Buoyancy is the ability of an object to float in a fluid (liquid or gas)
An objects floats when in water if the buoyancy force of the water pushing the object up is greater than the force of gravity pulling the object down.
Weight measures the force of gravity acting on an object.
Buoyancy is a force equal to the weight of water displaced by an object when it is placed in water.
Two Forces Affect Buoyancy
- Force pulling an object in water down. This is the force of gravity, which is the weight of the object.
- Force pushing an object in water up. This is the buoyancy force of water, which is the weight of the water displaced by the object.
A. The purple ball in diagram A is falling through the air toward the colored water in a glass. The water level is marked on the glass.
B. In diagram B, the ball is submerged in the water. Note the second water level mark on the glass shows the water at a higher level. The volume of water in the glass has not changed, yet the water has risen in the glass. This is because the ball takes up space. The volume of the ball is equal to the amount of water the ball displaces. Point out that displace means to push aside.
C. Diagram C show the weight of the displaced water to be 1N. This means the buoyant force of water pushing up on the ball is 1 N.
- How much would the ball have to weigh to rises and floats on the surface of the water? The ball must weight less than the buoyant force pushing the ball up; Weight of ball < 1N.
- How much would the ball have to weigh to remain submerged below the water’s surface? The ball must have the same weight as the buoyant force; Weight of ball = 1N
- How much does the ball weight if it sinks to the bottom of the glass? For the ball to sink, the weight of the ball must be greater than the buoyant force of the water pushing it up; weight of ball >1N
Conclusion: A boat or any object floats on water if the object’s weight is less than the weight of the water displaced by the object.
Real Life Application
Shipbuilding provides Engineering Career opportunities for both men and women. You could be part of a testing program that designs and builds one of the most complex products on Earth: nuclear powered ships.
Engineers apply science, but first they study and understand why things float as well as many other science related topics. “Engineering for Every Kid” introduces different types of engineering and how it relates to science. Easy activities that makes learning fun.