I want to understand what happens to the energy inputted into a simple machine if there is no resistance or load. For example, lets say you have a U-shaped tube of water with greater surface area on the left side. If a piston applies pressure to the left side and there is no piston on the right side, what does the water apply pressure to on the left side? Where does the energy go?

Related Other Physics Topics News on Phys.org
NascentOxygen
Staff Emeritus
With no load, it will be to the water that the input energy is transferred; the water will be accelerated outwards, exiting the manometer.

But what if the water level simply rises on the other side, like shown in this video at 8:09 () ?

What about in the case of a simple lever, what happens if I apply force to the short end and raise the long end of the lever, where is the energy transferred? To the lever? If so, how come when a block is placed on the long end, we say that all the energy is transferred to the block?

I want to understand what happens to the energy inputted into a simple machine if there is no resistance or load. For example, lets say you have a U-shaped tube of water with greater surface area on the left side. If a piston applies pressure to the left side and there is no piston on the right side, what does the water apply pressure to on the left side? Where does the energy go?
The energy input (load) is exerted for the water level to rise at certain height at the otherside. If load is applied at the side where the height is elevated, then simultaneously the input load is increased by that much. There are no energy loss in the system. Energy conservation is still there (w/with out restriction)

What do you mean by the input load being increased? If the same volume of water is raised on the other side, there is no change in the potential energy of the water, correct?

And what about the lever case?

NascentOxygen
Staff Emeritus
In a simple lever, such as a seesaw, if you push one end down, with no load on the other end all your input energy gets transferred to the lever bar itself, causing it to spin up and over and clock you if there is nothing to stop it. You've seen comedy skits of someone stepping on the upright tines of a garden rake?

sophiecentaur
Gold Member
I want to understand what happens to the energy inputted into a simple machine if there is no resistance or load. For example, lets say you have a U-shaped tube of water with greater surface area on the left side. If a piston applies pressure to the left side and there is no piston on the right side, what does the water apply pressure to on the left side? Where does the energy go?
If there is no Load then the MA is zero (Load / Effort). There is another quantity - Ideal Mechanical Advantage or Velocity Ratio, which is the ratio of distance moved by the effort / distance moved by the load. That ignores resistance and dead weight.
That simple hydraulic system will consume finite energy, just sloshing the water about, of course and, with no load, the efficiency is zero. Efficiency = MA/VR

Yeah..but then it stops spinning once the side you apply force on touches the ground. At that point, where has the energy been transferred? It can't be friction because had there been a load, you would say the energy would be transferred to the load.

What do you mean by the input load being increased? If the same volume of water is raised on the other side, there is no change in the potential energy of the water, correct?
Wrong concept, potential energy is = mgh, even that the other side elevation has minimal change because of bigger cross section, see that mass-m is increased hence m=ρV.

NascentOxygen
Staff Emeritus
Circuses have seesaws that don't hit on the ground. If the end of a seesaw hits the ground, the hole it makes accounts for the energy loss.

But the same mass water has lost elevation on the other side, therefore there is no net change.

But the same mass water has lost elevation on the other side, therefore there is no net change.
I suggest you need more study on hydrostatic pressure.

But the same mass water has lost elevation on the other side, therefore there is no net change.

Circuses have seesaws that don't hit on the ground. If the end of a seesaw hits the ground, the hole it makes accounts for the energy loss.
What about if no hole is made? When there is a load, it still hits the ground, yet the energy is transferred to the load not the hole.

sophiecentaur
Gold Member
But the same mass water has lost elevation on the other side, therefore there is no net change.
No change in what? Work out the mgh on each side before you make assertions like that.

This is a pretty futile discussion. The Energy in any system like this can always be accounted for, one way or another. Things get hot and deformed. That always accounts for any suggested deficit one can calculate.

In potential energy. If the volume is the same, then Mg is the same. The only different quantity is height. However, on the side that loses elevation, some water molecules are at higher elevations than other. So the net change in potential energy should be 0...or am I missing something?

sophiecentaur
Gold Member
or am I missing something?
You are missing the am[litudes of the changes in mgh. One goes down by less than the other goes up but the mass transferred is the same. Always do the sums when possible, before coming to intuitive conclusions. Those numbers are what counts.

Ok..so the potential energy decreases because the water moves less height upwards on the side with no load?

So where does the energy go?

Dale
Mentor
If you have a mechanical device with no load and you input energy, then you will raise the internal energy of the device, usually to internal KE and PE, vibrations, and heat. The vibrations and heat can then dissipate the energy to the environment, sometimes after breaking the device.

Last edited:
sophiecentaur
Gold Member
The minimum Potential Energy is (of course) when the two water columns are leverl. To change that situation requires work to be done. Work out (in proper detail) what happens to the centre of mass of the whole mass of the water. Either do it with algebra (preferable) or for a specific example (which isn't a proof - its just an indication).
PS - did you actually work it out or are you just repeating what I wrote?

I worked it out. I found that M, and g, are same on both sides and that -h, height water displaced downwards, is greater than +h, height water displaced upwards. Isn't that sufficient to say potential energy is decreased?

If you have a mechanical device with no load and you input energy, then you will raise the internal energy of the device, usually to internal KE, vibrations, and heat. The vibrations and heat can then dissipate the energy to the environment, sometimes after breaking the device.
So essentially the water increases in temperature? What would happen in the lever case if no hole is created?

Dale
Mentor
I worked it out. I found that M, and g, are same on both sides and that -h, height water displaced downwards, is greater than +h, height water displaced upwards. Isn't that sufficient to say potential energy is decreased?

russ_watters
Mentor
I want to understand what happens to the energy inputted into a simple machine if there is no resistance or load. For example, lets say you have a U-shaped tube of water with greater surface area on the left side. If a piston applies pressure to the left side and there is no piston on the right side, what does the water apply pressure to on the left side? Where does the energy go?
You already asked this question in a slightly different form (a massless lever with nothing on it) and received the answer: if there is no resistance/load, there is no force and therefore no energy input (however, in the manometer example, there is a force and energy input....so you failed to construct an example that meets your requirements).

Oh, wait, there it is again:
What about in the case of a simple lever, what happens if I apply force to the short end and raise the long end of the lever, where is the energy transferred? To the lever? If so, how come when a block is placed on the long end, we say that all the energy is transferred to the block?
You really need to do a better job remembering and applying what you've learned from one week to the next. These concepts you are stumbling over are not difficult, but you seem to be getting stuck with a wrong idea in your head that you are unwilling to let go of.

Part of it, as Dale says, is your lack of showing your work that is preventing you from figuring out the exact answers on your own. For example, in your lever example you are being too vague to answer it properly: you haven't said which side you are pushing down on. In your previous version of the question you asked about pushing down on the long side, which, of course, you can't do if there is nothing sitting on the short side (it just falls under its own weight).

Last edited:
Heres my calculation for the manometer

Attachments

• 161.2 KB Views: 266
Heres a diagram for the lever.

russ_watters
Mentor
Heres a diagram for the lever.View attachment 91468
What is the mass of the lever? This will determine how much force can be/has to be applied to move it.