I was reading about why lifting an object in water is easier then in air.

The explanation goes on the line that the water which was above the object will fall downward and sideways, taking the place of the object which was moved.

Now, if we assume the presence of a gravitation field, like earths' one, shouldn't the same logic apply considering air and vacuum?

If this seems legit in a sense, I see it as counterintuitive in another: Would this mean that lifting an object in a denser substance will make it easier?

I don't know what assumption make on the system to prove this statement, and if it could change the answer.

It helps me to think as air (or water) in a limited quantity and in a constrained region, but can't tell if these assumptions would affect teh results.

  • 1
    $\begingroup$ Are you asking whether an object immersed in Earth's atmosphere experiences a buoyant force. If so, then the answer is yes, but the force is much smaller than what the same object would experience if immersed in water. $\endgroup$ – Solomon Slow Apr 3 '18 at 15:15
  • $\begingroup$ Yes I think it's quite the same question, but mine is put in a let's say "basic" way. I had to check up what buoyant force was ahah. I also compared to what would happen in vacuum and asked how it generalize to denser fluids $\endgroup$ – Gabriele Scarlatti Apr 3 '18 at 15:26
  • 3
    $\begingroup$ You say you find it counterintuitive that lifting an object in a denser substance is easier. Perhaps this is because you're thinking of 'denser' as 'more viscous'? If you try to lift an object through a very viscous medium, you have frictional forces to contend with that make your life harder. But viscosity and density are not related. $\endgroup$ – gj255 Apr 3 '18 at 15:28

Compare lifting in water to lifting in air, even with the increased viscosity of the water, the object will still usually feel far lighter. The body also has far less weight acting on it when in the dense fluid.

The classic description of this is Archimedes Principle, where the weight of fluid displaced by the object will be equal to the buoyant force; which acts opposite of gravity. This reduces the apparent weight of the object; because there is a force acting upwards from the water which counteracts some of the gravitational force. For objects less dense than the medium, it counteracts it completely and the object is able to float on the surface.

A helium balloon is an example of something that is less dense than air, and therefore begins to rise even though air has low density.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.