How difficult would it be for a human to survive on a planet with an atmosphere 2-5 times as thick as Earth's but with gravity like Venus?
Would such an atmosphere be survivable? If yes, then how hard it would be to breathe, move and see/hear on the surface?
Are you talking about a human walking around with or without an Protective Gear? also is the make of the atmosphere the same as earth? 78% Nitrogen, 21% Oxygen and 1% other?
I'm going to presume "thick" is basically density
The density of Air at sea level is 1.225 kg per cubic metre, humans regularly breath at 5x atmospheric pressure (6kg/m3) when they go scuba diving, the density of water is roughly 997 kg/m³ and as a diver goes down 33 feet (10.06 metres) they add an atmosphere of pressure to themselves, the scuba gear they were in, specifically the regulators are designed to allow the correct amount of air from the tank for the pressure they are at. This is why divers must return to the surface very slowly, as they have been breath air at much higher density then their bodies can handle if not at the same atmospheric pressure. Do remember that the equipment is not designed to help them breath at these pressure as much as it is designed to allow them to breath underwater… at these pressures
So at your upper estimate: 5x the density, humans are able to breath perfectly fine, they would be limited slightly in terms of movement, only because there is more air resistance to push against.
However
The issues come from the science behind a planet being like this, for a planet to be the same size as Venus, but 5x the atmosphere, would mean either 5x the gravity, or the composition of the atmosphere being something other than what we have here on earth.
5x the gravity
This would mean the bulk of the core of the Earth which is Iron, would have to be Californium or Einsteinium or some other similar heavy, very radioactive and usually very rare elements, which at these pressures and quantities would probably be close to if not exceeding critical mass in some cases. A core made up mostly of gold or lead would make the planet roughly 4x as dense. With this greatly increased density would come the side effects, the planet would not spin as fast, if it did then it would completely shed its out layers or worse than that, it would have either a much weaker or completely non-existent magnetic field, exposing the inhabitants to solar radiation constantly.
Atmosphere
If the composition was different, then it would mean that certain PPE was required only because something like Krypton roughly is 5x the density of nitrogen, therefore make the atmosphere 5x as dense (Yes… I am generalising again) however if humans where to breath this then they would not be able to effective remove it from their lungs, so would not be able to take a full breath to receive enough oxygen. Among other issues, however there would be some good points krypton, is often used in lighting and lasers… so an electrical arc would be exceptional bright. Imagine a lightning storm on that planet!!! So if it was atmosphere composition then PPE would definitely be a must
If the reason for this happening is “it just is stop asking questions this is science fiction” then the only effect would be increased air resistance, that’s about it
If you're talking about an atmosphere the same composition as our existing one that's at 2-5 times the pressure of Earth's then no humans are will never survive that, oxygen at partial pressures over 0.3bar is toxic the minimum you're talking about is 0.42bar which will kill most people in a matter of days; the top end you're over 1 bar of partial pressure of oxygen and it'll kill you in hours.
But you'd have to muck about with the laws of physics to maintain the same gas mix at those pressures. A real atmosphere will have Hydrogen as a free gas which is toxic to most life as we know it at very low levels. You're also going to have increased levels of certain sulfurous and nitrous compounds since they're more stable at higher pressures breathing those is also going to cause short and long term damage, low altitude Ozone will be an issue that way too. In fact between the oxides of nitrogen, sulfur and oxygen the lower atmosphere will probably be pretty corrosive.
I've said this before in relation to questions about atmospheric composition but it bears repeating: there is no known relationship between the size of a planet or it's composition and the atmosphere it holds onto.
Survivable/Breathing
Yes, humans can handle things like being deep underwater relatively well. Breathing pressurised air should not really be a problem as long as it has the same properties as air on earth (enough oxygen etc.). I cant think of a reason why breathing should be much harder.
Moving
For most human movement air resistance is not really an issue, running will be harder but most things should be fine.
Seeing
That's a tricky one, the short answer is that pressure doesn't affect the basic functioning of the eye and thus you could see just fine. BUT there are a lot of other effects with regards to pressure and environmental effects that might influence sight (think about the effect pressure has on moisture contents in the air).
Hearing
Your hearing is not really bothered by pressure since the pressure in the inner ear is the same as the air outside it. BUT sound would spread quicker in the thicker atmosphere (just like it would in water). AND you will have a squeaky helium voice (due to physics of the standing wave in your throat). Again there are many other factors which are related to pressure which can influence hearing, but these are the most obvious ones for pressure.
