Pressure & Upthrust & Atmospheric Pressure

What is a fluid?

A liquid or gas.

What is pressure?

The force acting normally per unit area.

What is the unit of pressure?

Pascals (Pa).

How does pressure change when force increases but area stays constant?

Pressure increases.

How does pressure change when area increases but force stays constant?

Pressure decreases.

In what direction does fluid pressure act on a surface?

At right angles (normal) to the surface.

Why do snowshoes reduce the pressure on snow?

They increase the contact area, so the same weight produces less pressure.

How does liquid pressure change with depth?

Pressure increases with depth.

Why does pressure increase with depth?

A greater depth means a taller column of liquid above the point, producing greater pressure due to its weight.

How does density affect pressure at a fixed depth?

A denser liquid produces greater pressure.

Why is pressure greater at the bottom of a swimming pool?

There is a greater weight of water above each unit area at greater depth.

Does the pressure-due-to-liquid equation give total pressure in an open liquid?

No, it gives pressure due to the liquid column; total pressure also includes atmospheric pressure.

What is upthrust?

The resultant upward force exerted by a fluid on an immersed object.

Why does upthrust occur?

Fluid pressure is greater at the bottom of an immersed object than at the top, producing a resultant upward force.

What determines whether an object floats?

It floats in equilibrium when upthrust balances its weight.

What happens if an object's weight is greater than upthrust?

There is a resultant downward force, so it accelerates downwards.

What happens if upthrust is greater than weight?

There is a resultant upward force, so it accelerates upwards.

Why can a large steel ship float even though steel is denser than water?

Its hollow structure gives it a low overall average density, allowing enough water to be displaced for upthrust to balance weight.

What causes atmospheric pressure?

Air molecules collide with surfaces, exerting forces that produce pressure.

What happens to atmospheric pressure as altitude increases?

Atmospheric pressure decreases.

Why does atmospheric pressure decrease with altitude?

There is less air above a surface at greater altitude, so the weight of air above it decreases.

How does air density change with altitude?

Air density generally decreases.

Why is atmospheric pressure greater near sea level?

There is a greater weight of air above the surface, and the air is denser.