If any problems involving circular motion, which way does the acceleration vector point?

Acceleration in uniform circular motion is always towards the center of the circle.

Explanation:

When object revolves in the circle then during the uniform circular motion the speed of the object in circular path must remains constant. So the acceleration in the tangential direction must be zero.

Now we also know that the direction of velocity is continuously changing with time so here the acceleration must be there which will help to change the direction of velocity.

So here the acceleration is always perpendicular to the velocity which is towards the center of the circle which help to change the direction of velocity always and since there is no tangential acceleration so here speed of the particle will remain constant.

d. air pressure decreases as altitude increases.