Density

= m ÷ V - rho

Density = mass ÷ volume

Density is measured in kg/m3 or kgm-3.

It is a scalar quantity ( mass and volume are both scalar)

Density - to do with the mass of each atom and how tightly they are packed together.

Each material has its own density which can be used to identify it.

You have 4 activities to do

the density of a solid

the density of a liquid

the density of a gas (air)

how the density of solids liquids and gases compare

Tutorial questions Density Qu 1 to 9

SAQs 90 to 93

Purple book Chp 3.1

Solid liquid gas

Density 1000 1000 1

Volume 1 1 1000

Spacing 1 1 10

Pressure

Pressure = force ÷ area

P = F ÷ A

Pressure is measured in N/m2. It is a scalar quantity.

1 N/m2 = 1 Pascal = 1 Pa

Which would sink into mud further?

weight

weight

Pressure

Example: Explain why a person can lie on a bed of nails but can’t on a single nail.

Note: it is good to use an equation in an explanation

Experiment A: Find your pressure on the floor when you stand on one foot.

Experiment B: Find out how a liquid’s pressure varies with its depth

Experiment C: Find out how pressure of a liquid varies with its density

Tutorial questions 10 to 14

Examine a Bourdon Gauge and draw a labelled diagram of it.

From the previous experiment

P = gh where h is depth

Pressure increases with depth

Sidewise forces cancel out.

Up force due to water pressure is bigger than downward force.

F = PA

Upthrust or buoyancy is the difference in the upward and downward forces.

Buoyancy equals the weight of the displaced liquid.

FB- buoyancy

Fw - weight

Fw

FB

Fw

FB

Fu = FB + Fw if FB = - FW object is stationary

Fu = FB + Fw if FB > - FW object is rises

Fu = FB + Fw if FB < - FW object is sinks

Floating or Sinking

FB – Fw is sometimes called the apparent weight

For balanced forces there is no change in movement.

Force due to water - buoyancy

Force due to gravity - weight

Going down Decrease weight Going up

Increase weight

1. An object with mass 0.5 kg and density 4000kg/m3 is totally immersed in water. Water density 1000 kg/m3

What is(a) its volume(b) the apparent weight.

2. A hot air balloon is tethered to the ground. Draw a free body diagram showing the forces on it.

SAQ pages 20 to 26

Purple book Chp 3.8

3. Calculate the apparent weight of a 1kg mass of volume 150ml.

Then try it out.

Tutorial questions 15 to 20