Transcript Newton`s Laws and Universal Forces HS Printout

Newton’s Laws of Motion and
Universal Forces
Unit 1
Physical Science HS Credit
Mrs. Rubel
Newton’s Laws of Motion
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Begins Here!!
Great Scientists
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Aristotle- Incorrectly said
force was required to keep an
object in constant motion
Galileo- with no resistance
objects would move
indefinitely
Newton- based on Galileo’s
findings he came up with his
law’s of motion
Newton’s 1st Law of Motion
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AKA The Law of Inertia
Inertia- an objects tendency to resist
changes in motion
 an object at rest will remain at rest, and
an object in motion will remain in motion
in the same direction at a constant
velocity until acted on by another force.
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Remember:
The greater the mass of
an object the greater the
inertia
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Have you heard of the FORCE of gravity?
Gravity: the force that pulls objects towards each
other
Since gravity is a force it also obeys Newton’s
second law
F=ma
Since objects fall at
the same speed, their
acceleration is the
same.
All objects accelerate
at the rate. Here on
Earth the rate is:
Ag=9.8 m/s2
Or
Ag=32 ft/s2
With this experiment, Galileo
proved Aristotle wrong
With this experiment, Apollo 15
astronauts proved Galileo right.
(link to You Tube)
Weight and Mass
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Weight is the force of gravity acting on an object
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Weight (N) = mass (g) x acceleration of gravity (m/s2)
Note: just a version of F=ma, F = mass x gravity
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FYI:
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1 pound = 4.448 Newtons
so 1/4 lb is slightly more that 1 N
so a 1/4 lb burger is a "Newton" burger
Newton’s 2nd Law & Weight
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F=ma
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So, weight is a type of Force
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Remember:
The formula for weight: Weight = mass x Ag
1 newton = 0.22 pounds
2
Since Ag= 9.8 m/s then
Weight = mass x 9.8 m/s2
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Try This:
A bicycle takes 8.0 seconds to accelerate
at a constant rate from rest to a speed of
4.0 m/s. If the mass of the bicycle and
rider together is 85 kg, what is the net
force acting on the bicycle? (Hint: First
calculate the acceleration.)
Newton’s
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rd
3
Law of Motion:
For every action there is an equal & opposite
reaction.
This means every time a force is applied in one direction
an equal force is applied in the opposite direction.
If an object is not in motion, then all forces acting on it
are balanced and the net force is zero!
Action-Reaction Forces
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If you push against a
wall it pushes back
These forces can
cause motion, like a
swimmer
They do not cancel
each other out,
because they act on
different objects.
2 ice skaters
Questions that make you go
Hmmmmmmm….
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1. Why can you exert greater force on the pedals of a bicycle if you
pull up on the handlebars?
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When you pull up on the handlebars, the handlebars push down
on you, and this force is transmitted to the pedals.
2. Lets say you are weighing yourself on a set of bathroom scales.
You are standing next to the sink in the bathroom. If at the same time
while you are standing on the scales you reach under the sink and pull
up on the sink, will the scales register your weight to be more or less
than what they would register if you didn't pull up on the sink?
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the scales would register heavy. Since you are lifting up on the
sink, the sink is pushing down on you with an equal but
opposite force and this would be transmitted to the scales.
Hmmmmmm….
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3. Lets repeat the question in the situation above
except that you push down on the top of the sink
instead of pulling up on the bottom of the sink.
What will your weight be this time compared to
what it would be if you did not push down on the
sink?
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The scales would register lighter. Since you are
pushing down on the sink it is pushing up on you
with an equal force which tends to lift you up off the
scales some and therefore they register light.
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By definition, a Force is a push or a pull.
Just like Velocity & Acceleration
Forces have both
magnitude and direction
components
Natural Forces
There are 3 large natural forces that exist on
Earth:
1) Gravity
2) Friction
3) Air Resistance
Forces
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A force causes an
object to move,
accelerate, change
speed, or direction
Forces are
represented by:
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Arrows
Direction
Strength
represented by the
length of the arrow
Balanced &
Unbalanced Forces
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Balanced forces –
opposite and equal forces
acting on the same object
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result in NO motion of the
object
Unbalanced forces – two
or more forces of unequal
strength or direction
acting upon on an object
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results in motion of the
object
Balanced or Unbalanced Forces?
balanced
unbalanced
Force/Free body diagrams
(to show forces)
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used to show the relative magnitude and
direction of all forces acting upon an object
in a given situation.
a special example of vector diagram
size of the arrow represents
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amount of force
Force / Free-body Diagrams
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direction of the arrow shows the direction which the force
is acting
each force arrow is labeled
The object is usually represented by a box. Force arrows
are drawn from the center of the box outward in the
direction which the force is acting.
http://www.physicsclassroom.com/Class/newtlaws/u2l2c.
cfm#1 (look at types of forces also)
Combining Forces
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Net force: All forces
are added or
subtracted to create
one total force
If net force is Zero
then there is no
change in motion.
