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Q & A: Why is it work to hold a weight straight out from your body?

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Most recent answer: 12/19/2012
Having a fun conversation with my son...the farther you hold a weight away from your body, the harder your body has to work to hold it. What is the physics explanation/term for this? Thank you!!
- Jimmy Clark (age 44)
Chaska, MN USA
Hello Jimmy,
The answer to this question involves not only physics but also the science of how the body works, physiology.   When you lift a weight you do mechanical work in overcoming the gravitational force.  When the weight is stationary at arms length you no longer do any mechanical work but, as we all know, it's tiring.  The reason is physiological.  In order to keep a muscle under tension it must be supplied by chemical energy, ATP or Adenosine-5'-triphosphate in chemical jargon.  When the local supply of ATP runs short you start to feel fatigue. 
Now as to why it's more tiring to hold the weight farther away from your body, the torque, or torsional force, is proportional to the distance from the weight to the point of suspension, your body.    More force --> more ATP required --> more tiring.

Best regards to you and your son, and keep having fun conversations.

p.s. For more discussion, see

(published on 12/19/2012)

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