Transcript Chapter 4 Force

Chapter 4
Force and The Laws of Motion
Physics
Teacher: Mrs. R. Williams
Background

Sir Isaac Newton (1643-1727) an
English scientist and
mathematician famous for his
discovery of the law of gravity
also discovered the three laws of
motion. He published them in
his book Philosophiae Naturalis
Principia Mathematica
(mathematic principles of
natural philosophy) in 1687.
Today these laws are known as
Newton’s Laws of Motion and
describe the motion of all
objects on the scale we
experience in our everyday lives.
Newton’s Laws of Motion
1. An object in motion tends to stay
in motion and an object at rest
tends to stay at rest unless acted
upon by an unbalanced force.
2. Force equals mass times
acceleration
(F = ma).
3. For every action there is an
equal and opposite reaction.
SECTION 4.1--Force
◦ Definition: Force is the cause of an acceleration or the
change in an object’s velocity
◦ A force can be a push, a pull, and pressure; force can
act directly through physical contact or at a distance
like a magnetic field.
◦ We measure force in NEWTONS.
 A newton is the force needed to accelerate a 1
kilogram mass by 1 meter per second squared
1N = 1kg x 1m/s2
Newtons

◦ Newtons ARE NOT units of weight or mass!
◦ Weight: What you weigh is a measure of your mass on the
planet earth. Your weight can change relative to your
environment! Mr. Price weighs 200 pounds on earth, but on
the moon he would weigh only 33 pounds!
◦ While your weight can change your mass is constant
regardless of your environment. Although Mr. Price would
weigh 33 pounds on the moon his MASS would still be 200
pounds on the moon, Jupiter or in a black hole!
◦ Newtons are a measure of force—we can use newtons to
measure how much force the earth is exerting on your
body (your weight) but newtons are units of force!
◦ Conversion Factors
 1 pound = 4.448 newtons
 1 newton = 0.225 pounds
Forces


Two Types of Forces
1. Contact Forces: The force that arises
from the physical contact of two objects.
2. Field Forces: The force that can exist
between objects even in the absence of
physical contact (like gravity and magnetism)
The 4 Fundamental Forces of the Universe
◦
◦
◦
◦
Gravity
Electromagnetism
The Strong Nuclear Force
The Weak Nuclear Force
Force Diagram

Force Diagrams
◦ FORCE IS A VECTOR with both a magnitude
and a direction
◦ Forces are represented by arrows which indicate
the direction of the force. The length of the
arrow indicates the magnitude of the force.
◦ Diagrams are often used to analyze situations
where more than one force is acting on an object.
These are known as “force diagrams”
◦ Simple force diagrams of single objects and the
forces acting on them are called “free-body
diagrams.”
Section 4-2—Newton’s First Law
◦ Inertia: “An object at rest remains at rest, and an
object in motion continues in motion unless the
object experiences a net external force.”
◦ Acceleration is determined by net external force.
 Newton’s first law implies that the net external forces on
an object (with a constant velocity) must be equal to
zero.
 “net external forces” means the sum of all of the
individual forces—add those vectors!
 The Greek letter sigma (Σ) is used to designate a sum.
What does this mean?
Basically, an object will “keep doing
what it was doing” unless acted on
by an unbalanced force.
If the object was sitting still, it will
remain stationary. If it was
moving at a constant velocity, it
will keep moving.
It takes force to change the motion
of an object.
What is meant by unbalanced
force?
If the forces on an object
are equal and opposite,
they are said to be
balanced, and the object
experiences no change in
motion. If they are not
equal and opposite, then
the forces are
unbalanced and the
motion of the object
changes.
Some Examples from Real Life
A soccer ball is sitting at rest. It
takes an unbalanced force of a
kick to change its motion.
Two teams are playing tug of
war. They are both exerting
equal force on the rope in
opposite directions. This
balanced force results in no
change of motion.
Newton’s First Law
◦ Mass is a measurement of inertia
 The inertia of an object is proportional to its mass—
bigger objects are harder to move and harder to stop.
 Don’t confuse “size”, “mass” and “density”
◦ Equilibrium
 Equilibrium exists when the net forc3es on an object
equal zero.
 Equilibrium can occur when an object is at rest or moving
with a constant velocity.
Section 4-3--Newton’s 2nd and 3rd Laws
Newton’s 2nd Law: Force = Mass x Acceleration
ΣF = ma
 For objects at equilibrium a=0
Newton’s 3rd Law: For every action there is an equal
and opposite reaction.
 Newton’s third law implies that forces always exist in
pairs.
 We often split these pairs up into the action force and the
reaction force, which are always have the same magnitude
but opposite directions.
 Action/reaction pairs can cancel each other out which
results in equilibrium.
 Action/reaction pairs can also accelerate objects (see
action/reaction handout)
◦ Field Forces also Exist in Pairs
Section 4-4—Everyday Forces
◦ Weight
 “Weight” is defined in physics as the
force of gravity exerted on a mass.
Weight has magnitude but no direction
making it a scalar quantity.
 Remember the force of gravity is g =
9.81m/s2
 This value changes inversely to the
distance from the center of the earth
(you weigh less at higher altitudes!)
THE NORMAL FORCE
◦ The Normal Force
 The “Normal Force”: A force exerted by one
object on another in a direction perpendicular to
the surface of contact. (“The Floor Force”)
 *The mathematical definition of normal is
“perpendicular”.
 The normal force is always perpendicular to the
surface of contact…NOT always opposite the
force of gravity.
 Fn = m g cosΘ
FRICTION
◦ The Force of Friction
 Whenever two objects come into contact some energy is
lost to friction.
 Friction is defined as the resistive force that opposes the
relative motion of two contacting surfaces.
◦ Two Types of Friction
 Static Friction is the friction experienced by two or more
objects that are in contact and at rest.
 Kinetic Friction is the friction experienced by two or
more objects that are in contact and are moving relative to
each other.
 Kinetic friction is less than static friction
 Friction must always be considered in order to
exactly calculate net forces.
Force of Friction
◦ The force of friction is proportional to the normal force
(i.e. more massive objects experience more friction and a
larger normal force)
◦ Friction depends on the surfaces in contact (i.e. smooth
surfaces experience less friction than rough surfaces.)
◦ The Coefficient of Friction (the Greek letter mu –μ)
 The coefficient of static friction (μs) is the ratio of the
maximum value of the force of static friction to the
normal force.
 The coefficient of kinetic friction (μk) is the ratio of the
force of kinetic friction to the normal
μ = Friction force
Normal force