Question

1.       Prepare an explanation of the nonconserved work of pulling and friction forces. Observe what happens when you rub your hands together quickly. Also, observe what happens when you hold a heavy book out to your side for as long as you can. What form do you think the friction and muscle work would take?

Answer 1

for both the cases answer is heat, when we rub our hands quickly then heat is produced.

The work done by a non-conservative force does depend upon the path taken. Friction, for example, does less work on a book dragged across a table if the book is dragged straight across than if the book is dragged in a curved path

when we hold a book to our for a long time, our muscles feels strained.

While you do spend some body energy to keep the book lifted, it's important to differentiate it from physical effort. They are connected but are not the same. Physical effort depends not only on how much energy is spent, but also on how energy is spent.

Holding a book in a stretched arm requires a lot of physical effort, but it doesn't take that much energy.

• In the ideal case, if you manage to hold your arm perfectly steady, and your muscle cells managed to stay contracted without requiring energy input, there wouldn't be any energy spent at all because there wouldn't be any distance moved.

• On real scenarios, however, you do spend (chemical) energy stored within your body, but where is it spent? It is spent on a cellular level. Muscles are made with filaments which can slide relative to one another, these filaments are connected by molecules called myosin, which use up energy to move along the filaments but detach at time intervals to let them slide. When you keep your arm in position, myosins hold the filaments in position, but when one of them detaches other myosins have to make up for the slight relaxation locally. Chemical energy stored within your body is released by the cell as both work and heat.