5.5.4 Upthrust

Upthrust

• When an object is submerged in a liquid, it experiences pressure due to the pressure in the liquid.
• Because pressure increases with depth, the bottom surface of this object will experience more pressure than the top surface.
• Because this bottom pressure is larger, there is a resultant force on the object upwards (remember that pressure and force are related). This resultant force upwards is called upthrust.
• The upthrust is equal to the weight of the water displaced by the object. This is just the weight of the water with the same volume as the object.

• If the upthrust is equal to or more than the weight of the object, the object floats because the forces are balanced.
• If the upthrust is less than the objects weight, the object sinks. This is because floating or sinking depends on the density of the object. Upthrust is the weight of the displaced water. So if the density of this water is less than the object, then the weight of the water is less than the object as well. This means that the upthrust is less than the weight of the object. Hence the object would sink.