A tunnel is dug through the centre of the earth. Show that a body of mass ‘m’ when dropped from rest from one end of the tunnel will execute simple harmonic motion.
A tunnel is dug through the centre of the earth. Show that a body of mass ‘m’ when dropped from rest from one end of the tunnel will execute simple harmonic motion.

Answer:

Let g’ be the acceleration due to gravity of the earth inside the earth

g’ = g(1-d/R)

R-d = y

g’ = g y/R

F = -mg y/R

F is proportional to (-y)

The motion is SHM when the body is in a tunnel.

ma = -mg’

T = 2π √R/g