Buoyant Force
3 Minutes
Buoyant force explained for kids using a lemon in regular water and salt water to show why objects float or sink. An English science lesson.
- A lemon sinking in regular water but floating in salt water
- How buoyant force and weight determine if objects sink or float
- Mixing normal and salt water so the lemon stays in the middle
What this lesson teaches
Buoyant force is an upward force that acts on an object placed inside water. This force, also called buoyancy, pushes upward against the weight of the object and determines whether it sinks to the bottom or floats on top.
How buoyant force works
Buoyant force acts inside liquid whenever an object is placed in it, pushing upward rather than downward like gravity. If an object's weight is less than this upward buoyant force, the object stays on the surface and floats. If the object's weight is greater than the buoyant force, the upward push cannot support it, and the object sinks to the bottom.
Floating and sinking objects
- Light materials like leaves, sticks, rubber, plastic, and thermocol float because their weight is lower than the upward buoyant force.
- Dense materials such as stones and iron sink because their weight is greater than the upward buoyant force of the water.
- A lemon placed into regular tap water sinks directly to the bottom because its weight is greater than the buoyant force of that water.
Difference between regular and salt water
Salt water has a greater buoyant force than regular water because salt water is heavier. In salt water, the water molecules and the salt molecules are bonded together more tightly. This tighter bonding makes salt water weigh more than plain water. Because salt water has more weight, it provides a stronger upward buoyant force, which allows the lemon to float instead of sinking.
Mixing regular water and salt water
When regular water and salt water are mixed together, the lemon does not sink to the bottom or float to the top. The buoyant force in regular water is lower, which makes the lemon sink, while the buoyant force in salt water is higher, which makes the lemon float. Because of these differences in upward force, the lemon comes to rest right in the middle of the mixture.