EK General Physics Book #60

[desi0chick]

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I'm not sure if there's a typo for this question but for number 60, regarding torques how is the solution 3kg? The key states the equation setup as: (0.2)(3)=(0.3)(x), I just don't get how they got 0.3m, if it's a meter stick shouldn't the other end be 0.8? 😕

 

[brothersport]

Can you post the whole problem?

 

[WalterWhite10]

what is the whole question?

 

[desi0chick]

here's the problem, the answer is B.

 

[cloak25]

First of all, recognize that the pivot point or fulcrum is at 0.2m. Since the board is 1m long, the center of mass of the board is at 0.5m. So you can imagine that there's a weight at 0.5m, which is 0.3m away from the pivot point (this is the moment arm for the board's mass). Now all you have to do is balance the left torques with the right torques and you get 0.2(3) = 0.3(x). x = 2

 

[stevenz]

Gravity always acts on the centre of mass. You can think of the plank as weightless with all of its weight concentrated as a block hanging off its centre.