Recent content by lemd

I know it, I just want to emphasis on the difference between positive and negative work parts. OK, so the turbine example did not deliver the message. Let say we have a Carnot engine: 1/ Receive heat 2/ Expand Now, if we stop here, we will get 100 percent efficiency. So why don't we stop here...

Thank you for your brilliant answer. Yes, that why, because it needs to comeback to the initial state. Maybe I should rephrase a better question: why do all heat engines need to work in cycles, why don't just work in a receiving heat process only, so we get 100 percent efficiency? For...

Yes, all, even more than all, isn't it? Carnot cycle expansion includes isothermal and isentropic expansion. The heat is received in isothermal process, which is a constant temperature process, so the internal energy of gas does not change. Because the internal energy does not change, all the...

At least in the expansion part of Carnot cycle, all heat is transferred to work WITHOUT absolute zero, isn't it? It is the compression part of the cycle that does the negative work, if we don't do this part, e.g. stop the engine after its expansion, then all heat is converted to work. Of course...

Every thermodynamics cycle needs to do negative work to the environment, which lower its total positive work. For example, in Carnot cycle, the most efficiency possible: 1/ Engine receives heat from hot reservoir, expands and do positive work to surround 2/ Surround does work to engine...

Let say a plane flights at 1km/s, shoots a 2kg projectile at 1km/s, so to the Earth, the projectile flights at 2km/s with 2*2*2/2 = 4MJ. The same gun when fixed on Earth, shoots the same 2kg projectile at 1km/s, achieves 2*1*1/2 = 1MJ Assume that the energy comes from burning propellant or...

Please see the above picture, I need a mechanism to connect handle to a controlled object below. While the handle moves from a to b (~10cm) the object needs to move from c to d (about 25 - 30 cm) and when handle moves back from b to a, the object needs to moves back from d to c. They are both...

According to relative theory and quantum physics, what is the present moment? How long does it last? What is the meaning of the present tense that human feel in the sense of quantum/ relative physics? Is that we are seeing the Plank time moment to moment?

Yes, I knew that, but the problem is, I can not get it. Imagine a car moving at a constant speed with no slip on tire. So there is only static friction, no kinetic friction. Now the engine accelerates the wheel and the wheel should rotate faster. If this accelerated force is under the static...

Thanks, So let say car speed and wheel speed are not zero, slip ratio is still zero when car speed equals wheel speed, and friction is zero. I can't get it, how can friction be zero?

Most graphs show that when wheel slip is zero then the friction is zero. But I can't get it, as when the vehicle stand still on slope, the friction keeps the vehicle from going down hill Could someone explain for me? http://www.albi-engineering.nl/Electronics/Traction_Control/slipgraph.jpg

Thanks I remember I learned things like that a long time ago. Problem is physics is not my profession and so English is not my language when I learned them. I just want some short answers because I already tried google. So here are something I found but can't confirm they are true or not. When...

Hi I know F=ma is applied when force pushes through object's center of mass. But in case force does not go through center of mass, how will the object react? I think object will rotate a little but don't know how to calculate things such as rotation and translation speed like the case with...

Many thanks for the clarifications

Thanks, I am just curious about normal natural gas stove, as there is a countinous stream of natural gas from the tank to the stove, and there is fire, why doesn't the fire burn back through that fuel stream into the gas tank? Is it because there is no oxygen in the tank?