Moments / torque etc ( gravity is considered ? )

[sharkey1314]

The whole point is whether gravitational pull is taken account in moments ?

Homework Statement

The apparatus below is set up on Earth. It shows a piece of soft iron X of mass 0.6kg fixed on one end of a uniform beam AB which is pivoted at its midpoint. X is prevented from being pulled down by a fixed magnet by a load of 1.00 kg hunt at A.

The experiment is now conducted on the Moon where the acceleration due to gravity is 1/6 of Earth. The mass of the load at A required to maintain equilibrium of the beam is ?

Homework Equations

Principle of moments ? Sum of clockwise moments = sum of anti clockwise moments.

The Attempt at a Solution

Firstly, i decided to find the force due to the magnet.

Taking moments about the midpoint,

1kg x 10 x 0.5m = 0.5m x ( 0.6kg x 10 + force due to magnet )
Which leads to the force being 4N.

Subsequently, i went on to solve to question in moon.
mass of load at A x 10/6 x 0.5m = 0.5m x ( 0.6kg x 10/6 + 4 )
Which i got 3kg but the answer said that it was 1kg because gravity is not taken in account. ?! Explain please thanks.

 

[jbsteele]

The answer is indeed 1 kg. The reason for this is that gravity is not taken into account when calculating moments. Moments are calculated solely based on the forces applied to the beam. In this case, the only forces to consider are the force due to the magnet and the mass of the load at A. Since gravity does not apply any forces to the beam, it is not taken into account when calculating moments.