Mechanics
Unit 5: Motion and Forces
5.3 Mass and Weight
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Fundamentals of physics - Mechanics
Mass and Weight
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MASS — A MEASURE OF INERTIA
 If an object is speeding up, slowing
down, or changing course it is undergoing
acceleration.
 It resists changing its motion and this
is called inertia.
 The forces applied to it will determine
how much it accelerates.
The amount of matter in the object
determines its inertia. If we could add
up the masses of all the atoms in the
object that would give us its mass.
 More matter means more inertia.
 The amount of matter an object
contains is called its mass.
The greater the
mass of an object,
the greater its
inertia.
In a material
object, mass is a
measure of its
inertia.
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Fundamentals of physics - Mechanics
Mass and Weight
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MASS AND WEIGHT
 Some people confuse Mass and weight.
 Our experience is that more matter
means more weight.
 But we define each as follows:
 Mass: The amount of matter in an
object. It is also the measure of the
inertia or resistance to:
 starting a stationary object moving
or rotating it.
 stopping it, speeding it up, slowing
it down or change the direction it is
moving.
 Weight: The force upon an object due
to gravity.
... Inertia isn’t
a force...
... all matter
resist changes
in its motion....
... A kind of
laziness?...
Fundamentals of physics - Mechanics
Mass and Weight
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Mass and weight have a direct
proportion. Doubling the mass of an
object doubles its weight and halving its
mass is halves the weight.
In mathematics it is acceptable to
measure the weight (gravitational
attraction to Earth) in objects by their
mass (quantity of matter) but this is
unacceptable in science.
 However mass is a fundamental
quantity that doesn’t depend on gravity.
 Gravity on the Moon, for example, is
1/ that on Earth so you would weigh .
6
 However on Jupiter Gravity on would
be 2.3 times that on earth.
... Remember
weight changes
with gravity... But
mass does not...
Your weight will
change if you go
to other words or
space... But your
mass will be the
same...
Fundamentals of physics - Mechanics
Mass and Weight
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For every pound or kg on earth calculate
your weight on different celestial
bodies:
... A good way to
loose weight...
... If lived on
Mars you will
weigh 2/5 that on
earth... But will
still have the
same mass...
Fundamentals of physics - Mechanics
Mass and Weight
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Weight is measured in a unit called
Newtons (N).
Mass is measured in a unit called
kilograms (kg).
Their relationship is:
Weight (N) = Mass X acceleration
due to gravity (ms-2)
So if your mass is 50kg and
acceleration due to gravity on
Earth is 9.8ms-2.
Then your weight is 50kg X 9.8ms-2
Weight = 450N.
...
Demonstration
of Inertia ...
... The magician
promises that
she can break
the thread at
A or at B at
will...
Fundamentals of physics - Mechanics
Mass and Weight
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Weight is measured in a unit
called Newtons (N).
Mass is measured in a unit called
kilograms (kg).
Their relationship is:
Weight (N) = Mass X acceleration
due to gravity (ms-2)
So if your mass is 50kg and
acceleration due to gravity on
Earth is 10ms-2.
Then your weight is 50kg X 10ms-2
Weight = 500N.
A 1kg chicken
weighs 9.8N
Fundamentals of physics - Mechanics
Mass and Weight
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A slow, steady increasing downward
force breaks the string above the
massive ball at A, whereas a sudden jerk
will break the lower string at B.
Explanation:
The tension on the string with the slow
steady force at B is just the force
applied whereas the tension on the
string at A adds the weight of the ball
to the applied force so since the tension
at A is greater it will break there.
When the string is jerked there is
almost no tension at A as the inertia of
the ball opposes its motion so it breaks
at B...
Fundamentals of physics - Mechanics
Mass and Weight
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This astronaut in outer space (beyond
the solar system where gravity is
negligible) will have to use the same
force as he would on earth to move this
anvil. If the anvil’s mass is 500kg and
the astronaut is 100kg, what do you
think will happen?
What is the
weight of the
astronaut?
What is the
weight of the
anvil?
... On the sun
your weight
would be 28
times that on
earth... ... Better
go there at night
to avoid being
burned...
Fundamentals of physics - Mechanics
Mass and Weight
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Summary of Terms
 Force - a push or a pull.
 Inertia - The ability of
objects to resist changes in
motion.
 Kilogram - The unit of mass.
One kilogram (kg) is the mass
of 1 litre of water at 4oC.
 Mass - The amount of
matter in an object.
Technically, it is a measure of
the inertia.
 Newton - The unit of force.
 Weight - the force of
gravity on an object.
... A block of polystyrene is
bigger than a steel ball...
... Which
has more
inertia?
Which has
more
mass...
Fundamentals of physics - Mechanics
Mass and Weight
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Testing the concept of Mass
and weight...
 An astronaut in outer space
finds the labels of two
identical storage boxes have
fallen off. One label reads log
books and the other pillow
foam.
 How can she determine
which label applies to each box
without opening them?