Per forum rules, you must show an attempt.ktsa said:Homework Statement: how much work is done to hold a 0.80 kg book at a height of 1.3 m above ground for 5.0 s ?
Relevant Equations: Please can anyone help me with problem? I don't know if the work is 0 J
Please can anyone help me with problem? I don't know if the work is 0 J
What is the change in potential energy? What is the change in kinetic energy?ktsa said:why it is not possible to use the change in potential energy to fing the work done when holding the book at that height.
Not quite correct. This really should be one equation W = (KEf + EPf) - (KEi + EPi). There are circumstances, like pendulums and orbits where the change in kinetic energy is exactly compensated by an opposite change in potential energy, with no net work done. Those formulas only work if you specify that the other sort of energy doesn't also change.ktsa said:W= KEf- KEi
W= EPf- EPi........ and so one.
I believe that the key is in the question:ktsa said:...
But I ask for this because one other tutor said that the answer is 0 J because there is no displacement done, which can also be true. That is why I was wondering why it is not possible to use the change in potential energy to fing the work done when holding the book at that height.
I think this question is just a tad ambiguous because it does not specify the force that does the work. It could be the work done by gravity or by the hand holding the book or by the net force. Of course, in this case it doesn't matter because, as has already been argued, the displacement is zero hence neither force does work on the book and adding the two to get the work done by the net force yields zero.ktsa said:Homework Statement: how much work is done to hold a 0.80 kg book at a height of 1.3 m above ground for 5.0 s ?
Here is where it becomes important to know which force does the work. The change in potential energy is by definition the negative of the work done by a conservative force. You can use the definition here if the problem specifically asked you to find the work done by gravity but not if it asked you to find the work done by the hand.ktsa said:That is why I was wondering why it is not possible to use the change in potential energy to fing the work done when holding the book at that height.
I find this statement misleading. Take a simple pendulum that swings from maximum height ##h## to the lowest point of the motion. There are two forces acting on the bob:DaveE said:There are circumstances, like pendulums and orbits where the change in kinetic energy is exactly compensated by an opposite change in potential energy, with no net work done.
DaveE said:There are circumstances, like pendulums and orbits where the change in kinetic energy is exactly compensated by an opposite change in potential energy, with no net work done. Those formulas only work if you specify that the other sort of energy doesn't also change.
Yes. I shouldn't have said that. The work-energy theorem...kuruman said:I find this statement misleading. Take a simple pendulum that swings from maximum height ##h## to the lowest point of the motion. There are two forces acting on the bob:
1. Gravity that does work ##W_{\!g}=mgh##.
2. Tension that does zero work, ##W_T=0##, because the force is always perpendicular to the displacement.
We have a system that conserves mechanical energy yet the net work, i.e. the sum of the works done by all the forces, is non-zero, ##W_{net}=W_{\!g}+W_T=mgh.##
Work is defined as the amount of force applied to an object multiplied by the distance over which the force is applied. In this case, the force is the weight of the book and the distance is the height at which the book is held.
Yes, work is being done because the book is being held against the force of gravity. The force applied to the book is equal to its weight, and the distance over which the force is applied is the height at which the book is held.
The weight of the book directly affects the amount of work done, as it is the force that is being applied. The heavier the book, the more work is required to hold it at a constant height.
If the book is moved to a higher height, the amount of work done will increase because the distance over which the force is applied has increased. This means more force is needed to hold the book at the higher height.
Yes, the person holding the book is doing work. They are applying a force to the book to hold it at a constant height, and this requires energy to be expended. However, the amount of work done by the person is less than the work done by the book's weight, as they are not lifting the book but only holding it at a constant height.