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Consider the following situation:

Two balls are inside a jar with diameter $D$. The balls are uniform in mass and both have the same diameter $d$.

enter image description here

Express the forces that acts at the colored dots in the figure.

Could any body give me a hint? It seems to me that I can't express one force without using the other.

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    $\begingroup$ You know there is a force $mg$ acting downwards at the centre of each ball. You known the balls aren't rotating so the moments of the forces add up to zero, and the balls aren't accelerating so you know the forces on each ball must add up to $mg$ upwards. So you can down a series of simultaneous equations relating the forces. $\endgroup$
    – John Rennie
    Commented Aug 29, 2014 at 10:43

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Consider this for the upper ball:

enter image description here

The angle of the inclined plane is simply derived from geometrical considerations. Looks pretty easy to solve, right? Once you know the normal force of the inclined slope, reflect and apply it to the bottom ball to complete the problem.

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  • $\begingroup$ Very good approach. The right diagram makes all the difference. $\endgroup$
    – Floris
    Commented Aug 29, 2014 at 12:13

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