as time goes by. To avoid errors due to mis calibration, make sure you go to authorized centers and...

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