Transcript ISNS4371_011607_bw

ISNS 4371 - Phenomena of Nature
ARISTOTLE - 350 B. C.
Developed laws of motion based on force producing a velocity in a
body. No force, no velocity.
GALILEO - 1564-1642
Developed laws of
motion discordant with
those of Aristotle.
Force produces an
acceleration.
ISNS 4371 - Phenomena of Nature
A New View of Nature
Sir Isaac Newton (1642 - 1727)
- followed Galileo’s lead - developed fundamental laws of motion
- revolutionized mathematics and science
- experienced moment of inspiration at 24 years old - saw apple
fall from tree and suddenly understood gravity
- published most famous book in science in 1687 - Philosophiae
Naturalis Principia Mathematica - Principia for short
- built first reflecting telescope
- invented calculus
ISNS 4371 - Phenomena of Nature
DEFINITIONS (CONTINUED)
INERTIA: Resistance a body offers to a change in
its state of motion.
MASS: A measure of a body's inertia
MOMENTUM: Mass x velocity
ISNS 4371 - Phenomena of Nature
Newton’s First Law
A body remains at rest or moves along a straight line with
constant velocity so long as no external force acts upon it.
I.e., things tend to keep on what they are already doing.
Also called the law of inertia.
ISNS 4371 - Phenomena of Nature
Examples:
Pulling a table cloth out from under a
table setting
The reaction of coffee in a cup when
accelerating or decelerating in a car
Tightening of a hammerhead by
banging hammer on the ground
Getting ketchup out of a bottle
Not wearing a seatbelt during a headon car crash
Headrests in a car to prevent whiplash
during a read-end collision
ISNS 4371 - Phenomena of Nature
Galileo demonstrated that all objects accelerated at same rate
regardless of mass - supposedly dropped balls of different mass
from Leaning Tower of Pisa
Gravity and Acceleration Animation
ISNS 4371 - Phenomena of Nature
Acceleration of Gravity
All objects in a gravitational field fall at a constant acceleration - g regardless of mass
On Earth - g = 9.8 m/s2 or 32 ft/s2
On the Moon - g = 1.63 m/s2 or
1/6th that of Earth
Remember: distance = 1/2at2
So,
to calculate height of building drop a rock and time
its fall - h = 1/2gt2
ISNS 4371 - Phenomena of Nature
Hammer and Feather on the Moon
ISNS 4371 - Phenomena of Nature
The Pendulum
The Pendulum can be used to calculate g
P = 2(L/g)1/2
P is the period and L is the length of the pendulum
g = 42L/P2
This formula will be derived later when we discuss conservation of energy
ISNS 4371 - Phenomena of Nature
Newton’s Second law
A body (m) acted upon by a force (f) will accelerate (a) in the
direction of the applied force. The greater the force or the smaller
the mass, the greater will be the acceleration .
F = ma
ISNS 4371 - Phenomena of Nature
Newton’s 2nd Law
F = ma
or
a = F/m
Pushing Cart Animation
ISNS 4371 - Phenomena of Nature
Mass and Weight
•INERTIA: Resistance a body offers to a change in
its state of motion.
•MASS: A measure of a body's inertia - mass resists
acceleration
A = F/m
Acceleration is inversely proportional to mass
•WEIGHT: Gravitational force on a body—
Proportional to its mass.
Mass is not weight!
Weight is as force - the force of gravity.
W = F = ma = mg
1 lb = 4.44 N
ISNS 4371 - Phenomena of Nature
1 kg on the Earth weighs 9.8 N or 2.2 lbs
F = W = mg
W = 1kg X 9.8 m/s = 9.8 kg m/s = 9.8 N
Newton’s 2nd Law Explains the Feather and the Ball
Take a 1 kg rock and a 10 kg rock and drop them from the same height
a1 = F1/m1 = W1/m1 = 9.8 N/1 kg = 9.8 m/s = g
a2 = F2/m2 = W2/m2 = 98 N/10 kg = 9.8 m/s = g
ISNS 4371 - Phenomena of Nature
Apparent Weight
apparent weight - weight force that we actually sense not the downward
force of gravity, but the normal (upward) force exerted by the surface we
stand on
- opposes gravity and prevents us falling to the center of the Earth
- what is measured by a weighing scale.
For a body supported in a stationary position, normal force exactly
balances earth's gravitational force
- apparent weight has the same magnitude as actual weight.
If no contact with any surface to provide such an opposing force - no
sensation of weight (no apparent weight).
- free-fall - experienced by sky-divers and astronauts in orbit
who feel "weightless" even though their bodies are still subject to
the force of gravity - also known as microgravity.
A degree of reduction of apparent weight occurs, for example, in elevators.
In an elevator, a spring scale will register a decrease in a person's
(apparent) weight as the elevator starts to accelerate downwards. This is
because the opposing force of the elevator's floor decreases as it
accelerates away underneath one's feet.
ISNS 4371 - Phenomena of Nature
Apparent Weight Animation
ISNS 4371 - Phenomena of Nature
Friction is a Force That Affects Motion
Ff
Fg
FN
W

The force due to friction, Ff, is equal to the normal force, FN.
The block will slide when the friction force is equal to the component of the
gravitational force in the direction along the plane, Fg.
From simple trigonometry:
Ff = Wcos()
Where  is the coefficient of friction
Fg = Wsin()
So the block will slide when Wcos() = Wsin()
So
 = sin()/cos() = tan()
The larger  is the larger  must be for the block to slide