Download Section 12.1, Forces

Section 12.1, Forces
 What
Is a Force?
 A force is defined as a push or a pull
that acts on an object.
–Forces can cause resting objects to
move, or accelerate moving objects by
changing their speed or direction.
–Sometimes forces are easily measured,
like the force of gravity is measured by
weight.
Section 12.1, Forces
 What
is Force?
– Newtons are the units we use to
measure force.
–The abbreviation for the newton is N.
–A newton is the amount of force it takes
to accelerate one kilogram of mass at a
rate of 1 meter per second each second
or (1 m/s2)
–The newton is named for Sir Isaac
Newton.
In this example
we see how the
force of gravity
affects the scale.
As more fruit is
added to the
scale, the force
increases.
Section 12.1, Forces
 What
Is Force?
–In many cases, arrows are used to
represent both the magnitude and the
direction of a force.
Section 12.1, Forces
 Combining
Forces
–Forces moving in the same direction add
together.
Example: People pushing a car. The
force of each person pushing adds up.
–Forces moving in opposite directions
subtract from one another.
Example: People playing “tug of war”
the forces of people pulling on the
rope subtract.
Section 12.1, Forces
 Combining
Forces
–Balanced Forces
When the forces acting on an object
are balanced, the net force is 0, and
there is no change to the objects
motion.
Notice the arrows on the rope between the players.
They are the same length and go in opposite directions.
At this time they cancel each other and the net force is
equal to 0. The forces are balanced.
Section 12.1, Forces
 Combining
Forces
–Unbalanced Forces
The net force of an unbalanced force
is not 0.
When an unbalanced force acts on an
object, the object accelerates.
Net force equals the size of the larger
force minus the size of the smaller
force.
Section 12.1, Forces
 Friction
 Friction
is a resistance to motion by
objects that touch each other.
 There are four types of friction.
–Static Friction – The friction force that
acts on objects that are not moving.
–Static friction always acts in the
direction of the applied force.
–The friction between the ground and
Section 12.1, Forces
 Friction
 Friction
is a resistance to motion by
objects that touch each other.
 There are four types of friction.
–Static Friction
–Sliding Friction
–Rolling Friction
–Fluid Friction
Section 12.1, Forces
 Static
Friction
–It acts on objects that are not moving.
–Example – the friction that occurs
between the ground and the sole of
your shoe which keeps you from sliding
each time you take a step.
Section 12.1, Forces
 Sliding
Friction
–The force that opposes direction of
motion of an object as it moves over a
surface.
–Sliding friction is less than static friction
because once the object starts moving,
it takes less force to keep it moving.
–Example: Sliding an object across a
table top.
Section 12.1, Forces
 Rolling
Friction
–When a round object rolls across a flat
surface, both the object and the surface
are slightly bent out of shape. This
change at the point of contact is called
rolling friction.
–Example: A bowling ball rolling down
the lane of a bowling alley.
Section 12.1, Forces
 Fluid
Friction
–The force of fluid friction opposes the
movement of an object through a fluid
substance.
Remember that gases and liquids are
both conisidered to be fluids.
–Example: The resistance to motion of a
submarine moving through the ocean.
–Example: Aircraft experience fluid
friction from Earth’s atmosphere.
Section 12.1, Forces
 Gravity
–Gravity is a universal attractive force
that acts between any two masses.
–Earth’s gravitational force attracts
objects to its center.
–It is important to understand that we
know how gravity works and that it
exists, but we do not know why it
works.
Section 12.1, Forces
Notice how the
supporting force of the
bottom rock equals the
force exerted on the top
rock by gravity. The
opposing arrows are the
same length, so the net
force is 0.
Section 12.1, Forces
 Falling
Objects
–Air resistance and gravity are the two
main forces that affect falling objects.
Gravity causes objects to accelerate
downward
Air resistance acts in the direction
opposite the downward motion and
reduces the acceleration
Section 12.1, Forces
 Falling
Objects (continued)
–Terminal velocity is the velocity a falling
object reaches when it no longer
continues to accelerate.
 At this point, velocity remains constant.
 The force of gravity and the force exerted
by the air resistance become equal.
Notice that the arrow
above “Rocky”,
representing the force
of gravity is longer
than the one below
him, representing air
resistance. What does
this tell you about the
direction “Rocky’s”
flight is taking?
Chapter 12 Quiz
 1.
 2.
 3.
 4.
 5.
