# Search results for query: *

• Users: leafy
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1. ### I The Job of the Demon: Compressing Gas Without Work

Yes, I'm only interest in something new. I find that the argument against Maxwell's demon is a little short. I also read Feyman's ratchet.
2. ### I The Job of the Demon: Compressing Gas Without Work

Suppose we have a cylinder with one wall guard by a demon. Inside is one gas molecule moving left and right for simplicity. Now we want to closer the wall (compress the gas) without doing work. The job of the demon is push the wall closer when the gas move left and lock the wall when the gas...
3. ### B Why does a DC motor have a restricted speed?

Net current = 0
4. ### B Why does a DC motor have a restricted speed?

I think solid state is harder.
5. ### B Why does a DC motor have a restricted speed?

But there is current through it. Each time the switch close, current flow to speed up the wheel. Each time current flow to the 24V, curren flow slow down the wheel. Each cycle, there is a speed up and slow down. The net is zero current with is steady state.
6. ### B Why does a DC motor have a restricted speed?

Unload motor require current to overcome friction. Loaded motor require current for the load. We assumed ideal, no friction, so the only requirement is current for the load.
7. ### B Why does a DC motor have a restricted speed?

I do not get what you mean. How is it not zero?
8. ### B Why does a DC motor have a restricted speed?

Hm... you might have read the diode wrong. The bottom 24V has diode that only allow current for charging direction . The bottom 24V cannot be the supply. When the switch close, current is draw from the 12V through the coil, speeding up the wheel. Then the switch open, it'll close again on...
9. ### B Why does a DC motor have a restricted speed?

The current input has to be equal to current output have steady state speed. If there is only input current, the wheel would keep speeding up. And when we draw output current, that is a load. Charging battery is a load.
10. ### B Why does a DC motor have a restricted speed?

Base on the replies I think the answer is yes. Let's assume that everything is ideal. The current going into the magnet - current induced by magnet = 0 for steady state. Let's assume all batteries are 12Vs. So from the input we have IV = current of 12V x 12V = Power input From the output...
11. ### B Why does a DC motor have a restricted speed?

Tell me if this circuit possible. The switch close and open to spin the magnet. The magnet attain enough speed to charge two batteries in series.
12. ### B Why does a DC motor have a restricted speed?

Well, my main purpose is to see if we can attain an EMF greater than the supply voltage. When the magnet pass by the coil, let's say it's supper fast that exceed the supply voltage, we aren't apply voltage to the coil because if we do, the magnet slow down in transfering energy to the supply...
13. ### B Why does a DC motor have a restricted speed?

Yes, I guess we have to assume the coil can be on instantly without being choke. What I'm trying to say is can we attain EMF greater than the supply voltage.
14. ### B Why does a DC motor have a restricted speed?

As I drew in the voltage waveform, when the magnet change direction from incoming to outgoing, the back EMF is so small that we can inject current into the coil. Of course the wave form get bigger as speed increase, so we have to shorten the ON time of the coil. But we can always inject...
15. ### B Why does a DC motor have a restricted speed?

I read that the factor that limit motor speed is back EMF. I might have to go to an electrician for this one.

17. ### B Can a static force do work?

Thanks guys. I did the experiment and measurements. And the result is conservation of energy win, again. Efficiency is around 40s%. I turn the wheel upside down and efficiency doesn’t change much. So yes, you guys are right. Sorry giving you a hard time Dale. I should have done this before...

It's ok.
19. ### B Can a static force do work?

You have no regard for translation speed of the wheel.
20. ### B Can a static force do work?

If the wheel is moving at 1m/s, imparting a force while moving is equivalent of throwing at rock at greater than 1m/s and transfer energy. Matter of fact, let make the wheel move at 100m/s. You have to throw rock faster than that. Let's say the station pusher throw rock too to move the wheel...
21. ### B Can a static force do work?

You have to understand that imparting a force while moving cost a lot of energy.
22. ### B Can a static force do work?

oh? Let's say we have 10 pushers station along the path. The wheel translate at 1m/s. The pusher push on material moving at small v(almost zero). P=Fv x 10 Or we have one pusher moving at 1m/s also push on the material moving at small v. P=Fv x 10 + F(1m/s) It is a big difference.
23. ### B Can a static force do work?

I think there're two methods to replace the tension force on the wheel. One is you have to station pushers along the path the wheel travel and each time the wheel pass by, each pusher going to push the wheel. Second is you only need one pusher, but he must chase the wheel to impart a...
24. ### B Work and Friction: How Does Fxv Apply?

I see that you always take the absolute(earth) frame. But to the person on the train, the power is not 0.
25. ### B Work and Friction: How Does Fxv Apply?

