What is the reasons for leaving small gapps between the rails while laying the railway lines ?

What is the fusion line between two similar bones called ?

Two small balls having equal positive charges Q coulomb on each are suspended by two insulating strings of equal length L metre from a hook fixed to a stand. This whole setup is taken in a satellite into space, where there is no gravity (state of weightlessness). What is the angle between the strings? Find out the tension in each string.

The direction cosines of a straight line in two neighbouring positions are (l,m,n)and . If dtheta is a small amgle between them then prove that - (delta theta)^2 = ( deltal)^2 + (deltam)^2 + (delta n)^2 .

What is the angle between the lines x/1 = y/-2 = z/1 and x/4 = y/1 = z/-2 .