1. Mass, Weight and Density
Lesson Objectives:
At the end of this lesson, you should be able to:
State that mass is a measure of the amount of substance in a body.
State that mass of a body resists a change in the state of rest or motion of the body.
State that a gravitational field is a region in which a mass experiences a force due to gravitational attraction.
Define gravitational field strength as gravitational force per unit mass.
Recall the relationship weight = mass x gravitational field strength.
Recall the relationship density = mass / volume.

2. Metric System Developed by the French in the late 1700’s.
Based on powers of ten, so it is very easy to use.
Used by almost every country in the world, with the notable exception of the USA.
Especially used by scientists.
Abbreviated SI, which is French for Systeme International.

3. MASS Mass is a measure of the amount of substance in a body.
SI Unit: Kilogram (kg)
Mass cannot be changed by the location, shape and speed of the body.
Large masses are measured in tonnes while small masses are measured in grams.

4. Measuring Mass We will use a triple beam balance and
digital top-loading scale
to measure mass.

5. Examples of Masses of Objects
Mass in kilogram (kg)
Electron
A fine grain of sand
A pea
An apple
A medium-sized car
Earth
Sun
10-30
10-6
10-3
10-1
103
1024
1030

6. Newton’s First Law
An object at rest tends to stay at rest and an object in motion tends to continue in motion at constant speed in a straight line (uniform velocity)…….. unless
an external resultant force acts on it.

8. INERTIA
The inertia of an object is its reluctance to move when it is at rest or to slow down or stop when it is moving.
That is, it is its reluctance to change its state of rest or motion

9. Inertia is ... measured by mass
The more massive an object, the more inertia it has, hence, the larger the force is required to change its motion
Conversely ...

10. Gravitational Field Strength
The gravitational field is a region in which a mass experiences a force due to gravitational attraction.
Gravitational field strength is gravitational force acting per unit mass on an object.
On earth, the gravitational field strength is
10 N kg-1 or 10 ms-2.

11. Volume Volume is the amount of space an object consumes.
The base unit for volume is the liter.
There are two methods for finding volume.

12. Volume of Regular Objects
We can find the volume of box shapes with the formula Volume = length x width x height.
In this case the units would be cubic centimeters (cm3).
So a box 2 cm x 3 cm x 5cm would have a volume of 30 cm cm
5 cm
3 cm

13. We measure volume of irregular objects with a graduated cylinder.
This method is called water displacement
Liquids form a curved surface in graduated cylinders.
Take your reading at the low point of the curve or MENISCUS

14. Water Displacement
Pour 7 ml of water in a graduated cylinder. If a rock causes the level to rise to 9 ml, the rock must have a volume of 2 ml.

15. Weight is . . . Gravitational force acting on an object W = mg
where m is the mass of the object
g is the gravitational field strength.
Weight is measured in newtons (N)

17. Measurement Instruments
Weight is measured using a balance such as the
spring balance
compression balance

18. Differences between Weight and Mass
pull of gravity on the body
has both magnitude and
direction
measured in newtons
changes from place to place
amount of matter in the
body
has only magnitude but no
measured in kilograms
is constant regardless of
place or location

19. Story of a naked man
In the first century BC the Roman architect Vitruvius related a story of how Archimedes uncovered a fraud in the manufacture of a golden crown commissioned by Hiero II, the king of Syracuse. The crown (corona in Vitruvius’s Latin) would have been in the form of a wreath. Hiero would have placed such a wreath on the statue of a god or goddess. Suspecting that the goldsmith might have replaced some of the gold given to him by an equal weight of silver, Hiero asked Archimedes to determine whether the wreath was pure gold. And because the wreath was a holy object dedicated to the gods, he could not disturb the wreath in any way.

20. Story of a naked man
The solution which occurred when he stepped into his bath and caused it to overflow was to put a weight of gold equal to the crown, and known to be pure, into a bowl which was filled with water to the brim. Then the gold would be removed and the king’s crown put in, in its place. An alloy of lighter silver would increase the bulk of the crown and cause the bowl to overflow.

21. DENSITY Q) Which weighs more:- A kilogram of feathers or a kilogram of iron?

22. Density is the Mass per unit Volume
What is Density?
If you take the same volume of different substances, then they will weigh different amounts.
Wood
Water
Iron
1 cm3
1 cm3
1 cm3
IRON
0.50 g
1.00 g
8.00 g
Q) Which has the greatest mass and therefore the most dense?
Density is the Mass per unit Volume

23. Density Equation: = m V m V  Mass Density = Volume Example:
g or kg m V 
Mass
Density =
Volume
gcm-3 or kgm-3
cm3 or kg3
= m V
Example:
Q) Liquid water has a density of 1000kgm-3, while ice has density of 920kgm-3.
Calculate the volume occupied by 0.25kg of each.
V = m = 0.25 = m3 
V = m = 0.25 = m3 

24. DENSITY OF A REGULAR SOLID
Find the Mass of the solid on a balance.
Measure the three lengths and calculate the Volume.
(ie V = l x w x h )
Calculate the Density.
m = 240 g
2.0 cm
 = m = 240 =10.0 g/cm3 V
3.0 cm
4.0 cm

25. Density Formula Wheel
Formula wheels make it easy to solve density problems.
Cover the property you are trying to find, and do what is left over.
To find density, cover the word density. You have mass over volume remaining. So divide mass by volume to find density
Density
Volume

26. Material
Mass
in g
Length
in cm
Width
Height
Volume
in cm3
Density
in g/cm3

27. DENSITY OF AN IRREGULAR SOLID
m = 360 g
 = m = 360 =12.0 g/cm3 V
Find the Mass of the solid on a balance.
Fill the Measuring Cylinder with Water to a known Volume.
Add the Object.
Work out the Volume of Water that is displaced.
Calculate the Density.
80 cm3
50 cm3

28. DENSITY g/cm3 Aluminium 2.70 Iron 7.86 Brass 8.50 Wood 0.50 Slate 2.80
Glass
2.50
Lead
11.3
Marble
2.70
Wax
0.89

29. DENSITY OF AN IRREGULAR SOLID
OR use a Eureka Can to find the Volume.
Find the mass of the solid on a balance.
Add water until just overflowing.
Place a Measuring Cylinder under the spout.
Add the Object.
Collect the Water and read off the Volume.
Calculate Density
m = 440 g
40.0 cm3
 = m = 440 =11.0 g/cm3 V

30. DENSITY g/cm3 Aluminium 2.70 Iron 7.86 Brass 8.50 Wood 0.50 Slate 2.90
Glass
2.50

31. DENSITY OF A LIQUID  = m = 20 =1.00 g/cm3 V 20
Mass of Liquid = Mass of Measuring Cylinder and Liquid – Mass of empty Measuring Cylinder
Find the Mass of an empty Measuring Cylinder.
Add a certain Volume of Liquid.
Find the Mass of the Measuring Cylinder and Liquid
Calculate the Mass of Liquid.
How?
Calculate Density of Liquid.
45.0 g
25.0 g
20.0 cm3
45 – 25 = 20 g
 = m = 20 =1.00 g/cm3
V 20

32. Mass of empty Measuring Cylinder
Liquid
Mass of empty Measuring Cylinder
in g
Mass of Measuring Cylinder and Liquid
Mass of Liquid
Volume
in cm3
Density
in g/cm3

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Forces and Fluids