Density

1
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)
2
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
3
Solid liquid gas
Density 1000 1000 1
Volume 1 1 1000
Spacing 1 1 10
4
Pressure
weight
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
5
Pressure
6
Example Explain why a person can lie on a bed of
nails but cant 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 liquids pressure
varies with its depth
Experiment C Find out how pressure of a liquid
varies with its density
Tutorial questions 10 to 14
7
From the previous experiment P ?gh where
h is depth
Pressure increases with depth
F PA
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.
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Floating or Sinking
FB- buoyancy
Fu FB Fw if FB - FW object is stationary
Fu FB Fw if FB gt - FW object is rises
Fu FB Fw if FB lt - FW object is sinks
FB Fw is sometimes called the apparent weight
9
For balanced forces there is no change in
movement.
Force due to water - buoyancy
Force due to gravity - weight
Increase weight
Decrease weight Going up
Going down
10

• 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