Good Morning! 04/21/23

• Starter: why is it that a 1000 mega gram cruise ship can float when a ½ kg rock sinks? (there are two answers really)

• Today we will be…

– And discussing why and how things float this is the beginning of our new unit on Fluids and Gases.

Why is it that a huge heavy ship can float but a small light rock

sinks?• Answer this question to the best of your

ability in your group.

• The real answer is all about Buoyancy Force.

What is the force?• The force is called the

Buoyancy force, it is the force pushing up on objects keeping them afloat.

Where does it come from?

• The force comes from the liquid itself pushing up on the floating object.

Buoyancy• The buoyant force acts in the direction

opposite to the force of gravity, and that is why you feel lighter in the water.

Buoyancy • The pressure on the bottom of a submerged object is greater than the pressure on the top (pressure increases the deeper you go).

• This is because there is all that water above pushing down to increase the pressure.

• The result of all this is a net force in the upward direction. We call this Buoyant Force!

How much force does it push up with?

• It pushes up with the same force that it would push up to support the water that was there before the object moved it out of the way. So the Buoyancy force is equal to the weight of the water that used to be there.

Buoyancy• Achimedes’ principle states that the buoyant

force acting on a submerged object is equal to the weight of the fluid the object displaces.

How can you increase it?

• The force is equal to the weight of the water displaced (moved out of the way) so if you move more water out of the way then you will have more of a Buoyancy force.

Buoyancy• A solid block of steel sinks when placed in

water. A steel ship with the same weight floats. The difference is the amount of water

that each displaces that is

the important thing.

• A submarine changes the water it displaces by bringing it in or pushing it out this allows it to dive, rise, or float.

To make the submarine rise compressed air is released to push water out of the hull this displaces more water and increase the buoyancy force.

Relating Cause and Effect• As you read, identify the reasons why an

object sinks. Write them down in a graphic organizer like the one below.

Object sinks

Weight is greater than buoyant force.

Object is denser than fluid.

Object takes on mass and becomes denser than fluid.

Object is compressed and becomes denser than fluid.

Causes

Effect

• Please write down the starter below, but before we discuss it we will handout books and a homework assignment on Pressure.

• Starter: Some guy on the street says he has pure gold necklaces for $100. He says they are worth $1000.00 and that this is a “steal”.

• Truth be told, if they are gold they would be worth 1000 dollars.

• How do you KNOW if they are gold, how can you tell?

You are not the first to have face this dilemma, the first

recorded incident was posed to Archimedes.

Archimedes• Hiero, king of Syracuse in the third century B.C.,

gave a jeweler a bar of gold and ordered the jeweler to make it into a crown. When the King got the crown he was suspicious that the jeweler had cheated him by substituted some less precious metal for the gold.

• He measured the mass and found that the crown had the same mass as the gold bar he gave the jeweler, But this didn’t convince him.

• So he asked Archimedes to figure it out how to prove if it was gold or wasn’t gold.

He figured out that the jeweler had snuck in some cheaper metals. How did he do it?

• Oh Yeah, the King won’t let you cut into it or destroy it in any way?

• Want a hint?

• Well think about it two metals can have the same mass but not the same density.

Dude figured it out.• Even though they had the same mass, they

couldn’t also have the same volume unless they were both gold.

• So he put the crown and a bar of gold onto a balance and as expected the equaled out.

• But then he put the whole thing under water and the crown floated up.

• What did that tell him?• Crazy story but true, he realized that the

crown went up because it was had a greater volume and was displacing more water.

Buoyancy Force• Before we can connect why having a larger

volume caused the crown to go up we need to talk about why anything floats up in water.

• So what gives why do things float?

• Think about it like this, before you got into the swimming pool what was in the space that you now occupy?

Buoyancy Force

• Yea, Water, so what was supporting the weight of that water?

• You’re right the other water, so if it could support that weight of water how much of your weight will it support?

• The same amount of weight ! Does that make sense?

Almost there…• So if the water could hold up the weight of the

water that you moved out of the way, It can proved a force up on you that is equal to the weight of the water you moved out of the way.

• So if you move 100 liters of water the water will push up on you with a force equal to the weight of 100 liters.

• It is this force that allows you and boats and everything else to float.

• We call this force Buoyancy Force

The final answer!• Now the question, did the crown gold and the

bar gold have the same Force pushing up on them?

• NO!, so they must have moved different amounts of water out of the way! This means that the goldsmith used a metal that’s mass takes up more space than gold’s mass.

The Final Answer Continued

• The fact that he figured it out is why we remember his name.

• It doesn’t hurt that the story goes…Archimedes made this discover while in the bath and ran through the streets naked screaming “Eureka” which means “I’ve found it”