Hi, this is my first post here.
I have a degree in physics and i do some maths and physics tutoring.
Recently I realised that I have some holes in my understanding of work, based on a question I was doing with a student (this is not the question I'm asking here though).
I'll use a really simple situation that highlights my problem.
A block is lifted up, using 1000 Joules of energy. How much work is done on the block?
The way I would have thought of this previously is, 1000 J of mechanical work is done on the block. It gains 1000 J of gravitational potential energy. Therefore energy is conserved for the block.
But the correct answer is NO work is done on the block. +1000 J of mechanical work is done on it, and -1000 J of gravitational work is done on it for a net work of 0 J.
Initially, this seemed odd to me and made me think, if no work is done on the block, how can it be 1000 J better off in terms of total energy? Then I thought that although "the block" is better off by 1000 J, the source of the 1000 J of mechanical energy is 1000 J worse off, so everything still balances, and there are really 4 flows of energy when you count everything.
BUT the correct textbook answer is to count only a particular two of those flows. The textbook answer that I read is that a change in GPE does not count in terms of work done ON something, because GPE is a property of the whole system, not just "the block", because there is another mass (in this case, the Earth) that is required for GPE to exist.
The book only seems to count kinetic energy as belonging to the object itself. But that made me think, doesn't KE also require another object, or at least another reference frame, external to the object you are considering, for there to be a v in 1/2mv[itex]^{2}[/itex], the v must be measured with reference to something else. So the idea of "only counting KE" as work done ON something seems really arbitrary to me now.
E.g. if the original situation is changed so that now the mechanical force is applied sideways instead of up (and everything is frictionless and lossless etc), and now 1000 J of work is done ON the block, legitimately, and the block has gained 1000 J of KE.
What I am asking here is for any thoughts that might help clear up my understanding of this topic!
Or, more specifically, some general rules for answering the question of how much work is done ON an object - which works count as done on the object, and which do not? Also, is there anything fundamental and intuitive to the answer to my question (which works count as done ON the object and which do not), or is it just a matter of convention and what is the accepted way that we all agree to define "on the object"?
I think that the proper answer is going to be something in terms of identifying what forces are acting on the object. i.e. in the first situation, the mechanical lifting force, and gravity are the two forces acting ON the block. In the second (horizontal) situation, only one force is acting, the mechanical pushing force. But something still seems like its missing in my understanding.....
I have a degree in physics and i do some maths and physics tutoring.
Recently I realised that I have some holes in my understanding of work, based on a question I was doing with a student (this is not the question I'm asking here though).
I'll use a really simple situation that highlights my problem.
A block is lifted up, using 1000 Joules of energy. How much work is done on the block?
The way I would have thought of this previously is, 1000 J of mechanical work is done on the block. It gains 1000 J of gravitational potential energy. Therefore energy is conserved for the block.
But the correct answer is NO work is done on the block. +1000 J of mechanical work is done on it, and -1000 J of gravitational work is done on it for a net work of 0 J.
Initially, this seemed odd to me and made me think, if no work is done on the block, how can it be 1000 J better off in terms of total energy? Then I thought that although "the block" is better off by 1000 J, the source of the 1000 J of mechanical energy is 1000 J worse off, so everything still balances, and there are really 4 flows of energy when you count everything.
BUT the correct textbook answer is to count only a particular two of those flows. The textbook answer that I read is that a change in GPE does not count in terms of work done ON something, because GPE is a property of the whole system, not just "the block", because there is another mass (in this case, the Earth) that is required for GPE to exist.
The book only seems to count kinetic energy as belonging to the object itself. But that made me think, doesn't KE also require another object, or at least another reference frame, external to the object you are considering, for there to be a v in 1/2mv[itex]^{2}[/itex], the v must be measured with reference to something else. So the idea of "only counting KE" as work done ON something seems really arbitrary to me now.
E.g. if the original situation is changed so that now the mechanical force is applied sideways instead of up (and everything is frictionless and lossless etc), and now 1000 J of work is done ON the block, legitimately, and the block has gained 1000 J of KE.
What I am asking here is for any thoughts that might help clear up my understanding of this topic!
Or, more specifically, some general rules for answering the question of how much work is done ON an object - which works count as done on the object, and which do not? Also, is there anything fundamental and intuitive to the answer to my question (which works count as done ON the object and which do not), or is it just a matter of convention and what is the accepted way that we all agree to define "on the object"?
I think that the proper answer is going to be something in terms of identifying what forces are acting on the object. i.e. in the first situation, the mechanical lifting force, and gravity are the two forces acting ON the block. In the second (horizontal) situation, only one force is acting, the mechanical pushing force. But something still seems like its missing in my understanding.....