Assume the ruler has length of 2l and thickness of d.
Consider only a half part of the ruler. The ruler does not move, so we can say that the total force and torque on the ruler is zero.

First, consider when the ruler is pulled with a force F.If we pull the ruler with a force F, the other part of ruler will exert a same force, F, to this part in order to cancel the pulling force.

In other hand, if we bend the ruler with a force F at edge of the ruler, the other part will exert a torque with the same magnitude of

Therefore, we can calculate roughly the magnitude of force exerted by the other part

Because a ruler is commonly very thin compared to its length, the value of l/d will be very large.
So the force exerted by the other part is much larger if we bend the ruler, and it will break easily.

That's my solution.
If you have any question, correction, or comment, please leave a comment below.
Thank you.

Solution - Breaking A RulerSocialTwist Tell-a-Friend


Anonymous said... @ March 4, 2009 at 1:13 AM

awesome logic!!!!

Anonymous said... @ January 28, 2011 at 6:32 PM

the upward force should be 2F and not F to net external force zero.

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