Question

the value of acceleration due to gravity is less on the top of the mountains than on plains . give reason

Answer 1

this is because acceleration due to gravity decreases(-ve) when we move upward from the earth and increases(+) when we move towards the earth. that's why it's less at the top of mountains than plains.or √ atmospheric pressure is also a reason behind this decrease !

hope it helped a little !
thank you !

Answer 2

This happens with Mountains too, also slowly, but mountains are formed over millions of years so the adjustment has time to take place. Mountains are formed when two continental plates push against each other, one slides under the other, so both what forms the mountain, and what goes under the mountain is lighter than basalt under the Earth's crust. When you climb a mountain, you're not actually on top of more mass because much of the rock under you is lighter rock. Mountains are actually puffy (I always wanted to say that). If mountains were actually greater mass relative to the (relatively) spherical Earth, then they'd sink into the Earth, which they do to some extent, but, as I said above, Mountains are puffy buoyant compared to the Earth's crust, so the mass is pretty much the same on top of the mountain and at the foot of the mountain.