centre of mass

Classical Mechanics Level 3

Two children Tony and Chris of same mass M M (including their caps) are sitting on either end of a see saw. Initially, the beam is horizontal. At once, Chris throws away his cap (of mass M / 25 M/25 ) which falls at point Q Q , midpoint of the left half of the beam, due to this the balance of beam is disturbed. To balance it again we must put a mass m m at point P P , which is located halfway from the balance point to the right end of the beam.

What is m / M m/M ?


The answer is 0.12.

Diksha Verma by DIKSHA VERMA , 22, India

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2 solutions

Sriram Balaji
Oct 24, 2014

Let the length of the see saw be 4x. Then taking the moment of masses, 

    M(2x)+M/25(x) = M(1-1/25)(2x)+m(x)

On solving x = 3/25 = 0.12

4 Helpful 0 Interesting 0 Brilliant 0 Confused

Where in the question does it say where point P lies. The mass 'm' required to balance depends on the distance from point 'P' to the pivot (centre).

David Baker - 6 years, 7 months ago

no. u r wrong. when the child throws away his cap, mass at left most point will decrease and will become 24/25 M. so the equation u have written is wrong. the question is also not clear.

abhijeet raj - 6 years, 7 months ago

net torque = 0

A Former Brilliant Member - 6 years, 7 months ago
Diksha Verma
Jan 2, 2015

1 Helpful 0 Interesting 0 Brilliant 0 Confused

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