Chopstick Physics

Classical Mechanics Level 2

Suppose Takeshi's hands hold the chopsticks correctly as shown above, where the fingers face at the shortest tips. He wants to grip the sushi by two shortest tips of the chopsticks.

Given a certain minimum force on the sushi piece exerted by the tips of the chopsticks (required to even pick the piece up), where may the hand be positioned for maximum bending moment along the length of the chopsticks?

Details and Assumptions:

Neglect chopstick slippage.
Assume that Takeshi is a chopstick master.

Image Credit: Mental Floss

Close to the shortest chopstick tips. Far from the shortest chopstick tips At the center of the chopsticks

1 solution

Sam Gilbert
Feb 27, 2017

A maximum torque is desired to apply the most gripping force on the sushi. Since torque equals force times the radius of seperation and the force applied on the chopsticks is constant, we want the radius to be as great as possible, thus being the furthest from the tip.

@Michael Huang "shortest tips" instead clear to me. Thoughts on how we can improve the clarity of the problem?

Calvin Lin Staff - 4 years, 3 months ago

I believe picking up the sushi should be the part of the question.

Michael Huang - 4 years, 3 months ago

I first got confused by "where the fingers face at the shortest tips".

Since "shortest tips" is a unique term that you're defining, the definition should occur before its usage.

Calvin Lin Staff - 4 years, 3 months ago

Assuming the hand applies a constant torque about the rotational center, the torque applied by the sushi is constant, so it doesn't really make much sense to maximize the torque. Rather, I think we should maximize the gripping force under the assumption the torque is constant, which means we want to minimize the lever arm.

Brian Moehring - 4 years, 3 months ago

This is such a confusing question, but I think I get it now.

Originally I thought the question was asking how to exert the greatest amount of force on the sushi. In that case, we can assume a fixed amount of torque applied by the hand and thus the force on the sushi is torque/distance, so we would want the hand as close to the sushi as possible.

Instead, the question is asking about the force applied to the chopsticks. Assuming he is a master, he will apply only enough force on the sushi as is required to hold it firmly (doesn't want to squish it.). Therefore, the further back he holds the chopsticks, the more force he needs to apply in order to exert the same amount of pressure on the sushi.

So it's kind of a backwards question - instead of asking how to make it easy on him, we're trying to find out how to make it hard on him.

Gregory Lewis - 4 years, 3 months ago

Chopstick Physics

Image Credit: Mental Floss

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