Who Decided that Work = Force * Distance?

  • #1
68
1
In other words, why is change in kinetic energy defined this way? (In contrast to momentum being defined as Force times time?) I'm just trying to anchor my understanding of kinetic and potential energies into something intuitive?
And, since Force is a vector, why isn't Work a vector quantity?
Thx, Jon
 
  • Like
Likes m.reza

Answers and Replies

  • #2
Orodruin
Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Gold Member
16,829
6,652
Force multiplied by distance is the work done on a system. This does not imply that this turns into kinetic energy, but it could also become othher energy forms.

It is simply how we define work. There is another quantity which is force times time, namely impulse. They both have distinct relations with other kinematical quantities, such as energy and momentum. You could of course choose what you call what, but this is the standard definition and if hou define it differently you will get different relations to the other kinematical quantities.

Force is a vector and so is the displacement. If you take the force x distance in several spatial dimensions, the product is an inner product between force and displacement, which gives you a scalar.
 
  • Like
Likes jon4444
  • #3
jtbell
Mentor
15,727
3,879
why is change in kinetic energy defined this way?
For constant force and therefore constant acceleration, it comes from the following standard relationship for one-dimensional kinematics with constant acceleration: $$v_2^2 - v_1^2 = 2ad$$ Start by writing Fd = mad, then use the equation above to substitute for a.
 
  • #4
68
1
For constant force and therefore constant acceleration, it comes from the following standard relationship for one-dimensional kinematics with constant acceleration: $$v_2^2 - v_1^2 = 2ad$$ Start by writing Fd = mad, then use the equation above to substitute for a.
I'm not sure I understand your point. I get how the kinematics equation leads you to 1/2mv^2, but my question was directed to why you begin with Fd (or mad) as definition of work/energy in the first place
 
  • #5
jtbell
Mentor
15,727
3,879
I think the concept of force times distance being "special" goes back to studies of simple machines, in which you have a small "input" force acting over a large distance producing a large "output" force acting over a small distance. A long time ago (can't find the thread now), I posted a reference to a book published around 1500 (maybe 1600?) which was apparently the first to use "force times distance" in connection with simple machines.
 
  • Like
Likes ulianjay
  • #6
atyy
Science Advisor
14,302
2,543
A force is a push or a pull.

And energy is the ability to do work.

By defining work as how much a force moves and object over some distance, we can get an equation about how how pushing or pulling produces work :) This is called the work-energy theorem.

Work is not a vector because it is the integral over a path of the dot product between force and displacement - it is the dot product that produces a scalar from two vector quantities.
 
  • Like
Likes jon4444
  • #7
jtbell
Mentor
15,727
3,879
  • Like
Likes OmCheeto, jon4444 and Nugatory
  • #8
220
152
A table which supports a book is applying a force to that book. Because the book isn't going anywhere (in this book/table/earth system) there is no change to its kinetic or potential energy. No work is being done to the book.
 
  • #9
68
1
A table which supports a book is applying a force to that book. Because the book isn't going anywhere (in this book/table/earth system) there is no change to its kinetic or potential energy. No work is being done to the book.
But in that case, I'm assuming work is being converted to heat within your muscles.
 
  • #10
jtbell
Mentor
15,727
3,879
Right, the muscle fibers sort of ratchet back and forth, doing work which increases the thermal energy of the muscles.
 
  • #11
220
152
Work is still not being done to the book, however. (At least in an ideal system where the book is stationary in your arms.
 
  • #12
russ_watters
Mentor
20,234
6,793
Work is still not being done to the book, however. (At least in an ideal system where the book is stationary in your arms.
That isn't correct. The situation is analogous to a car revving its engine with the parking brake on. No work is being done on the car as a whole and all of the mechanical energy generated by the engine is dissipated as heat, mostly in the transmission.

Whether your muscles are continuously vibrating or not (I don't think they are), the amount they are would not necessarily bear any relation to the energy they consume.
 
  • #13
220
152
That isn't correct. The situation is analogous to a car revving its engine with the parking brake on. No work is being done on the car as a whole and all of the mechanical energy generated by the engine is dissipated as heat, mostly in the transmission.

Whether your muscles are continuously vibrating or not (I don't think they are), the amount they are would not necessarily bear any relation to the energy they consume.
I have failed to see where we are in disagreement.
 
