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dpsguy
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Its been bugging me for a while now-can pseudo forces do work? Is the calculation of amount of work done dependent on our frame of reference?
The difference between inertial (pseudo) and "real" forces is that real forces have agents (something that exerts the force) while inertial forces do not. Inertial forces are simply due to viewing things from a non-inertial reference frame. There is no reaction to an agentless force!dpsguy said:But do pseudo forces really exist? I mean, a force should have an equal and opposite reaction,no? Now consider a small block placed on top of a large frictionless block. The large block is accelerating.If we analyse the situation from the frame of reference of the small block ,we use a pseudo force in a direction opposite to the acc.,so that we can apply Newtons laws.But where is the reaction to this pseudo force?
And if the force does not exist,how can it do work?
From the frame of the water/observer, the turbine (and the Earth itself!) is rushing towards him with plenty of energy! And the speed of the water with respect to the Earth will change when it hits the turbines, of course. If you want to analyze the interaction from the inertial frame moving with speed v, you must stick to it. (Since the water is really accelerating as it falls, this is a messy frame to use.)As for the work done being dependent on the frame of reference, I have just one question.In a hydroelectric plant water falls from a great height from a dam and causes the blades of a turbine to rotate. Suppose water falls with a terminal vel. v.If an observer falls along with the water,(assuming that he does not die on falling!),the water is at rest with respect to him and hence its energy does not change.So how would he explain the turning of the blades given that he knows the turbine is fixed to the ground and cannot come up to meet the water. Where would the blades get the energy from if the water does not lose any?
Then gravity is a pseudo-force ... because for someone in free fall gravitational force doesn't exist (GR). I'm not sure of me that's why i asked it before but i didnt get any answer :(Inertial forces are simply due to viewing things from a non-inertial reference frame.
Sorry for ignoring your question, but I was purposely trying to stick to classical mechanics and avoid GR considerations. While you are right that gravity is a "pseudo-force" in a GR context, you can't simply treat it like an ordinary inertial force (like centrifugal force) within classical mechanics.BioBen said:Then gravity is a pseudo-force ... because for someone in free fall gravitational force doesn't exist (GR). I'm not sure of me that's why i asked it before but i didnt get any answer :(
cyrusabdollahi said:It seems that I wasent all that far off in my reasoning then, right doc al? Could you explain the problem I faced when I came to your conclusion. If we look at the water falling at speed v, the m in mv^2 is small, but if we look at the earth-wheel system, the m in the mv^2 is HUGE. This frame seems to have too much energy associated with it. We would expect the water to be vaporized upon contact, since so much energy is being transfered. Is the total energy generally conserved from one frame to another, or does it change? Also, does the massive kinetic energy somehow get reduced by a similarly huge potential energy?
Not at all. (I hope you realize that the amount of fuel used cannot depend on the frame of reference!) Calculation of work and kinetic energy does depend on frame of reference. Why don't you work through that example with the car and see for yourself.dpsguy said:Also,Doc Al, by saying that fuel needed does not depend on the RF, are you not contradicting what you said earlier-that work depends on our reference frame?
Doc Al said:Not at all. (I hope you realize that the amount of fuel used cannot depend on the frame of reference!) Calculation of work and kinetic energy does depend on frame of reference. Why don't you work through that example with the car and see for yourself.
That's the problem I asked you to figure out! To do that, you must consider that the work done on the car (by friction of the road) is different in different frames. And the fact that work is done on the earth. Give it a shot.dpsguy said:Sorry,Doc Al, but I don't see your point.Suppose x l of fuel are used in accelerating from 0 to V. Then 3x l are used up in going from V to 2V.If our RF is moving at a vel. V,then the increase in KE is only 0.5m(V^2). But we know that 3x l of fuel have been used. How would the observer explain this anomaly?
How can kinetic energy be independent of RF? Speed (obviously) depends on the reference frame.Also, in the turbine example,the final KE of the turbine would be the same irrespective of the RF(obviously?).
When I get some time later I will post comments on your water/turbine example. But it's a similar issue as with the car.Hence whether we work in the RF of water or the turbine,the work done on the turbine will be same(see mathematical approach).Please correct me if I am wrong.
Doc Al said:That's the problem I asked you to figure out! To do that, you must consider that the work done on the car (by friction of the road) is different in different frames. And the fact that work is done on the earth. Give it a shot.
How can kinetic energy be independent of RF? Speed (obviously) depends on the reference frame.
When I get some time later I will post comments on your water/turbine example. But it's a similar issue as with the car.
The rotational KE of the turbine with respect to its axis will not be frame dependent. (But the translational KE of a turbine blade will be.)dpsguy said:Say the electricity produced in the plant is proportional to how fast the turbine rotates.If the KE of turbine is dependent on the RF isn't the electricity produced too?
dpsguy said:Sorry,Doc Al, but I don't see your point.Suppose x l of fuel are used in accelerating from 0 to V. Then 3x l are used up in going from V to 2V.If our RF is moving at a vel. V,then the increase in KE is only 0.5m(V^2). But we know that 3x l of fuel have been used. How would the observer explain this anomaly?
Eaxctly! When applying Newton's 2nd law to calculate [itex]F \Delta x[/itex], realize that you must include the "work" done on all the objects involved (Earth and car) and that [itex]\Delta x[/itex] is frame dependent. (For example, in the Earth frame the Earth doesn't move.)jtbell said:You have to take into account the change in KE and momentum of the earth, produced by the reaction force of the tires against the road. In the second reference frame, the Earth's KE changes by a larger amount than in the first reference frame.
No, pseudo forces are only apparent forces that arise due to the non-inertial reference frame. They do not have a physical origin and can only be experienced by an observer in a non-inertial frame. Therefore, they cannot perform any physical work.
Pseudo forces are different from actual forces in two main ways. Firstly, pseudo forces do not have a physical origin and only exist in a non-inertial reference frame. Secondly, pseudo forces do not transfer energy or cause any physical displacement of a body, whereas actual forces can do work and cause physical changes in a body.
No, pseudo forces cannot be measured or detected directly as they do not have a physical origin. They can only be observed and calculated by an observer in a non-inertial frame.
Yes, pseudo forces follow Newton's laws of motion just like actual forces. However, they are not real forces and do not have a physical origin. They only appear to exist due to the acceleration of the reference frame.
Yes, pseudo forces can affect the motion of an object in a non-inertial frame. However, they do not cause any physical changes in the object and cannot perform any work. The object's motion is still ultimately determined by the actual forces acting on it.