Question

A body hanging from a spring stretches it by 2 cm at the earth's surface. How much will the same body stretch the spring at a place 800 cm above the earth's surface? Radius of the earth is 6400 km.

Answer 1

Spring stretched due to act of it gravitational force . let if mass of spring is m then Fg = mg° ( weight )

e.g Fs = Fg

Kx = mg° ----------(1)

but we know,

g is varies with height from earth surface. e.g

g = g°/( 1 + h/r)²

where h is height from the surface of earth .

at h = 800 km , and r = 6400 km

g =g°/( 1+ 800/6400)²

=g°/( 1 + 1/8)²

=g°/(9/8)²

=64g°/81

now, spring stretched act by gravitational force = m64g°/81

F"s = F"g

Kx" = 64mg°/81 --------(2)

equation (1) and (2)

x/x" = 1/(64/81)

x" = 64x/81

hence 64/81 times stretched spring at h = 800 km

e.g stretching of spring = 64