Transcript Newton`s First Law of Motion- Inertia

Newton’s First Law of MotionInertia
Chapter 4
The History of the Concept of Motion
 Aristotle
Fourth Century B.C.
Divided motion into two types
 Natural (gravity)
 Violent (imposed)
 Copernicus
First to interpret astronomical observations as
movement of the Earth around the sun
Worked in secret to escape persecution
 Galileo
Supported Copernicus’ ideas and was placed under
house arrest
Demolished the notion that a force was necessary to
keep an object moving
Newton’s First Law
Called the law of inertia
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 on it
Desk will remain at rest relative to the floor unless a
force moves it
A hockey puck will slide along a frictionless surface
at a constant velocity unless acted on by a force to
make it stop or change direction
Question
If the force of gravity between the sun and
planets suddenly disappeared, what type
of path would the planets follow?
Mass-A measure of Inertia
The amount of inertia an object has
depends on its mass
The more mass an object has, the greater
its inertia and the more force it takes to
change its state of motion
Empty can
Can full of sand
Can full of lead
Mass is not volume
The SI unit of measurement for mass is
the kilogram
Objects can have a large mass and small
volume (more dense) or a small mass and
large volume (less dense)
Which has more mass a car battery or a
pillow?
Which has a greater volume?
Which has more inertia?
Mass is not weight
 Mass is often confused with weight
 Mass is more fundamental than weight
 Mass is a measurement of the amount of
material in a given object
 Weight is a measurement of a gravitational force
acting on an object
Weight depends on an objects location
The same force is needed to shake a bowling ball
on Earth, on the Moon, or in outer space even
though gravity varies
Questions
 Does a 2-kg iron block have twice as much inertia as a
1-kg iron block?
 Twice as much mass?
 Twice as much volume?
 Twice as much weight at the same location?
 Does a 2-kg bunch of bananas have twice as much
inertia as a 1-kg loaf of bread?
 Twice as much mass?
 Twice as much volume?
 Twice as much weight at the same location?
One kilogram weighs 9.8 Newtons
 The SI unit for force is the Newton (kg·m/s2)
 A Newton is equal to slightly less than a quarterpound
 In Newtons (N) a 1-kg object weighs 9.8N
Due to the pull of gravity on objects near the
surface of Earth at 9.8m/s2
 Weight (N) = mass (kg) x acceleration of gravity
1-kg x 9.8m/s2 = 9.8 kg·m/s2
 w=mg
Net Force
In the absence of force, objects at rest
remain at rest and objects in motion
remain in motion
More specifically, in the absence of a net
force, objects do not change their state of
motion
If equal and opposite forces act upon an
object at rest, will the object remain at
rest?
Net Force
 Forces combine to
produce a net force
 It is the net force that
changes an objects
state of motion
 Applied forces in the
same direction are
added
 Applied forces in
opposite directions
are subtracted
Equilibrium
 Equilibrium occurs when the net forces on an
object equal zero
 When an object at the surface of the earth is at
rest, more than one force is acting on the object
 Gravity is one force
 The other is the support force called the normal
force (N)
 A book at rest on a table
The book is pushing down on the table
The table is pushing back on the book with an equal
and opposite force
 The book is in equilibrium
Question
 When you step on a bathroom scale, the
downward force supplied by your feet and the
upward force supplied by the floor compress a
calibrated spring. The compression of the spring
gives you your weight. In effect, the scale
measures the floor's support force. What
happens if you stand on two scales with your
weight divided equally between them? What
happens if you stand with more of your weight
on one foot than the other?
Equilibrium
 Spring scales are used to measure
compression, like in the bathroom scale, and
tension, like when an object hangs from a rope
 Tension occurs when the atoms in the rope are
stretched rather than compressed
 How much tension is in a rope when you hang
from it?
 How much tension is in two ropes if you grab
one with each hand?
 The total tension force upward will balance your
weight, which acts downward
 You will be in equilibrium
Vector addition of forces
When looking at non-vertically orientated
spring scales, vector addition is needed to
determine the tension
Tension and weight are forces and, like
velocity, are vector quantities
They have both magnitude and direction
As the angles increase from the vertical,
the tension in the spring scales will also
increase
Vector addition of forces
The Moving Earth
 Consider a bird resting at the top of a tall tree
and a big, juicy worm on the ground
 If the Earth is revolving around the sun at
30km/s, why doesn’t the ground (and the worm)
zoom by the bird if it flies down from the branch?
 Is the Earth at rest?
 Invoke the idea of inertia to understand this
scenario: the ground, the tree, the worm, and the
bird are all moving at 30 km/s
 All these objects are in motion and will remain in
motion until unbalanced forces act on them
 The bird catches the worm without noticing the
motion of its total environment
The Moving Earth
If Joey stands next to a wall and jumps off
the ground, why doesn’t the wall slam into
him before he lands?
The speed of 30 km/s is the Earth relative to the
sun, not of Joey to the wall
If Katie is flying on a high-speed airplane
and she flips a coin over her head in the
cabin, where will the coin land?
The vertical force of gravity affects only the
vertical motion of the coin