A _________ is defined as a push or a
pull that acts on an object.
The units we use to measure force are
__________.
__________ is a universal attractive
force that acts between two masses.
A resistance to motion by objects that
touch each other is called ________.
Name the four types of friction we have
studied in this section.
Sec. 12.2, Newton’s 1st and 2nd Laws of Motion
 Aristotle,
Galileo, and Newton
–These three scientists lived in a time
period that covered over 2000 years,
but each one was instrumental in
developing our modern concepts of
force and motion.
Sec. 12.2, Newton’s 1st and 2nd Laws of Motion
 Aristotle,
Galileo, and Newton
–Aristotle (384 B.C. – 322 B.C.)
Incorrectly proposed that force is
required to keep an object at a
constant speed.
Though it held back the progress of
motion studies, it gave man
something to disprove, which is far
better than having nothing at all.
Sec. 12.2, Newton’s 1st and 2nd Laws of Motion
 Aristotle,
Galileo, and Newton
–Galileo (1564 – 1642)
Studied how gravity produces
constant acceleration.
He concluded that without friction,
moving objects would continue to
move indefinitely (forever).
Sec. 12.2, Newton’s 1st and 2nd Laws of Motion
 Aristotle,
Galileo, and Newton
–Sir Isaac Newton (1642 – 1727)
Newton is often referred to as
England’s greatest scientist.
His work included research in gravity,
motion, forces, light, and telescopes.
Newton even served for several terms
in England’s Parliament.
Sir Isaac Newton
1642 - 1727
Sec. 12.2, Newton’s 1st and 2nd Laws of Motion
 Sir
Isaac Newton – Newton’s First Law
–Newton’s first law of motion: “The state
of motion of an object does not change
as long as the net force acting on the
object is 0.”
–This law is sometimes called “The Law
of Inertia”.
–Put simply, objects at rest tend to stay
at rest, while moving objects continue
to move with the same speed and
direction.
Sec. 12.2, Newton’s 1st and 2nd Laws of Motion
 Sir
Isaac Newton - Newton’s 2nd Law
–Newton’s second law of motion states
that the acceleration of an object is
equal to the force acting on it divided by
the objects mass.
–Acceleration = Net force or a = F
mass
m
Newton’s 2nd law also applies when a
net force acts in the opposite direction
of the object’s motion, causing deceleration.
Sec. 12.2, Newton’s 1st and 2nd Laws of Motion
 Weight
and Mass
 Sometimes we try to use the terms
weight and mass interchangeably, but
this is incorrect.
–The mass of an object is defined as the
amount of matter it contains.
–Weight is the force gravity exerts on an
object (a mass). W = mg
–Or, Weight = Mass x Acc. due to gravity
Notice that in the picture below, the weight of
the object decreases, the farther it moves from
the Earth’s center, but its mass stays the same.
Sec. 12.2, Newton’s 1st and 2nd Laws of Motion
 Weight
and Mass
–It would also be correct to state that
“Mass is a measure of the inertia of an
object and weight is a measure of the
force of gravity acting on an object.
Section 12.2 Quiz
 1.
Newton gave us _______ (a number)
laws of motion.
 2. The motion of an object does not
change as long as the net force acting
on the object is ____ (a number).
 3. The acceleration of an object is equal
to the net force acting on it divided by
its ________.
 4. The force of gravity acting on an object
(matter) is its _________.
Section 12.2 Quiz
 5.
An object has a mass of 10 grams and
is accelerating at a rate of 9.8 cm/s2.
What is its weight?
Getting Started Quiz #5
 1.
Perform the indicated operations in
scientific notation.
9.5 x 1020 + 5 x 1021
(6 x 1015)(3 x 105)
Getting Started Quiz #5
 2.
A gas at a temperature of 27 oC has a
volume of 6 L. The volume decreases to
3 L. What is the new temperature?
 Balance
the following equations and name
the type of reaction taking Place.
–NaOH + CO2
NaCO3 + H20
–MgCl2 + AgNO3
Mg(NO3)2 + 2AgCl
Section 12.3, Newton’s Third Law
 Newton’s
Third Law
–Newton’s third law involves motion and
momentum.
–Newton’s third law states that whenever
one object exerts a force on a second
object, the second object exerts an
equal and opposite force on the first.
Section 12.3, Newton’s Third Law
 Newton’s
Third Law
–Action and Reaction Forces
When you press your hand against the
wall, you exert a force against it. The
wall, in turn, exerts an equal and
opposite force against your hand.
