Why is there vacuum above the level of mercury in a mercury barometer?

Is it possible to use the energy from two atoms colliding to create a clean burning lasting fuel source with a large amount of force? is it also possible to harness that energy from the two atoms colliding to create a clean energy source that will last for years from just two atoms colliding. is there a possibility to control and store that energy for years but slowly let the energy out enough to power cities cheaper and faster.

After a big snowfall, you take your favorite rocket-powered sled out to a wide field. the field is 195 m across, and you know that your sled accelerates at a rate of 3.65 m/s2 when the rocket is on. how much time will it take the sled to cross the field starting from rest, assuming the rocket is on the whole time?

Apulley with mass mp and a radius rp is attached to the ceiling, in a gravity field of 9.81 m/s2 and rotates with no friction about its pivot.