Skip to content
Know.itKnow.it
Create an animated lesson
On request

Read the answer free. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

Why do some objects float in water while others sink?

Class 9 ScienceNCERT chapter: Gravitationइसे हिंदी में पढ़ें

Exam-ready answer

Whether an object floats or sinks in water depends on its density compared to water. If an object's density is less than that of water, it floats; if greater, it sinks. The upward buoyant force (upthrust) acting on the object decides this, as explained by Archimedes' principle.

Explanation

When an object is placed in water, it experiences an upward force called buoyant force or upthrust. This force is equal to the weight of the water displaced by the object (Archimedes' principle). If the upthrust is equal to or greater than the object's weight, it floats; if less, it sinks.

The density of an object is its mass per unit volume. Water has a density of about 1 g/cm³ (or 1000 kg/m³). Objects like wood (density ~0.6 g/cm³) float because their density is less than water's. Objects like iron (density ~7.8 g/cm³) sink because their density is greater.

Even materials denser than water can float if shaped to displace more water. For example, a steel ship floats because its hollow shape increases volume, lowering its average density below that of water. A solid steel ball sinks because its density remains high.

Points to remember

  1. Buoyant force (upthrust) = weight of fluid displaced (Archimedes' principle).
  2. Density = mass/volume; SI unit: kg/m³ (or g/cm³).
  3. Float if density(object) < density(water); sink if density(object) > density(water).
  4. Average density of a ship is less than water due to its hollow shape.
  5. Relative density = density of substance / density of water (no unit).

Exam tip

While answering, write the formula for density and mention that floating or sinking depends on the comparison of densities. Draw a simple diagram showing the buoyant force and weight.

Questions students also ask

What is buoyant force?
Buoyant force is the upward force exerted by a fluid on an object immersed in it. It acts opposite to gravity and is equal to the weight of the fluid displaced by the object.
Why does an iron nail sink but a ship floats?
An iron nail is solid and has density greater than water, so it sinks. A ship is hollow and has a large volume, so its average density is less than water, allowing it to float.
What is Archimedes' principle?
Archimedes' principle states that when an object is fully or partially immersed in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by it.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.