So there is no different between motion of material or motion of the force here because they're both exactly the same. What about if you're on a train moving right at 1 m/s and pushing a mass left. When the mass reach 1m/s left. What is the mechanical power? Fx1m/s or 0?
26. ### B Can a static force do work?

Let's say you're right that the motion of the material is important. How can you be sure that that at time t+delta(t) it will be the same as the time t? The point of material also move upward and at time t+delta(t), or half delta(t), the force already jump to the next point in the arc.
27. ### B Work and Friction: How Does Fxv Apply?

Suppose you're a point on the floor and a block mass slide pass you. The block mass said " I have friction acting on me during my motion so the mechanical power is friction x my speed". When ask the point on the floor, it said "well, I don't have motion so there is no mechanical power". Yet...
28. ### B Can a static force do work?

And I said the motion of the point of application of the force is also important and gave you example which you said irrelevant.
29. ### B Can a static force do work?

Thanks guys. I just don't get where I misunderstood things. Of course I would continue to prove or disprove myself when I have a chance. My thought is instead of a tension force pulling on the wheel, you can imagine yourself running and pushing on the same point on the axle. Now compare...
30. ### B Can a static force do work?

The only solution to our argument is to let the block drop a height h, after height h, measure the rotional energy of the wheel, the translational energy of the wheel, and the kinetic energy of the weight. If it add up to the drop height potential, then I submit.
31. ### B Can a static force do work?

You're only half correct. Consider the case of a block mass moving through a flat friction surface. Yes, there is the motion of the block at the point of application of the friction force, a relevant quantity. But, there is also the motion of the point of application of the friction force...
32. ### B Can a static force do work?

Here I drew the the system. The cylinder moved a distance D at a later time. The force (in red) moves the object for a distance D. The weight on the right also displaced a smaller distance d. work done by the weight = mgd work done on the cylinder = mgD
33. ### B Can a static force do work?

Not really, if you think a force moving through space, both ends have the same force moving through different space length. That's why the string wound up, to compensate for the translation movement.
34. ### B Can a static force do work?

Let's say I was pulling the string. I felt a tiny displacement. So my work done is the force I pull times the displacement my hand made. On the cylinder end, the same force carry by the string, but the displacement is many times higher. The work done is then force time big displacement...
35. ### B Can a static force do work?

I can understand spring does work, but string doesn't store energy. Work is force x distance, so one end of the string the displacement is zero. The other end of the string, it displaced with the cylinder through space. But what is the source of energy?

37. ### B Unlocking the Mystery of Blackbird: Exploring Wind Power Leverage

I meant we can gear it to turn the blades clockwise or counterclockwise. Of course one direction will cause the cart to go faster than the wind.
38. ### B Unlocking the Mystery of Blackbird: Exploring Wind Power Leverage

I think what intrigued me about this is that I often think when the wind hit a windmill, the blade turn with the wind. It baffles me when the blade turns the opposite of the wind. So here I drew up a system of how we can make the blade turn any direction. we have blades and a large wheel...
39. ### B Why is angular momentum not conserved in this case?

Suppose we have the mass and string orbiting like the outer ramp. Then we pull the string to achieve the inner ramp orbit. We can say that the the axis never changed. Can we then make the ramps exactly as the path of the string and consider the axis never changed?
40. ### B Why is angular momentum not conserved in this case?

#2 also stated the force is always orthogona. If torque = Frsin(theta) and theta always zero, then torque is always zero?
41. ### B Why is angular momentum not conserved in this case?

We can make the ramp smooth. The question is there isn’t any torque acting on the system, yet angular momentum not conserved.

44. ### B Space propulsion without mass ejecting

When the pulse is given at A, the box move to the right. When it is absorbed at B, the box stop.
45. ### Determine the final train speed (mass and slinky on a train)

It should. That's why I say there is no speed change in the train.
46. ### Determine the final train speed (mass and slinky on a train)

initially the two mass stretched the slinky, after m1 release, the slinky contract toward the second mass. nevermind
47. ### Determine the final train speed (mass and slinky on a train)

the stretched slinky, when unstretched, provides the force. It's resembled. The second mass still move in circle even when the first mass disconnected because pressure wave need time to reach the second mass.
48. ### Determine the final train speed (mass and slinky on a train)

yes, slinky are masses and springs connected in series. What I meant by delay momentum is slinky action redistribute the center of mass without an external force. If you see a slinky drop from a rooftop, the top part fall down faster, which means the center of mass change as it drop. After...
49. ### Determine the final train speed (mass and slinky on a train)

We have two masses hitting the train north, so the speed increase. However, after rethinking about the slinky mass (since mass is require for a force) has momentum component heading south. The overall speed of the train remain the same. But the interesting thing I'm thinking is can we...
50. ### Determine the final train speed (mass and slinky on a train)

A, the train speed up?
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