An object is made of glass and has the shape of a cube 0.11 m on a side, accordi

Question

An object is made of glass and has the shape of a cube 0.11 m on a side, according to an observer at rest relative to it. However, an observer moving at high speed parallel to one of the object's edges and knowing that the object's mass is 3.2 kg determines its density to be 7800 kg/m3, which is much greater than the density of glass. What is the moving observer's speed relative to the cube?

Possible answers:

a. 0.951c b. 1.250c c. 0.814c d. 0.713c e. 0.524c f. 0.496c

I keep on getting 0.737c for my answer. Its clearly not right..

Explanation / Answer

let the speed be kc

so,

L=Lo(1-(c/v)^2)^0.5

or x=0.11*(1-(k^2))^0.5

also

7800=m/volume

but m=3.2

so,

7800=m/v

or 7800=3.2/(0.11^3*(1-k^2)^(1.5)

the above equation has the solution k=0.737 c

so my friend you are 100% correct

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