Section 1 of 4What Upthrust Is
Any object placed in a fluid feels an upward push from that fluid.
Upthrust
- Upthrust is the upward force exerted by a fluid on an object that is partly or wholly immersed in it.
Origin
- Pressure increases with depth, so the fluid pushes up on the lower surface harder than it pushes down on the upper one.
Size
- The upthrust equals the weight of the fluid that the object pushes out of the way, called the displaced fluid.
Direction
- Upthrust acts vertically upwards, opposite to the weight of the object.
Section 2 of 4Floating or Sinking
Floating or Sinking
Floats
- If the upthrust can equal the weight of the object, the forces balance and the object floats.
Sinks
- If the upthrust, even with the object fully under, is smaller than the weight, the object sinks.
Density test
- An object less dense than the fluid floats, and one denser than the fluid sinks.
Partly submerged
- A floating object sinks until the weight of fluid it displaces equals its own weight.
| Material | Density in g/cm³ | In water (1.0 g/cm³) |
|---|---|---|
| Polystyrene | about 0.05 | Floats high |
| Wood (typical) | about 0.9 | Floats low |
| Iron | 7.9 | Sinks |
Section 3 of 4Reasoning Through an Example
A solid block of volume 0.020 m³ and mass 15 kg is lowered into water of density 1000 kg/m³ (g = 10 N/kg).
1
The weight of the block is W = mg = 15 × 10 = 150 N, acting downwards.
2
If fully submerged it would displace 0.020 m³ of water, whose weight is 0.020 × 1000 × 10 = 200 N.
3
The maximum upthrust of 200 N is larger than the weight of 150 N, so the block cannot stay under water.
4
The block rises until it displaces water weighing 150 N, which happens when 75% of its volume is submerged.
5
The density of the block is 15 ÷ 0.020 = 750 kg/m³, which is less than water, confirming that it floats.
Section 4 of 4Exam Checklist & Tips
1.3.5 – Key Points for the Exam
- Upthrust The upward force from a fluid on an immersed object.
- Displaced fluid The fluid pushed out of the way by an object placed in it.
- Density Mass per unit volume of a substance, in kg/m³.
- Weight The force of gravity on an object, found from mass × gravitational field strength.
Common Mistakes
- Believing heavy objects always sink;
- it is density compared with the fluid that matters, and steel ships float.
- Writing that upthrust equals the weight of the object in every case; this is true only when it floats.
- Forgetting that the object's own weight acts downwards and must be compared with the upthrust.
- Using the volume of the whole object for a floating one, when only the submerged part displaces fluid.