[News]

News: Welcome to The Oktave Forum.

[sri]

sri

sri

Show sri's last posts.

Show general stats for sri.

• 
• Administrator
• Sr. Member
• Karma: 7
• Posts: 264

We all know that frames of reference are relative in physics. Hence if there is a train A accelerating at 20 km/s2 towards train B which is stationary, train B appears to accelerate at 20 km/s2 towards train A. In the absence of an absolute frame of reference, there is no way for us to know which train is accelerating and which is stationary (if at all).

Now, here is a thought experiment: How to knock down an opponent without even touching him. We know that if the human body is accelerated to forces greater than 4G, it experiences a black out. Hence if there is a stationary opponent somewhere, and since there is no absolute frame of reference, all I need to do is to rush towards him with an acceleration of 4G or more. Because there is no absolute frame of reference, this is equivalent to saying that my opponent is rushing towards me at 4G or more. Hence he will be knocked out without me even touching him!

Cool, isn't it? What is the flaw?

[mandar]

mandar

mandar

Show mandar's last posts.

Show general stats for mandar.

• 
• Global Moderator
• Newbie
• Karma: 2
• Posts: 36

Good one.

I think here is the flaw:

When train B is stationary, even though it may seem like it is accelerating towards train A, it is not doing any work. It is not displaced.

Similarly, when I rush towards my opponent with an acceleration of more than 4G, I am doing the work. Not my opponent. So, even though it can be argued that he is rushing at me (because of the absence of a frame of reference), I will be the one to be knocked out. Because the force of that acceleration displaced me and not my opponent.

[sri]

sri

sri

Show sri's last posts.

Show general stats for sri.

• 
• Administrator
• Sr. Member
• Karma: 7
• Posts: 264

Good one.

I think here is the flaw:

When train B is stationary, even though it may seem like it is accelerating towards train A, it is not doing any work. It is not displaced.

Not really. If you and your opponents are spacecrafts in outer space (with no other references), there is no way of finding out who is getting "displaced" and who is doing work -- there is nothing against which we can refer.

[sanket]

sanket

sanket

Show sanket's last posts.

Show general stats for sanket.

• 
• Global Moderator
• Full Member
• Karma: 4
• Posts: 104

When a physical phenomenon occurs what an observer can hope to observe (and conclude) is only one part of the story: the effect of the physical phenomenon on the environment in which it is occurring. Such observations include displacement, passage of time etc.. However, unbeknownst to the observer, the systems/bodies involved in the phenomenon might be experiencing other effects that might or might not be observable. So, an uninvolved observer cannot make conclusions about the bodies involved without knowledge about them.

When train A is approaching train B, all the observer can see is that there is displacement is space. In the absence of an absolute reference frame, and in the absence of enough knowledge about the bodies involved, there is no way to say which train is accelerating and which stationary. If the observer says it was train B that was accelerating, it cannot be invalidated. But let's say, the initial separation between the two trains is extremely great. Train A starts accelerating towards B, and eventually it runs out of diesel and stops. At this point, there is still no way to say which train was accelerating until a moment ago. But say one can go and check which train ran out of diesel and then conclude which train was in motion.

Similarly, in the knocking out experiment, all that an observer can conclude is that the separation between the two bodies will go on diminishing and eventually become zero. This is the effect of the motion on the environment. The flaw is that the observer is making conclusions about the effect of motion on the bodies themselves without knowledge (or without any way to observe) of such effects.

[sri]

sri

sri

Show sri's last posts.

Show general stats for sri.

• 
• Administrator
• Sr. Member
• Karma: 7
• Posts: 264

Hmm.. clever..

But can we explain the flaw assuming a closed world? Suppose we are talking about hypothetical heavenly bodies, which have no diesel, not any other way to determine what makes it move (i.e. there is no rocket propulsion or any observable entity required to thrust something forward..)

[sri]

sri

sri

Show sri's last posts.

Show general stats for sri.

• 
• Administrator
• Sr. Member
• Karma: 7
• Posts: 264

OK, a quick answer. It is got to do with inertial frames of reference. Relativistic reasoning w.r.t. frames of reference is possible only when the frames are inertial -- i.e. stationary or moving with a constant velocity.

To illustrate this, consider two spaceships in a vacuous outer space (where there is no other frame of reference). If they are both stationary or moving at a constant velocity, objects inside the spaceships will be floating around. There is no net force on them. In such a case it is impossible to know who is moving and who is stationary.

On the other hand, if one of the spaceships is accelerating, objects inside the spaceship will stick to the ground (in the opposite direction of the acceleration). There is a net force acting on the objects. It is possible to now identify which spaceship is accelerating and which is not.

In a frame of reference that is not inertial, Newton's first law will not hold anymore (unless the acceleration of the frame of reference is also taken into consideration). Newton's first law is about inertia -- stationary or constant velocity objects will continue to in their state unless acted upon by an external force. This law will appear to be violated if we view an object in inertia from an accelerating frame of reference.

So the answer is, your opponent won't get knocked out from the Gs if you are accelerating and he is stationary