Why do you get more tired when you stand up?

This is a very interesting question that actually has 2 answers to it! The first one is that when you stand up your center of mass rises and so does your potential energy. When your potential energy rises according to the conservation of energy, you must accommodate it with applying more force with your muscles which is an uncomfortable activity. More mathematically, potential energy is equal to (for our purposes) -mgh. Potential energy means the work that gravity does if we let it do its thing. However that would mean falling to the ground in our case, so we apply a counter work which MUST be equal but opposite in direction so that the net energy is zero(non zero energy would mean motion or in this situation falling). When h increases, the work must be done also increases. The second reason is that when you stand up the area of your touch to the floor decreases, resulting in a smaller force from the ground (better known as the normal force) leading you to do more work by your muscles to actually stand up. The was muscles do work is by contracting by the use of ATP which is a valuable resource for other uses. So why do you feel tired when you do more work? This is due to natural selection. The organisms who did not feel discomfort when faced with work, did loads of it, spending precious ATP for no good reason ending in their demise. Your ancestors were wether clever or were subjected to random but lucky behavior which caused them to get tired when subject to work, preserving their internal energy to create offsprings which led to your creation. These genes trough offsprings passed down to generations and later to you! QED

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