When a clock controlled by a pendulum is taken from planis to mountain, it becomes slow but a wrist watch controlled by a spring remains unaffected. Why ?

Explanation

Time period of a pendulum clock, `T = 2 pi sqrt((l)/(g))`.
At mountain, the value of `g` is less than that at plains.
So time period of pendulum clock increase. Due to it, the pendulum clock becomes slow.
Time period of a spring clock, `T = 2 pi sqrt((m)/(k))`
when `k` is a spring constant. As the value of `m` and `k` are same on plains as well as on mountains, hence time period of spring clock remains unchanged. that is why spring clock gives accurate time on mountains.