Buoyancy Chapter 3.4

What is buoyancy? There is some force balancing the object in the water. Gravity is pushing down… Buoyant Force: the upward force on objects in a fluid

Think about it… Why are you able to carry something heavy (like a person) in water but not on land? Because of buoyant force, objects seem lighter in water. For example, it is easier to lift a heavy rock in water than on land because the buoyant force pushes upward on the rock, reducing the net force you need to lift it.

Think about it… Why would an object like the ping pong ball float back to the surface after you push it under the water? Remember that in a fluid, pressure increases with depth. There is greater pressure acting on the bottom of the ping pong ball than on the top of it. The pressure difference between the top and bottom of the ball produces a net force that is pushing the ball upward.

Buoyancy and Density Whether or not an object floats in a fluid depends on the densities of both the object and the fluid.  If an object is less dense than the fluid it is in, the object will float. If an object is floating in a liquid, the buoyant force is balancing the weight. If an object is more dense than the fluid, the object will sink. A sinking object has a weight that is greater than the buoyant force on it.

Answer these questions in your notebooks

The motion of a fluid affects its pressure A faster-moving fluid exerts less pressure as it flows over the surface of an object than a slower moving fluid.  For example, wind blowing over a chimney top decreases the pressure at the top of the chimney. The smoke from the fire is more effectively pulled out of the top of the chimney.