I'm doing first year physics, and we've just stared a unit on Fluid Mechanics Our lecturer showed us a demonstration of suction, lifting up a table by attaching a rubber suction matt type device, with a handle on it. The point of the demonstration was to show that even though the rubber suction device didn't weigh much, the difference in air pressure between the normal atmosphere of the lecture hall and the air trapped beneath the suction cup was so great that considerably pore force had to be applied to break the suction. Now, my understand is, this is due to the fact that while the atmospheric pressure is exerting a downwards force on the suction object, due to the scarcity/low density of air trapped beneath it, that there is less upwards force being applied to the bottom of the suction cup, so the net force that one must overcome to lift it is greater. My question is, what is it about fluid that exerts a force upwards, or indeed, in any direction not aligned with gravity? The downwards acting force of the atmosphere I get, because it has mass and is under the force of gravity, but an upwards force exerted on an object I do not understand. What acceleration is happening at the surface of a fluid!? Sorry if I have explained this for me, let me know if you need clarification.