When we are carrying a weight there is no movement. Hence no work done. But we feel tiered. WHY?

Answer 1

You are correct that no work (force times distance) is being done on the weight.

However, in order to hold the weight, the muscle must be tense enough (not relaxed) to exert the appropriate force. In order to maintain the muscle tension, your body must continue to metabolize and burn energy (which it sends to the muscle). For whatever physiological reason, we can't keep our muscles tense indefinitely. That's why you feel tired.

Contrast this with setting the weight on a table. The table is exerting an upward normal force on the weight and does no work. The normal force comes from intermolecular forces in the table's structure and we do not need to add energy to the table to keep the force applied.

This is a case when we incorrectly generalize from our personal experience (energy is needed to cause muscles to exert a force) to other objects.

Answer 2

This is a really good question. It helps to realize that energy is a more general quantity than work. Say, heat is also an energy. We loose energy of coal when we burn it, and in result of the burning we may not get any work done (if we did not put some machinery to use the heat to create ordered macroscopic motion). Same is in your question: There is no work done (which is very narrow way of energy transformation). When you stand and have weights in your hands, you pass energy to environment not by making some macroscopic movement, but by burning your fuel to support your muscles. You do move things in your body, but you move molecules and ions. It is just a different way to transfer energy to environment and on the way to do something useful. If you want, you may call it 'microscopic work', in sense that instead of moving several large things you move many little things (like molecules and ions, and I am sure somewhat larger things too).

Answer 3

When the height of the carried weight is constant then no work is done against gravity on the weight (force x distance). However, the body is displacing the point of contact of various forces when moving, eg on each step muscles raise the body against gravity and then move it forward against friction. This does work and transfers energy and our muscles feel tired.

Answer 4

We are always resisting the force of gravity with what is called the Normal Force which is equal and opposite to the force of Weight:


Normal Force = Weight
Weight = Mass x Gravity

Increase the Mass you are holding or change planets, you have more/less weight therefore you have to a apply a bigger/smaller normal force with your muscles against the ground.

Answer 5

Your muscles are getting tired from having to support the weight of the load against gravity. There is no work being done because your force cancels out gravity's.

Answer 6

Because you expended energy in opposing gravity. And what happened to that energy? It was lost as heat to the environment, i.e. it was not added to the weight. According to the definition of work (in physics), you did not do any work.

Assume you have a big boulder in your front yard and hired me to move it to the back yard (for $200.00). I tried 6 hours but was not able to move it from its original position. After 6 hours, I'm very tired but how much work did I do? How much will you pay me?

Answer 7

There is work being done. You are opposing the gravitational acceleration with an equal and opposite acceleration in order to keep the weight stationary.

Answer 8

You are still holding the weight up, (against gravity) for a time. So it is still work!

Answer 9

Cause you must have failed English... how can you feel tiered? Like you feel like you have many levels?

Answer 10

Depends on the size of your butt cheeks and the Earths' gravitational pull on your hind parts.