- #1
kent davidge
- 933
- 56
The equation ##x = x_0 + vt + at^2/2## is derived assuming a constant acceleration ##a##. My question is , from what frame is this path ##x(t)## described? Can I use it from a non inertial frame?
Only in special cases and with great care. The initial position coordinate ##x_0##, coordinate velocity ##v## (which you should have written as ##v_0## to indicate that it is also an initial constant and not a function of ##t##) and coordinate acceleration ##a## are frame-dependent, so must be correctly represented in your non-inertial frame; that's the "great care" part. If ##a## is not constant in this frame, then of course the formula will fail; that's the "special case" part.kent davidge said:Can I use it from a non inertial frame?
You would use ##a+g## if the elevator is accelerating upwards - the fictitious force needed to make ##F=ma## work in your non-inertial frame is working in the same direction as gravity, pulling the dropped object towards the floor more strongly.kent davidge said:Ok. So let us consider that the acceleration is constant, what should we do next?
Suppose the frame is that of an elevator going up in Earths gravitational field. The accelerator itself has an acceleration ##a## and gravity gives an acceleration ##g## (assume this acceleration is constant too). Can we use a "resultant" acceleration in this case? That is, would we use ##a - g## in the formula for the particle's path?
I don't know how to do it?Nugatory said:It would be a good exercise to write down the coordinate transformations between the inertial frame in which the surface of the Earth is at rest (you posted this in classical physics so we're considering gravity to be a real force and an object resting on the surface of the Earth to be at rest with zero net force on it) and the non-inertial frame in which the floor of the elevator is at rest, then transform the formula for the trajectory of the object from one frame to the other.
kent davidge said:I don't know how to do it?
May be $$x' = x + v_e t \\ v' = v + v_e + a_e t \\ a' = a + 2 a_e$$ where primes denote the particle's coordinates in the elevator frame and unprimed ones denoted the particle's coordinates in the inertial frame of the Earth. And ##a_e## is the elevator's acceleration.
In the case in consideration, the particle is seen having an acceleration ##g## (from gravity) when it's dropped from the elevator's ceilling, so ##a' = g + 2 a_e##.
But that nasty factor of 2 in front of ##a_e## seems to be telling me that I'm wrong in all of this.
Here's the general process you'll go through. (For this particular problem it's overkill; once you understand the process you can skip the intermediate steps where you formally write down the coordinate transformations, skip straight to writing the trajectory in the primed coordinates as @PeroK just did).kent davidge said:I don't know how to do it?
A non-inertial frame is a reference frame that is accelerating or rotating. In contrast, an inertial frame is a reference frame that is not accelerating or rotating, and follows the laws of Newtonian mechanics.
In a non-inertial frame, the position formula must take into account the acceleration or rotation of the frame. This means that the velocity and acceleration of an object in the non-inertial frame will be different from those in an inertial frame.
The Coriolis effect is the apparent deflection of objects moving in a rotating frame. In a non-inertial frame, the Coriolis effect must be taken into account in the position formula to accurately describe the motion of objects.
No, the position formula is only applicable in non-inertial frames that follow a specific set of rules, such as those described by Newton's laws of motion. For example, a frame that is accelerating at a constant rate can use the position formula, but a frame that is accelerating at a non-constant rate would require a different formula.
The position formula in a non-inertial frame is used in various fields of science and engineering, such as aerospace and navigation. It is also used in the study of planetary motion and the behavior of objects in rotating systems.