1. Why do objects sink or float?
Buoyancy
Why do objects sink or float?
Why do

2. Buoyancy Why do some things sink, while others float?
When an object is placed in a fluid, the fluid presses on the object in all directions.
Buoyancy is the upward force that a fluid exerts on an object.
Buoyancy determines whether an object will sink or float.
Buoyancy depends on weight, fluid displacement, shape and density.

3. Weight and Fluid Displacement
When an object is put in a fluid, it displaces a certain amount of fluid.
The weight of the displaced fluid equals the upward buoyant force (Archimedes’ Principle).
An object floats if it displaces an amount of water greater than or equal to its own weight.
An object sinks if it displaces an amount of water less than its own weight.

4. Shape
An object’s shape affects the amount of fluid it displaces. Therefore, an object’s shape affects its buoyancy.
A screw does not displace much water, so the buoyant force acting on it is very small.
The buoyant force cannot support the weight of the screw, so the screw sinks.

5. Shape
A ship has a large hollow shell and displaces a lot of water, so the buoyant force acting on the ship is large.
The buoyant force can support the weight of the ship, so the ship floats.
Force of gravity
Buoyancy force

6. Density The density of the fluid also affects buoyancy.
Denser fluids exert a greater buoyant force.

7. Upward Lift You Will Need: 2 tall drinking glasses
Water and a container of salt
2 uncooked eggs
Procedure:
Fill both glasses half full with water.
Mix about 10 heaping teaspoons of salt into one of the glasses of water.
Add one egg to each glass of water. (Caution: Always wash your hands after touching an uncooked egg. It may contain harmful bacteria.)
Observations:
Explain what you observed and why you think this happened.

8. Observations
A denser liquid has a greater upward lift or buoyancy. Salt makes water denser.
The egg floats in the salt water because it is less dense than the salt water.
The egg sinks in the fresh water because it is denser than the fresh water.
This is why ships ride higher in ocean water than in fresh water and why it is easier to swim in the ocean than in a lake.