Density
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.
Density = mass ÷ volume
=m÷V
 - rho
Density is measured in kg/m3 or kgm-3.
It is a scalar quantity ( mass and volume are both scalar)
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
SAQs 90 to 93
Purple book Chp 3.1
Density Qu 1 to 9
Solid
liquid
Density 1000
Volume 1
Spacing 1
1000
1
1
gas
1
1000
10
Pressure
Which would sink into mud
further?
weight
Pressure = force ÷ area
P= F ÷A
Pressure is measured in N/m2. It is a scalar quantity.
1 N/m2 = 1 Pascal = 1 Pa
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.
Examine a Bourdon Gauge and draw a labelled diagram of it.
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
From the previous experiment
P = gh
F = PA
where h is depth
Pressure
increases
with depth
Sidewise forces cancel out.
Up force due to water
pressure is bigger than
downward force.
Upthrust or buoyancy is the
difference in the upward and
downward forces.
Buoyancy equals the weight
of the displaced liquid.
Floating or Sinking
FB- buoyancy
Fu = FB + Fw if FB = - FW object is stationary
Fw - weight
FB
Fu = FB + Fw if FB > - FW object is rises
Fw
FB
Fu = FB + Fw if FB < - FW object is sinks
Fw
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
Increase weight
Going down
Decrease weight
Going up
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.
3. Calculate the apparent weight of a 1kg mass of volume 150ml.
Then try it out.
Tutorial questions 15 to 20
SAQ pages 20 to 26
Purple book Chp 3.8