Liquid Pressure and Upthrust

Liquid Pressure

Liquids also apply pressure on objects. The pressure in a liquid increases as the depth increases.

The closer the water spurt is to the bottom (greater depth), the water spurts out at a higher speed. Also, the further the water spurt is from the bottom (lower depth), the water spurts out at a lower speed.

When you place an object in water, most of the pressure exerted on the object will be from the water molecules surrounding it. This is because the water molecules will collide with the object. However, the weight of water above the object will provide a downward force on the object. The pressure will increase as the object falls deeper into the water because the amount of water above the object will increase.

Calculating pressure

The equation to calculate pressure is:

  • P = Pressure in pascals (Pa)
  • h = Height of column in metres (m)
  • ρ = Density in kilograms per metre cubed (kg/m3)
  • g = Gravitational field strength in newtons per kilogram (N/kg)

Density is an important factor when finding out the pressure, because the denser the liquid is, the higher its mass per unit of volume is. If an object’s mass increases while its volume remains constant, its density will also increase.

Gravitational field strength is another important factor as it determines the weight of a given mass. A higher gravitational field strength results in a greater weight.


When looking at why objects float, it is important to understand upthrust. If an object is partially or completely submerged in water, the surrounding water molecules will exert forces on the object. The object experiences greater pressure from the bottom than from the top.

If the resultant force is upwards, the object is pushed upwards. The force pushing upwards on the object is called upthrust and it is always in the opposite direction to the object’s weight.

For an object to be submerged in a fluid, it must displace some of the fluid. The force of upthrust depends on the weight of the fluid that was displaced by the object.

Floating and sinking

Weight and upthrust

If upthrust was the only factor, then all objects would float. So, we also have to consider the object’s weight. The weight of the object is the downward force that opposes the upthrust.

  • If the force of weight is greater than the force of upthrust, then the object sinks.
  • If the force of upthrust is greater than the force of weight, then the object floats.


We do not need to calculate the upthrust and weight each time. Instead, we just need to know the density of the object and the liquid that the object is in.

  • If the object is more dense than the liquid, then it will sink.
  • If the object is less dense than the liquid, then it will float.

This principle operates on the idea that if the object is more dense than the fluid, it will not displace enough fluid to generate enough upthrust to hold its weight. If the fluid is unable to hold its weight, then the object will sink. The sinking stops once the object reaches the bottom.