INERTIA AND EQUILIBRIUM
OF FORCES
Inertia
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Galileo, an italian scientist, is considered the father of
modern science since he started doing scientific
experimentation to prove or disprove his ideas.
He showed that moving things, once moving, continued in
motion without the application of forces.
A force is a push or a pull. A force is needed to start an
object moving, but once moving, no force is needed to keep
it moving (except to overcome friction).
A ball moving horizontally would move forever if friction
were absent as a result of inertia.
Inertia is the property by which objects resist changes in
motion.
Taken from Hewitt's "Conceptual Physical Science"
Mass
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Every material object has inertia. How much
depends on amount of matter/mass. The more
matter/mass, the more inertia.
The greater the mass, the greater the amount of
matter, the greater its inertia.
Mass: The quantity of matter in an object, and also
the measure of its inertia.
Weight: The force upon an object due to gravity.
Taken from Hewitt's "Conceptual Physical Science"
Mass and Weight
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Mass and Weight are directly proportional to each
other, or directly related. If you change one, the
other changes proportionally.
If the mass of an object is doubled, its weight is also
doubled. This is why mass and weight are often
interchanged. w = m x g
Mass is measured in kilograms, weight is measured
in units of Force (newtons).
Taken from Hewitt's "Conceptual Physical Science"
Review Questions
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Does a 2 kg iron block have twice as much inertia
as a 1 kg iron block? …yes
Does a 2 kg iron block have twice as much inertia
as 1 kg bunch of bananas?...yes Twice as much
mass?...yes Twice as much volume?...no Twice as
much weight when weighed in same location?...yes
How does the mass of a bar of gold vary with
location? It doesn’t vary.
Taken from Hewitt's "Conceptual Physical Science"
Weight Units are not kg but N
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1 kg of anything on Earth has a weight of 9.8N. If
gravity is less (like on the Moon) the material would
weigh less.
Weight can change with location (different planets),
but Mass doesn’t change.
Taken from Hewitt's "Conceptual Physical Science"
Net Force
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A force is a push or a pull. Objects don’t speed up,
slow down or change direction unless a force acts.
By force we mean the total force, or net force,
acting on an object.
When you kick a soccer ball, the force of gravity,
air friction, and the pushing force you apply with
your muscles all act on the ball.
Net force is the combination of all these forces.
Taken from Hewitt's "Conceptual Physical Science"
Net Force - continuation
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You pull on a box with a force of 5N. If your friend
pulls 5N in same direction, net force is 10 N.
If friend pulls on box with same force but opposite
direction, net force is zero.
If you increase pull to 10N and friend pulls
opposite 5N, net force is 5N in your direction.
Force is a vector quantity, meaning it has magnitude
(how much) and direction (which way). Arrow length
represents magnitude.
Taken from Hewitt's "Conceptual Physical Science"
The Equilibrium Rule
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When the net force on something is zero, we say
that the object is in equilibrium.
Exercise: If you hang from a trapeze at rest, what is
the tension in each of the two supporting vertical
ropes?
Answer: Assuming that the weight of the trapeze’s
seat is negligible, the tension on each rope would
be half of your weight.
Taken from Hewitt's "Conceptual Physical Science"
Support Force
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A book lying at rest on a table is in equilibrium. One
force acting on book is a consequence of gravity, the
book’s weight. As book is in equilibrium there must be
opposite force, of same magnitude.
This upward support force works as a tension force.
Imagine you pushing a spring. The balancing force is
the support force, or tension in the spring.
The weight of book pushing on the table actually
compresses atoms in the table, like microscopic
springs, and the atoms squeeze upward on the book.
Taken from Hewitt's "Conceptual Physical Science"
Review Exercise
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Suppose you stand on two bathroom scales with your
weight evenly divided between the two scales. What
will each scale read? How about if you stand with
more of your weight on one foot than the other?
Answer: Each scale would indicate half of your
weight. If more of your weight is on one side than the
other, that side would show more weight.
Taken from Hewitt's "Conceptual Physical Science"
Dynamic Equilibrium
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When an object isn’t moving, the forces on it add up
to zero- it’s in equilibrium, static equilibrium.
An object moving at constant speed in a straight-line
path is also in equilibrium, dynamic equilibrium.
Once in motion, it there is no net force to change the
state of motion, object moves at same speed (inertia).
In static or dynamic equilibrium, sum of forces = 0.
If you push desk and it moves steadily at constant
speed, the force of friction between desk and force
must be equal and opposite to our pushing force.
Taken from Hewitt's "Conceptual Physical Science"
The Force of Friction
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Friction is a force. A force is a push or a pull. All
forces have two properties: magnitude and direction.
Friction is the resistive force that opposes the motion
or attempted motion of an object when it rubs against
something else.
Friction occurs for solids, liquids, and gases.
Friction always acts in a direction to oppose motion.
When an object falls downward through the air, the
force of friction, air resistance, acts upward.
Frictions always acts in a direction to oppose motion.
Taken from Hewitt's "Conceptual Physical Science"
The Force of Friction cont.
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Amount of friction between two surfaces depends on
kinds of material and how much they are pressed
together.
Friction is due to tiny surface bumps and also to the
“stickiness” of the atoms on the surfaces.
Friction between a sliding desk you’re pushing and a
smooth linoleum floor is less than between desk and a
rough concrete uneven floor. If surface is inclined
friction is less because it doesn’t press as much on the
inclined surface.
Taken from Hewitt's "Conceptual Physical Science"
Types of Friction
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If you push horizontally hard enough on piece of
furniture to match friction of floor, the net force on it
is zero, and it slides across floor at constant speed.
What affects the amount of friction?
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Imagine that a ball is rolled over a carpeted floor and
another ball is rolled over a wood floor . In which case
would the ball move faster? Why?
There are two types of friction. Kinetic friction occurs
when object moves (kinetic means moving). Static
friction occurs when force is applied but object does
not move.
Taken from Holt's “Physical Science"
Changing Friction
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How can Friction be decreased?
To reduce amount of friction you can apply a lubricant
between two surfaces (like oil, grease, and wax).
 You can also roll rather than slide an object.
 You can smooth the surfaces that rub against each other
(skiers have their skis sanded to make them smoother).
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How can Friction be increased?
Increasing friction can be necessary, for example when
talking about the tires of a car so that it can stop easier.
 On icy roads sand can be used to make them less slippery.
 By increasing force (like cleaning a pan or pot).
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Taken from Holt's “Physical Science"