Your hand does not go through or
move the wall and the wall does not
change the position of your hand.
Section 12.3, Newton’s Third Law
 Newton’s
Third Law
–Action-Reaction Forces and Motion
All action-reaction forces do not
cancel. Some examples are:
–The thrust of a jet engine pushing
against the air and moving the plane
forward.
–An oar from a rowboat pushing against
the water and moving the boat forward.
In these examples acceleration occurs
and the net force is not 0.
Section 12.3, Newton’s Third Law
 Newton’s
Third Law
–Momentum
Momentum is an objects mass
multiplied by its velocity.
Momentum = Mass x Velocity
Momentum is expressed in kg•m/s
–Kg = the mass
–m/s = the velocity
Section 12.3, Newton’s Third Law
 Newton’s
Third Law
–Conservation of Momentum
If we view momentum through a
closed system, momentum is
conserved.
–A closed system is one in which the only
active forces are with the colliding cars.
Forces such as gravity, friction, wind
resistance, etc. are not a part of the
system, so aren’t considered in the
calculations.
Section 12.3, Newton’s Third Law
 Newton’s
Third Law
–The Law of Conservation of
Momentum
In a closed system, the loss of
momentum of one object equals the
gain in momentum of another object,
therefore momentum is conserved.
Take friction and wind resistance out
of the system and think of a billiard
ball striking another billiard ball.
Section 12.3, Newton’s Third Law
 Newton’s
Third Law
Section 12.4, Universal Forces
 Electromagnetic
Forces
 Electric and magnetic forces are the
only forces that can both attract and
repel.
Section 12.4, Universal Forces
 Electric
Forces
–Two positively charged particles will
repel each other.
–Two negatively charged particles will
repel each other.
–A positively charged and a negatively
charged particle will attract each other.
Section 12.4, Universal Forces
 Magnetic
Forces
–Magnetic forces act on certain metals,
on the poles of magnets and on moving
charges.
–Two like poles on a magnet repel.
N
N or S
S
–Two opposite poles of a magnet attract.
N
S
Earth’s Magnetic Field In Space
Section 12.4, Universal Forces
 Nuclear
Forces
–Strong Nuclear Force
A strong nuclear force that acts only
on protons and neutrons in the
nucleus
It holds the protons and neutrons
together in the nucleus.
It has a very short range of only about
10-15 of a meter.
Section 12.4, Universal Forces
 Nuclear
Forces
–Weak Nuclear Force
Weaker in strength than the strong
nuclear force.
Also an attractive force that acts over
a short range (about 10-18M).
Section 12.4, Universal Forces
 Gravitational
Force
 Gravitational force is a universal
attractive force that acts between any
two masses.
 Newton’s law of universal gravitation
states that every object in the
universe is attracted to every other
object.
Notice that the
strength of the
gravitational force
depends on the mass
and distance between
objects.
Section 12.4, Universal Forces
 Gravitational
Force
–Gravity Acts Over Large Distances
Gravitational force between two
objects is proportional to their masses
and decreases rapidly with distance.
–As the distance between objects
doubles, their gravitational forces
become ¼ as strong.
–Gravity is the weakest Universal Force,
but the most effective over long
distances.
Section 12.4, Universal Forces
 Gravitational
Force
–Earth, Moon, and Tides
Earth’s moon has the same general
gravitational relationship to Earth as
Earth does to the sun.
The same gravity/inertia relationship
exists between Earth and moon as
exists between Earth and Sun.
Section 12.4, Universal Forces
 Gravitational
Force
 Earth, Moon, and Tides
–The Moon and Sun, both exert a
gravitational force on Earth causing
tides to form.
–How great the “tidal range” (difference
in height between high and low tide) is
depends on how Earth, Moon, and Sun
are alligned.
When the Moon and Sun are at a 90o angle relative to
Earth, their gravitational forces act against each other
producing the lowest tidal ranges.
The Sun and Moon both affect Earth’s tides. When they
are aligned, their combined gravitational forces produce
the widest tidal ranges..
Section 12.4, Universal Forces
 Gravitational
Forces
 Satellites
–The same principles of gravity and
inertia work to keep satellites in orbit
around our planet.
–There are many manmade satellites
around our planet used for purposes
such as weather tracking, defense
communications, mapping, and many
others.
Section 12.4, Universal Forces