Transcript Newton`s First Law of Motion Inertia

Newton’s First Law of Motion
Inertia
Aristotle on Motion
Divided motion into 2 types:
• Natural Motion- Motion
straight up or down. Objects
would seek their natural
resting places. For example:
A boulder falling toward the
ground or smoke rising into
the clouds.
• Unnatural (Violent)
Motion- Motion resulting
from forces that pushed or
pulled. Motion due to some
external cause. For example:
Cart pulled by a horse or
ship pushed by the wind.
Aristotle on Motion Continued…
• Aristotle also believed
that circular motion was
natural for the heavens.
• He believed that the
planets and stars moved
in perfect circles around
the earth.
• Since the motions were
natural, they were not
caused by forces.
Copernicus and the Moving Earth
• Nicolaus Copernicus
(1473-1543) believed
that the earth and the
other planets moved
around the sun.
• Copernicus worked on
his ideas in secret in fear
of persecution.
• He published his ideas,
De Revolutionibus,
before his death on May
24, 1543.
Galileo
Galileo was a Italian
scientist during the laterenaissance.
Galileo on Motion
• Galileo demolished the notion that a force was
necessary to keep an object moving.
• Force-Any push or pull
• Friction-Force that acts between materials that
touch as they move past each other. If friction
were absent, a moving object would need no
force whatever to remain in motion.
• Galileo argued that only when friction is
present, is a force needed to keep an object
moving.
Galileo Tests His Idea
• He tested his idea by rolling balls along plane surfaces tilted at
different angles.
• Ball rolling down an inclined plane picks up speed and moves in
the direction of gravity.
• Ball rolling up an inclined plane, opposite to gravity, loses speed.
• Ball rolling on a level surface does not roll with or against gravity
and has almost constant velocity. Without friction, the ball would
move forever with no push or pull required to keep it moving.
Galileo Supports His Idea With
Another Line of Reasoning
Galileo Continued…
Galileo Comes to a Conclusion
• Galileo stated that the tendency of a moving
body to keep moving is natural and that
every material object resists change to its
state of motion.
• The property of a body to resist change is
called inertia.
Netwon’s First Law- Inertia
• Newton develops his famous laws at the age
of 24.
• 1st Law – Every object continues in a state
of rest, or of motion in a straight line at
constant speed, unless it is compelled to
change that state by forces exerted upon
it.
Inertia
For Example
• Dishes on a
tablecloth are in a
state of rest.
• They tend to
remain at rest, even
if you snap the
tablecloth out from
beneath them.
Inertia
For Example
• If you slide a hockey
puck along the surface
of a city street, the puck
will soon come to rest.
• If you slide it on the ice,
it slides a longer
distance. (Due to less
friction).
• In the absences of
forces, a moving object
tends to move in a
straight line
indefinitely!
More Examples of Inertia
• Blood rushes from your head to your feet when riding
on a descending elevator which suddenly stops.
• The head of a hammer can be tightened onto the wooden
handle by banging the bottom of the handle against a
hard surface.
• To dislodge ketchup from the bottom of a ketchup
bottle, the bottle is often turned upside down, thrust
downward at a high speed and then abruptly halted.
• Headrests are placed in cars to prevent whiplash injuries
during rear-end collisions.
• While riding a skateboard (or wagon or bicycle), you fly
forward off the board when hitting a curb, a rock or
another object which abruptly halts the motion of the
skateboard.
Mass-A Measure of Inertia
• Kick an empty can and it moves. Kick a can filled
steel nails and it will not move as much and you will
hurt your foot.
• The nail filled can has more inertia that the empty can.
The amount inertia an object has depends on its mass.
• Mass- the amount of material present in an object. If
an object has a large mass, it may or may not have a
large volume.
• The more mass an object has, the greater its inertia
and the more force it takes to change its state of
motion. Mass is the measure of the inertia of an
object.
More Mass = Greater Inertia
Quiz- Which Has More Inertia?
Mass is not Weight!!!
• Mass = the quantity of matter in an object. More
specifically, mass is a measure of inertia, or “laziness”,
that an object exhibits in response to any effort made
to start it, stop it, or change its state of motion. The
mass of an object is the same whether its on the moon,
on earth or in space.
• Weight = the force of gravity on an object. An objects
weight would be very different on the moon, on earth
or in outer space. On the moon, the object’s weight
would be one-sixth than that on earth. In space, the
objects weight would be zero.
One Kilogram Weighs 9.8 Newtons
• Weight = mass ×
acceleration due to gravity.
(W = mg)
• In the US the unit for weight
is pounds. 1-kg bag of nails
has a weight of 2.2 pounds or
9.8 Newtons.
• A Newton is the SI unit for
force named after Sir Isaac
Newton.
Net Force
• Net Force = all the forces acting
on an object.
• When you pull with a force of
10 N, the net force acting on the
object is 10 N.
• If a friend assists you and pulls
in the same direction with a
force of 5 N, then the net force
is the sum of the forces or 15 N.
• If your friend pulls with a force
of 5 N in the opposite direction,
then the net force is the
difference of the forces, or 5 N.
Equilibrium-When Net Force Equals Zero
• What forces act on
your book while it is
motionless on a
table?
• Normal force- Force
perpendicular to
surface. Often, not
always, equals weight
of the object.
Equilibrium Continued…
When you hang from a
rope, the tension of the
rope equals your
weight. The rope pulls
you up while gravity
pulls you down. Since
the forces cancel each
other out, you are
motionless.
Vector Addition of Forces
• The tension is greater
in a pair of nonvertical spring scales
and depends on their
angle from the
vertical.