Transcript Hon_Physics_ch4_Newtons_laws

Chapter 4
st
Newton’s 1 Law
Aristotle – Believed that in order for an
object to move, a force had to be pushing it.
“All objects have a natural state of rest.
Heavy things have a natural state on
The ground, and very light things have
A natural state in the air, such as smoke.”
Galileo Galilee
Galileo’s experiments:
A) Yes
B) No
Assume the ballfloor friction is
negligible
Which of these
images are or
are not accurate
Galileo’s ideas
Through his experiments, objects in motion want
to stay in motion, the reason objects come to
rest is because of friction. Friction is the force
that opposes motion.
Friction put a wrench in his calculations on motion – he needed a new
form of math – Calculus
Coincidentally, Galileo’s views on motion were taken with a lot of
criticism. Aristotle’s views had been accepted for 2000 years – how
could Galileo be “smarter” than Aristotle???
How would Aristotle view a ball
rolling across the table and slowly
coming to a stop?
How would Galileo say the ball stopped?
A)
B)
C)
Friction makes the ball stop
The ball wants to come to its “natural” resting
state
Gravity makes the ball stop
How does friction work?
Friction from the microscopic level:
The force that opposes motion in the
opposite direction is called friction
When objects are in contact with each
other, microscopic ridges and valleys
create the “resistance” to motion.
Smooth objects have less contact
points than rough objects.
Discuss these in your group
How could you get reduce the time
it takes you to go down a
waterslide?
Why are socks more slippery on the
floor than bare feet?
Why is rubber used for tires instead
of other cheaper materials
(plastic)?
Newton’s 1st Law
At age 24, Isaac Newton came up with his world
changing 3 laws of motion.
The first:
An object continues in a state of rest or in a
state of motion at a constant speed along
a straight line, unless compelled to
change that state by a net force
i.e. An object in motion stays in motion and an object as rest stays at rest unless changed by an outside force.
Concept Quiz

If the force of gravity between the planets
and the sun were to suddenly disappear,
what path would they follow? A
B
C
D
Inertia
Mass – A measure of matter (stuff) that makes up an
object.
Inertia – an objects resistance to change, which is related
to mass (it’s harder to move a train than it is a car).
The inertia of an object IS measured by its MASS.
Newton’s 1st law is sometimes referred to the law of inertia.
A measure of inertia = mass
Which has the most inertia?
A) An empty can
B) Can full of water
C) Can full of sand D) Can full of a vacuum
Will it be just as difficult
to shake a bowling
ball in space as it is
on earth?
A)
Yes
B)
No
Will it be just as easy to “wind
up” a bowling ball in space?
Does a person have less
inertia on the moon?
Does a 2 kg bunch of bananas have 2
times as much inertia as a 1 kg block
of steel?
Mass vs Weight
Mass is different than weight
Mass is the measure of matter in an object
(the number and type of atoms). This is
measured with a balance.
Concept Quiz
Will you have the same mass on the moon?
A) Yes
B) No
SI unit for Inertia and Mass:
Kilogram (kg)
Weight
Weight is the force of gravity on an object.
It is measured with spring scales. A scale
will compress due to gravity.
Weight IS a FORCE.
Weight
Weight is a force! It is measured by taking the
mass of an object multiplied by gravity (and
gravity can change!).
Weight/Force (N) = mass (kg) x gravity (m/s²)
The SI unit for weight is the Newton.
(One Newton is equal to about a ¼ burger)
Concept Quiz
At the doctor’s office, a balance measures
the future Mr. Shaffer to be 255 lbs of
pure solid muscle. What is his weight?
(2.2 lbs = 1 kg)
A)
B)
C)
D)
E)
116 kg
255 lbs
255 kg
1137 N
2499 N
Net Force
The combination of all forces acting on an
object
Equilibrium
When the net forces equal zero. There is no
acceleration.