FORCES AND FLUIDS

Some things to bring out of the recesses of your mind: Fluids: any matter that has the ability to flow = liquid gas Force: any push or pull exerted on matter Pressure: force exerted on matter by air molecules more air molecules=more pressure less air molecules = less pressure Let’s not forget DENSITY!!! Density is the amount of matter in a given volume DOES NOT CHANGE WITH CHANGES IN SHAPE OR SIZE!!!

Density of water is 1 g/ml The ability of an object to float is dependent on the Object’s density with respect to the density of the Fluid it is in. If an object sinks, its density would be greater than the Density of the fluid. In this case, greater than 1 if we Are placing the object in water. Styrofoam D=0.75g/ml

Buoyancy is the force exerted on an object by a fluid and it usually acts against gravity. This is why objects feel lighter in water. Some of the force of gravity (weight) is cancelled out by the force of buoyancy. If the buoyant force is stronger than the force of gravity, the object will float in the fluid.

Gravity Buoyant Force

Archimedes’ Principle states that the buoyant force of a fluid is equaled to the weight of the fluid displaced by the object. If an object can displace (push away) enough fluid to equal its weight, it will float. This is why a tremendous ship made of iron can float but a cannonball would sink. The ship’s shape allows it to push enough water away to equal the weight of the ship.

5.0 Pounds 0.0 lbs5.0 lbs If the object can push away an equal weight of fluid as its own weight, it will float in that fluid.

10.0 Pounds 0.0 lbs6.0 lbs If the object pushes away a lower weight of the fluid than its own weight, it will sink in that fluid.

The amount of water that was displaced by the floating boat weights the same as the boat itself. According to Archimedes’ Principle, this is why the boat floats and doesn’t sink. Displaced Water

Sinkable Rock If the rock is thrown overboard, how will the water level in the tub change?

Here’s why the water level lowered: Inside the boat the rock (10 lbs) is floating. That means the rock is causing the boat to displace 10 lbs of water to support it. Outside the boat the rock would sink (weighs 10 lbs but only can displace 6 lbs). The rock sinks because it cannot displace an amount of water equal to its weight.

Because 10 lbs of water takes up more space than 6 lbs of water, the water level lowers. Remember, when something floats, it is able to displace the same weight of fluid as its own weight. If it sinks, it means it weighs more than the fluid it is able to displace. WOW, that was a mouthful. Time for me SPINACH!

Press to view Buoyancy BRAINPOP movie