Fasten your belts, part II

Classical Mechanics Level pending

A horizontal platform has radius r r and is spinning with angular velocity ω \omega in relation to an inertial frame. Suppose ω r < < c \omega r << c . According to general relativity, any accelerated frame of reference may be compared to a frame of reference that is subjected to a gravitational field. Then, if F ( r ) \overrightarrow{\mathrm{F}}(r) is the centripetal force an object feels while on the platform, it may be associated with a potential field φ ( r ) \varphi(r) . So

Δ t ( r ) Δ t ( 0 ) A + φ ( r ) B c 2 \frac{\Delta t(r)}{\Delta t(0)} \approx A + \frac{\varphi(r)}{Bc^{2}}

where Δ t ( r ) \Delta t(r) is the time that passes for an observer at a distance r r from the center of the platform.

What is A + B A + B ?


The answer is 2.

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1 solution

According to Einstein

Δ t Δ t 0 = 1 1 v 2 c 2 1 + v 2 2 c 2 \frac{\Delta t'}{\Delta t_{0}} = \frac{1}{\sqrt{1- \frac{v^{2}}{c^{2}}}} \approx 1 + \frac{v^{2}}{2c^{2}}

if v < < c v << c . As v = ω r < < c v = \omega r << c , we'll be able to use this approximation. Now, of course Δ t = Δ t ( r ) \Delta t' = \Delta t(r) must be a function of r r , and Δ t 0 = Δ t ( 0 ) \Delta t_{0} = \Delta t(0) . Then the above equation becomes

Δ t Δ t 0 1 + ( ω r ) 2 2 c 2 \frac{\Delta t'}{\Delta t_{0}} \approx 1 + \frac{(\omega r)^{2}}{2c^{2}}

Now

F = m v 2 r = m ω 2 r = r V ( r ) = m r φ ( r ) \left | \overrightarrow{\mathrm{F}} \right | = \frac{mv^{2}}{r} = m \omega^{2} r = \frac{\partial }{\partial r} V(r) = m \frac{\partial }{\partial r} \varphi(r)

where V ( r ) V(r) is the potential energy associated with the force (and all forces have a "gravitational potential field" associated with it, according to Einstein). Solving for φ ( r ) \varphi(r) will give us

φ ( r ) = ( ω r ) 2 2 \varphi(r) = \frac{(\omega r)^{2}}{2}

And, of course,

Δ t Δ t 0 1 + φ ( r ) c 2 \frac{\Delta t'}{\Delta t_{0}} \approx 1 + \frac{\varphi(r)}{c^{2}}

Therefore A + B = 2 A + B = 2 .

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