# Evan's Space

## Mass Weight Density Videos

Density: Invisible water

Weight is equal to water displaced

Mass and Weight

Float and sink – Can drinks

Archimedes Principle

How ship floats

Stabilizers in ship

Inertia : ball and strings

## Unsual Behaviour of Less Dense Balloon in Air or Water

Of course if a heavy ball is suspended from the car we all know the ball will move in opposite direction of a decelerating car due to inertia. We are all familiar to this where inertia is applied to a body which is denser than the surrounding medium (air or liquid) which is less dense. That’s why this balloon’s behaviour surprises us!

Let’s assume the water molecules are initially moving at constant speed with the tank before the deceleration.  Due to inertia, when the tank decelerates, the water molecules continue its state of motion forward. Hence the water molecules gush to the right side of the tank, displacing (pushing) the balloon to the left. Hence the balloon moves to the left!

Refer to this Youtube video by Smarter Everyday and you can see the similar experiment of helium balloon (less denser) in the air (denser) of a car.

## Will the whole lake be frozen during winter? At what temperature is water at its max density?

During summer, though the surrounding is hot, the water in the lake will not reach very high temperature mainly due to its high specific heat capacity. In general, cool water sinks as it is denser and warm water rises as it is less dense, the bottom of the lake will always be lower temperature compared to near the surface. The bottom of the lake may not necessary be 4 degree Celsius.

## Bowling ball and feather in a vacuum room – which drops faster?

In a vacuum room, where is no air resistance, the bowling ball will have the same acceleration as the feather. That means to say that both will have the same speed at any time and reach the floor at the same time.

## Finding density of sand

In an experiment to determine the density of the sand, the measurements as shown below are obtained.

If the density of water is 1.00 g/cm3, what is the density of sand?

## Density (Units Conversion) and Floatation

Density is the mass per unit volume.

Common units for density are g/cm3 or kg/m3. SI unit is kg/m3.

Density of a pure substance is fixed, regardless of it’s size!

Floating and Sinking

A denser (heavier) substance will sink in liquid which is less denser (lighter).
A less dense substance will float in a liquid which is denser.
If substance has the same density as the liquid, it will be suspended in the liquid.