- #1
WraithGlade
- 19
- 0
One concept in physics that has never set well with me is the way work and energy are defined.
According to all the physics sources I've looked at, work is defined as:
[tex] W = \vec{F} \cdot \vec{d} [/tex]
(for a constant force over a distance)
However, intuitively the notion of taking the dot product of F with the displacement doesn't seem quite right to me. I am well familiar with the properties and interpretation of the dot product, but I still feel like something is missing.
Consider the following thought exercise:
Imagine we have two separate objects in space, object A and object B. Object A is at rest and object B is moving at a constant velocity through space. In other words, there is no net force acting on either object.
Suppose that we now apply a force [itex]F_1[/itex] to object A and a separate force [itex]F_2[/itex] to object B for an equal amount of time and that [itex]F_1 = F_2[/itex] and that both forces point in the same direction as the velocity of object B does. Object A will cover a distance [itex]d_A[/itex] and object B will cover a distance [itex]d_B[/itex] in this time interval.
Because A started at rest it will cover a smaller distance over the time interval than object B will cover. Therefore according to the definition of work the work done on object B over the time interval will be greater than the work done on object A, even though equal force was applied to both in equal time.
This in turn implies that the amount of kinetic energy applied to A to cause the resulting change in momentum would not be sufficient to apply the same change in momentum to object B, even though we already know that the same amount of force over time was applied to achieve it. This seems like a contradiction to me.
It seems to me that a more intuitively accurate formula for work might be something like this:
[tex] W = \vec{F} \cdot \frac{\vec{d}}{ \lVert \vec{d} \rVert}\, t [/tex]
(where t is the amount of time over which the force is applied to the object)
Notice that this alternate formula gives the same value for both objects A and B, whereas the standard formula does not.
It seems like you can't really cheat by just changing frames of reference, because in this example we're trying to calculate energy totals for our current frame of reference as the observer. The standard formula seems to imply that more kinetic energy must be put into changing the momentum of already moving objects than of objects at rest, which seems to violate the basic laws of physics.
Furthermore, I can't help but notice that the energy laws, such as conservation of energy are very similar to the laws of conservation of momentum. Perhaps they are in fact one and the same when proper analysis is applied. Perhaps change in momentum in a direction (as given by my alternate formulation of work above) is in fact the real unit of mechanical work. Granted, one would need to reformulate other equations in physics that depended on it because of the change, but maybe it could in fact work.
Energy is basically supposed to measure a systems "ability" to enact a change in momentum right (assuming we're considering only mechanical energy), and isn't it true that if a particle collides with a particle at rest then it transfers momentum into the other, and by so doing enacts a change in momentum? Thus doesn't this further support the argument that energy may in fact be a measure of momentum in disguise (in the purely mechanical cases at least).
Can you disprove my reasoning and clarify what work actually is and why the standard definition is what it is? Or perhaps, could my alternate definition be more correct somehow?
All the sources I've found on work and energy don't really discuss why it's defined to be the way it is; they instead just repeatedly reference other definitions and terms without really linking any of those terms to any real reason. How were these things originally thought up and why are the formulas what they are?
According to all the physics sources I've looked at, work is defined as:
[tex] W = \vec{F} \cdot \vec{d} [/tex]
(for a constant force over a distance)
However, intuitively the notion of taking the dot product of F with the displacement doesn't seem quite right to me. I am well familiar with the properties and interpretation of the dot product, but I still feel like something is missing.
Consider the following thought exercise:
Imagine we have two separate objects in space, object A and object B. Object A is at rest and object B is moving at a constant velocity through space. In other words, there is no net force acting on either object.
Suppose that we now apply a force [itex]F_1[/itex] to object A and a separate force [itex]F_2[/itex] to object B for an equal amount of time and that [itex]F_1 = F_2[/itex] and that both forces point in the same direction as the velocity of object B does. Object A will cover a distance [itex]d_A[/itex] and object B will cover a distance [itex]d_B[/itex] in this time interval.
Because A started at rest it will cover a smaller distance over the time interval than object B will cover. Therefore according to the definition of work the work done on object B over the time interval will be greater than the work done on object A, even though equal force was applied to both in equal time.
This in turn implies that the amount of kinetic energy applied to A to cause the resulting change in momentum would not be sufficient to apply the same change in momentum to object B, even though we already know that the same amount of force over time was applied to achieve it. This seems like a contradiction to me.
It seems to me that a more intuitively accurate formula for work might be something like this:
[tex] W = \vec{F} \cdot \frac{\vec{d}}{ \lVert \vec{d} \rVert}\, t [/tex]
(where t is the amount of time over which the force is applied to the object)
Notice that this alternate formula gives the same value for both objects A and B, whereas the standard formula does not.
It seems like you can't really cheat by just changing frames of reference, because in this example we're trying to calculate energy totals for our current frame of reference as the observer. The standard formula seems to imply that more kinetic energy must be put into changing the momentum of already moving objects than of objects at rest, which seems to violate the basic laws of physics.
Furthermore, I can't help but notice that the energy laws, such as conservation of energy are very similar to the laws of conservation of momentum. Perhaps they are in fact one and the same when proper analysis is applied. Perhaps change in momentum in a direction (as given by my alternate formulation of work above) is in fact the real unit of mechanical work. Granted, one would need to reformulate other equations in physics that depended on it because of the change, but maybe it could in fact work.
Energy is basically supposed to measure a systems "ability" to enact a change in momentum right (assuming we're considering only mechanical energy), and isn't it true that if a particle collides with a particle at rest then it transfers momentum into the other, and by so doing enacts a change in momentum? Thus doesn't this further support the argument that energy may in fact be a measure of momentum in disguise (in the purely mechanical cases at least).
Can you disprove my reasoning and clarify what work actually is and why the standard definition is what it is? Or perhaps, could my alternate definition be more correct somehow?
All the sources I've found on work and energy don't really discuss why it's defined to be the way it is; they instead just repeatedly reference other definitions and terms without really linking any of those terms to any real reason. How were these things originally thought up and why are the formulas what they are?