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Q & A: Why do you get tired when pushing against a wall?

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Most recent answer: 04/17/2013
Q:
When we push a wall the wall does not move so, no work is done but still we get tired Why? What energy changes occur in this process.
- Areeba (age 16)
Islamabad,Pakistan
A:
Nice question.   I thought about this for some time.   Fortunately, I have a friend in physiology who  is an expert in this business.   He said that muscles under tension, or compression actually consume energy by metabolizing ATP, the chemical energy carrier in the body.  In technical terms it's the difference between systolic and diastolic metabolism.
Although you are doing no work on the wall, you are still eating up ATP energy by keeping your muscles clenched.  Eventually you get tired.

LeeH

p.s. I think that inside the muscle, there's a little slippage of the myosin-actin bonds. You have to do work to overcome that slippage, just as the engine of a car on a slope has to do work to keep it stationary if the tires are slipping a bit on the slope. Mike W.

(published on 10/01/2012)

Follow-Up #1: work pushing on wall

Q:
Hello. I'd like to better understand the relationship between energy, work, and force. Why does a force being applied not necessarily involve expending energy? If I'm applying a force to a wall, but the wall doesn't move, aren't I still expending energy in attempting to move it? Why, then, is energy defined using force and distance. Similarly, doesn't the Earth expend energy to keep me from flying off its surface?
- Ben (age 22)
Washington, DC, USA
A:
Ben- I've switched which question yours follows-up. Perhaps this previous answer will suffice. If not, follow-up again.

Mike W.

(published on 04/17/2013)

Follow-up on this answer.