UNIT 1 - ENERGY & MOTION
Module
2
LESSON 1.5 - INERTIA
Overview
This lesson deals with inertia as a measure of mass and
the tendency of inertia to oppose acceleration. We also
discuss the difference between mass and weight. On
completion of the lesson, you should be able to explain
the difference between momentum and inertia. You should
also be able to calculate the weight of an object from
its mass.
ToDo
Watch the video presentation.
Carry out the activities.
Read through the lesson notes and do the exercises.
Refer to the solutions and check your answers.
At home: Prepare for Lab 1.5 by reading the instructions
and collecting the
necessary materials and equipment.
Prepare for the two activities in Lesson 1.6.
ACTIVITIES
Inertia Ball:
Break a piece of string above or below a heavy object by
pulling firmly or rapidly on the lower piece of string.
Marble Launcher: Use an elastic band to launch
different sized marbles across a carpet. See how mass
affects the distance covered by the marble.
Slingshot: Use slingshot with different
projectiles to see the effect of mass on maximum range
Activity
1.5.1 INERTIA BALL
Purpose: To show how inertia affects the amount of force
needed to accelerate an object.
Equipment :
- An Inertia Ball: This
can be purchased from a laboratory supply company
although any object with a mass of 500 grams or
more can be used. A method of attaching pieces of
string to apposite sides of the object must be
employed. Some string or wire wrapped around the
object may be sufficient.
- Three pieces of
string. The string should be strong enough to
support the object but should break quite easily
when a slightly larger force is applied.
Activity
Attach two pieces of string to the object.
Use one of the pieces of string to suspend the ball from
a suitable anchor point.
Pull gently on the piece of string below the object until
the string above the object breaks.
Repeat the experiment but this time, pull rapidly on the
lower piece of string. In this case, the piece of string
below the object should break.
Question
Why does the piece of string below the object break when
a force is applied rapidly?
Activity
1.5.2: MARBLES ON THE CARPET
Purpose: To show how inertia affects an object while
accelerating.
Equipment :
A board about 30cm x 30 cm with a V cut into it and two
nails hammered into the board near the upper ends of the
V. Stretch a piece of elastic or a large elastic band
between the two nails.
Activity
Turn the board upside down with the nails resting on the
floor. Pull the elastic towards the bottom of the V and
use it to launch different sized marbles across a carpet.
Start from the same position each time and note the
effect of the mass of the marble on the distance it
travels.
Question
Do the bigger marbles roll more easily on the carpet than
the smaller ones. How does this affect the results of
your experiment?
Activity
1.5.3: SLINGSHOT
Purpose: To show how inertia affects an object while
accelerating.
Equipment :
Slingshot and a number of items (such as marbles) with
different masses that can be launched from a slingshot
Activity
Select an area like an open field where it is safe to
launch objects with a slingshot. Ensure that no one is in
the firing range. Try to shoot each object as far as
possible and note whether there is any relationship
between the mass of an object and the distance it can
cover when launched from a slingshot.
Inertia
The inertia of an object is a measure of its mass and it
is its tendency to resist changes in velocity. The
greater an object’s inertia, the greater the force
needed to achieve a particular change in velocity.
Newton’s First Law
An object that is at rest will tend to stay at rest and
an object that is in motion will tend to continue its
motion in the same direction with the same speed unless a
force or set of unbalanced forces acts upon it. This is
sometimes referred to as Newton’s law of inertia.
Essentially, a force is needed to get a stationary object
to move. Once moving, a force is needed to change the
speed or direction of the object.
Momentum
Momentum involves velocity as well as mass. The momentum
of an object is the product of its mass and its velocity.
Momentum = mass x velocity.
Weight
The weight of an object is the force that a gravitational
field exerts on the object. In the SI system of units,
weight is measured in Newtons. Because the force of
gravity only varies slightly from place to place on the
earth, scales are often calibrated to indicate the mass
of the object being weighed.
Gravity
Gravity is a force that results from a force field that
exists around every object. The gravitational force
between two objects depends on the masses of the two
objects and their distance apart.
At the earth’s surface, 1 kilogram weighs 9.81
Newtons. At a distance of 10,000 km above the earth’s
surface, 1 kg weighs 1.5 N.
Gravitation on the moon
1 kilogram weighs roughly 1.6 N on the moon.
. .
. . . Mm
F = G ------ . . . . .Force
= mass x acceleration
.
. . . . .d 2
Where: F = force
between two masses, M and m (N)
M = mass of first object (kg)
m = mass of second object (kg)
d = distance apart (m)
G is the gravitational constant = 6.7 x10-11 (Nm2/kg2)
Momentum = mv
Where: m = mass (kg)
v = velocity (m/s)
Example 1.5.1
Weight on the moon
Estimate the weight of a space vehicle with a mass of
1000 kg when it is on the surface of the moon.
Solution
1 kg weighs roughly 1.6 N on the moon. The weight of 1000
kg would therefore be 1600 N.
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Review
Questions
- What is the
difference between inertia and momentum?
- How is inertia
defined? What is the SI unit of inertia?
- What is the
difference between mass and volume?
- What is the
difference between mass and weight? What is the
mass of 1 kg on the moon?
- State Newton’s
first law of motion.
- What does the
gravitational force between two objects depend
upon?
- What is the weight of
1 kilogram at the Earth’s surface?
- What is the weight of
1 kg on the Moon?
- If two astronauts
decide to practice throwing and catching a
baseball while outside their spaceship, what
difficulties might they encounter?
HOMEWORK
Select one or more of the recommended
activities for Lesson 1.6, collect the items needed and test the
procedure before demonstrating the activity during the
next theory lesson.
.
Momentum: Momentum
is inertia with velocity. It is the product of the mass
of an object and its velocity.
Volume: The
quantity of space that an object or body occupies.
Weight: The force
on an object due to gravity.
Gravity: A force
that results from a force field that exists around every
object.
g:
The symbol for the acceleration due to gravity at the
earth’s surface. g = 9.81 m/s2 (meters
per second squared)
g: The
gravitational field vector. The gravitational force on an
object in a vertical direction towards the earth. At the
earth’s surface this is equivalent to 9.81 N/kg.
Vector quantity: A
quantity that has magnitude and direction.
Vector: A line,
arrow or set of coordinates that represents a quantity
and its direction.
- No speed is needed
for an object to have inertia. Momentum is the
product of an object’s mass and velocity.
- Inertia is a measure
of an objects mass or tendency to oppose
acceleration. The SI unit of inertia is the
kilogram (kg).
- Mass is a measure of
the amount of matter. Volume is a measure of the
amount of space an object or body of matter
occupies.
- Weight is the force
that gravity exerts on a certain amount of mass.
1 kg has a mass of 1 kg anywhere but on the moon
it will have a weight far less than it would have
on earth where the force of gravity is much
greater.
- Newton’s First
Law: An object that is at rest will tend to stay
at rest and an object that is in motion will tend
to continue its motion in the same direction with
the same speed unless a force or set of
unbalanced forces acts upon it.
- Both of their masses
and their distance apart.
- 1 kg weighs roughly 9.81
N at the Earth's surface.
- 1 kg weighs roughly 1.6
N on the moon.
- Every time they throw
the ball, they will accelerate in the opposite
direction with nothing to stop them – ever.
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