Topic 6 Buoyancy Buoyancy is the tendency for

Topic 6: Buoyancy

Buoyancy is the tendency for materials to rise and float in a fluid. The upward force exerted on objects submerged in fluids is called the buoyant force. This is a force measured in Newtons (N) Buoyancy allows for the transportation of nutrients through our blood stream, pollen floating in the air and boats and planes moving around in our world.

The “Anti-Gravity” Force Floating occurs when an object does not fall in air or sink in water, but remains suspended in the fluid. Particles of a fluid exert a force opposite that of gravity. Buoyant force Gravity

Sinkers and Floaters If the density of our bodies is both greater than the density of air and water why don’t we sink? Is density the only factor that affects buoyancy? What about shape and design of the object? Figure 1. 34 and 1. 35

Average Density Large ships and aircraft can use average density to measure the total mass of all substances on board divided by total volume. A large hollow hull can ensure that the average density of the ship is less than that of the water. Ex. large cruise ships, oil rigs, fish (swim bladder), submarine, blimp.

Benefits of Average Density Average density is useful because it enables objects that would otherwise sink (large ships, oil rigs) to float Average density also helps floating objects to sink Ex. Swim bladder in fish

Blimps can float because its mass is relatively small in comparison to its enormous volume. Being filled with helium and having an average density that is slightly less than the air around allows for these massive air ships to float.

Archimedes Principle: The buoyant force acting on an object equals the weight (force of gravity N) of the fluid displaced by an object.

Neutral Buoyancy: when gravity equals buoyancy Archimedes stepped into the bathtub and sank because the amount of water he displaced weighed less than he did. He stepped into a boat and displaced a larger volume of water. The weight of this water far exceeded the weight of the boat and Archimedes combined. Therefore the boat has a greater buoyant force than Archimedes alone. F buoyant force = W weight in air – W weight in liquid

How are Buoyancy and Density Related? Objects float more easily in fluids that are more dense (sea water) Saltwater particles are packed together more tightly and can support more weight / volume than fresh water.

How do we measure the relationship between buoyancy and density? Hydrometer: an instrument designed to measure liquid density -If the liquid has a higher density, the hydrometer will rise farther out of the liquid -If the liquid has a lower density, the hydrometer will sink lower into the liquid