  • #14
CWatters
Science Advisor
Homework Helper
Gold Member
10,532
2,298
I think the concept of force times distance being "special" goes back to studies of simple machines....
Perhaps one example being ploughing a field? How much work it is depends on the difficulty with which the plough can be pulled through the ground and the size of the field.
 
  • #15
russ_watters
Mentor
20,234
6,793
I have failed to see where we are in disagreement.
Oops. I was tripped up by your reverse/negative wording. Sorry. (That's the second time that's happened to me this week!)
 
  • #16
416
169
To respond to your original question:
The definition of work dates back to Leibniz' insistence that vis viva (mv^2) was the true conserved quantity in nature and that Descartes 'quantity of motion' (mv) was not. Leibniz' argument stemmed from Galileo's work that vis viva is proportional to the height from which a falling body is released (very early idea of what we now call gravitational potential energy and kinetic energy).

Fast forward to the beginning of the industrial revolution: Coulomb pioneered work in the machine sciences and used mechanics to model worker productivity during a day of labor. One of the quantities that Coulomb used as a measure of machine efficiency was the product of force and distance, but he did not use the term 'work' to describe this quantity

Coriolis was the first person to coin the term work (travail) and he defined the quantity as the product of force and distance where the force must be the component along the direction of the displacement (as a previous post mentioned - this is elegantly written using the scalar product). I believe that Coriolis was also responsible for introducing the factor of 1/2 into the definition of vis viva, but I'd have to verify that (I'm not sure offhand if he called it by a different name, but I know that the term 'kinetic energy' first used by Lord Kelvin).

The road to the general principles of work and energy as they are stated today is long and winding.
 
  • Like
Likes ulianjay and jon4444
  • #17
It's because work is the integral of force.
Once you integrate force, it simply happens that W=FD.
I think this is probably correct.
 
  • #18
brainpushups is correct. But the formula cannot be proven theoretically. It is an empirically proven law. This is superior to any theorists postulate.
 
  • #19
phinds
Science Advisor
Insights Author
Gold Member
16,719
7,394
Who Decided that Work = Force * Distance?

I think it was probably the first guy who had to carry a big rock from one place to another and noticed that the farther he had to carry it, the more tired he got and the heavier it was the more tired he got and he said "gee, heavier and farther is more work!"
:smile:
 
  • Like
Likes nasu
  • #20
7
2
It's because work is the integral of force.
Once you integrate force, it simply happens that W=FD.
I think this is probably correct.
This is misleading.

Work is the integral of force over distance. If the force is constant over the distance then of course W = F • D.

If you integrate force with respect to some other quantity, such as time, you do not get work.
 
  • #21
416
169
Another 2 cents since this is still going:

Technically work is the line integral of the dot product of force and an infinitesimal displacement. For nonconservative forces work is path dependent (just to be clear).
 
  • #22
340
103
In other words, why is change in kinetic energy defined this way? (In contrast to momentum being defined as Force times time?) I'm just trying to anchor my understanding of kinetic and potential energies into something intuitive?
And, since Force is a vector, why isn't Work a vector quantity?
Thx, Jon
The change in the kinetic energy is not "defined" that way.

If the force is conservative and you define the work as the line integral of the force dot the distance then you can prove using vector calculus that the work is indeed equal to the change of the kinetic energy.
 
  • #23
A.T.
Science Advisor
11,003
2,481
If the force is conservative and you define the work as the line integral of the force dot the distance then you can prove using vector calculus that the work is indeed equal to the change of the kinetic energy.
Why does the force have to be conservative for this?
 
  • #24
416
169
Indeed. The work-energy theorem does not require the net work to be a result of conservative forces. For example, when the work done by kinetic friction is included in the total work is equal to the change in kinetic energy.
 
  • #25
340
103
I stand corrected.
 

Related Threads on Who Decided that Work = Force * Distance?

Replies
15
Views
418
  • Last Post
Replies
1
Views
825
Replies
18
Views
2K
  • Last Post
Replies
1
Views
540
  • Last Post
Replies
7
Views
2K
  • Last Post
Replies
5
Views
2K
Replies
2
Views
6K
Replies
21
Views
15K
Replies
3
Views
2K
  • Last Post
Replies
6
Views
874
Top