If there is a net force
there is an
acceleration
Force Problem
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What is the net force on an object being
pulled toward the west with a force of 30N
and another force pulling the object toward
the east with a force of 75N?
Friction
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All moving objects
encounter friction
Without it most
motion would be
impossible
4 types, static,
sliding, rolling, fluid
Static Friction
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The force that keeps an object from moving
It is the largest frictional force
Always opposite direction of the applied
force
Pushing a Car
Walking
Sliding Friction
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Once the object is in motion it experiences
sliding friction
Opposite direction from applied force
Less than static friction so less force is
needed to keep it in motion
Rolling Friction
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As something rolls, the
object and floor bend
slightly. This bend causes
rolling friction
It is a much smaller force
than static friction
 As much as 1000
times smaller
Allows you to move heavy
objects
Ball bearings reduce
friction
Fluid Friction
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It opposes the motion
in the liquid or gas
Like swimming, it is
hard to move
If you are in the air,
fluid friction is called
air resistance
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At higher speeds it is
very noticeable.
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Force
of
Gravity
A force that acts between 2 objects
an attractive force, pulls objects together
causes objects to accelerate downward
Falling objects, gravity pulls down,
What pushes up?
 air resistance acts in opposite
direction
Terminal Velocity- when Gravity and
air resistance cancel each other out
(velocity of falling body becomes
constant)
Terminal Velocity
Free Fall – Gravity Only
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Skydivers say they are in free fall, but that isn’t
true. The only way to have free fall is when
gravity is the only force working on you (no air
resistance.) You can have free fall in a vacuum.
Here’s the only example of a human being
experiencing free fall to the Earth:
Projectile Motion
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When an object not only goes up and down but
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It is a
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Air resistance and gravity are the only forces
acting on a projectile
What falls faster, an object that falls straight
down, or one that has a horizontal velocity?
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Forces Problems
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1.Two tugboats are moving a barge. Tugboat
A exerts a force of 3000 newtons on the
barge. Tugboat B exerts a force of 5000
newtons in the same direction. What is the
combined force on the barge?
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2.Draw arrows showing the individual and
combined forces of the tugboats in #1.
Force Problems
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3.Now suppose that Tugboat A exerts a force
of 2000 newtons on the barge and Tugboat B
exerts a force of 4000 newtons in the opposite
direction. What is the net force on the barge?
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4.Draw arrows showing the individual and
combined forces of the tugboats in #3.
Force Problem
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5.Could there ever be a case when Tugboat A
and Tugboat B are both exerting a force on
the barge but the barge doesn't move? Draw
arrows showing the individual and combined
forces in such a situation.
Universal Forces
Forces that can be found anywhere
in the universe
Electromagnetic Force
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Electric and Magnetic
Forces are the only forces
that can both attract and
repel
Electric forces- between
charged particle, positive
attracted to negative
Clothes in a dryer
Magnetic forces- N and S
poles, opposites attract,
likes repel
Nuclear Forces
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Two forces a strong and a
weak force hold an atom
together
The strong nuclear force
overcomes the protons
repulsion force
This force is over 100x
stronger than electric forces
The weak force is involved
in radioactive processes
Fusion vs Fission
(see page 113)
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In nuclear fission, a small amount of mass is
converted into large amounts of energy. The
nuclei of atoms are broken apart. Nuclear
power plants used this energy to generate
electrical energy.
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In nuclear fusion, a small amount of mass
becomes much larger when nuclei of atoms
combine. An example would be the Sun, when
hydrogen atoms combine to become larger
helium atoms.
Gravitational Force
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Attractive force between
any two masses
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This force is very small
compared to all the rest
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Newton came of with the Law
of Universal Gravitation,
which explains how we can
find the gravitational force of
any object
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A large mass is required for
gravity to be felt
Gravity is relational to distance and mass
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The further apart two
objects are the less the
gravitational force
The smaller the masses,
the less gravitational
force.
Gravity still has pull over
millions and millions of
miles apart
As distance doubles the
force gets quartered
The Earth Moon System
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The moon stays in orbit
because of Earth’s
gravity
It works very similar to a
centripetal force
It pulls in on an object
So as the moon follows
this circular path, it’s
gravitational force has an
affect of Earth.
Satellites
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Satellites stay in orbit with centripetal force
of gravity
If it were to slow down it would lose
altitude
Satellite motion requires energy for the
satellite to move forward, while orbiting
due to the centripetal force of gravity.