P5 - Upthtust and Atmospheric Pressure

What is upthrust?

When an object is submerged in a fluid, the pressure exerts a force on it from every direction, which increases with depth so the force exerted on the bottom is larger than the force acting on the top of the object. This causes a resultant force known as upthrust, equal to the weight of fluid displaced by the object.

What happens when an object less dense than a fluid is placed in the fluid?

It weighs less than the same volume of fluid so it floats.

What happens when an object more dense than a fluid is placed in the fluid?

The weight is greater than the upthrust so the object sinks as the fluid can’t displace enough fluid to equal its weight.

What is atmospheric pressure?

Created on a surface by air molecules colliding with that surface, as altitude increases, the atmosphere gets less dense so there are fewer molecules to collide with the surface so the atmospheric pressure decreases.