Download Unit 3 Forces

FORCES
The Dynamics of Newton’s Laws
Physics is
Dynamic
 We now shift our focus from
kinematics (a description of
motion) to dynamics (a study of
what causes motion).
 Newton’s Laws are at the heart
of the study of dynamics.
Dynamics is the study of
motion and the forces that
cause that motion.
 The 4 Forces of
Nature
 Gravitational
 Electromagnetic
 Strong Nuclear
 Weak Nuclear
 Common Forces in
our problem-solving
 Weight
 Normal force
 Tension
 Friction
The 4 Forces of Nature
 GravitationalCaused by anything
that has mass. Really
noticeable when there is a large
mass- like a star or planet. It is
the weakest of the forces but
never goes away.
The 4 Forces of Nature
 Electromagnetic- Caused by
electric charges or magnetic
poles. It never goes away, but
is much, much stronger than
gravity.
The 4 Forces of Nature
 Strong Nuclear- Only acts
within a very short distance
such as on the atomic level.
This is the force that holds the
nucleus together. It is the
strongest of the forces.
The 4 Forces of Nature
 Weak Nuclear- Responsible for
radioactive decay. This force has
recently been integrated within
the electromagnetic forces as one
and the same. It is called the
electro-weak force.
Isaac Newton
 Isaac Newton (1642-1727),
mathematician and physicist, is
one of the foremost scientific
intellects of all time. He was born
at Woolsthorpe, in England.
 Newton outlined his Laws of
Motion in his book the Principia,
which included how and why
objects move, the fundamental
force gravity, the motion of fluids,
and planetary motion.
 He invented the mathematics of
Calculus.
 Published Opticks which
explained the phenomena of the
color spectrum and light as a
particle.
Newton’s Laws of Motion
 1st Law of Motion
 An object at rest remains at rest, and an
object in motion remains in motion, unless
acted upon by a net external force
Inertia is directly proportional to an object’s mass
Newton’s Laws of Motion
 2nd Law of Motion
 An acceleration is imparted on a mass by a
non-zero net force. The acceleration is
proportional to the force and in the same
direction as the force.
The acceleration is inversely
proportional to the mass.
 F = ma
Newton’s Laws of Motion
 3rd Law of Motion
 For every action force
there is an equal and
opposite reaction force
The opposing forces are
called action-reaction pairs.
Forces are Vectors
 A force is a push or a pull. Sometimes
there is contact between the objects, but
sometimes there is non-contact, like
gravity.
 Force is a vector. It has magnitude and it
has direction. We represent a vector on
paper with an arrow drawn in the direction
of the force.
Free-Body Diagrams
 A diagram that only shows the forces that
are acting on one object. The forces are
shown by drawing arrows on a
representation of the object.
Shove
Pull
Push
Tug
Problem Solving
The 2nd Law
 Steps in all your dynamics problem-solving
 Draw a free-body diagram.
 Write a F = ma equation for each
dimension, both x and y (horizontal and
vertical.) Pay attention to the signs of the
vectors.
 Solve the equation for the unknown.
 Do any kinematics as necessary.
Units of Force
 The units of force are derived from
the units of mass and acceleration.
 SI units of force:
 Newton (N) = 1 kg•m/s2
 English System unit is the Pound.
Weight
 Weight is the force of gravity that acts on an
object when it is on (or near) a large
planetary-like body.
 Weight = mass x acceleration due to gravity
 On Earth:
 weight = mass x 9.8 m/s2
 Symbolically: Fg = mg Or w = mg
 Weight is always directed toward the center
of the earth which is the bottom of the
page/board.
Weight vs. Mass
 Mass is a universal quantity because the mass
of an object remains the same no matter where
it is measured in the Universe. It is the measure
of inertia that an object has.
 Weight is dependent upon the celestial body that
the object is near. It increases when on a larger
planet and decreases on a smaller one. It also
can change on the earth. The farther you are
from the center of our planet, the less the
object’s weight.
Normal Force
 The normal force FN is the force that a
surface applies to an object that is in
contact with it.
 The normal force is always perpendicular 
to the surface-no matter what the case.
Surface
pushing up on
box
FN
mg
Tension
 Tension is the force that is applied
to an object by a rope, string, cable,
wire, or chain.
 Tension is always directed along
the rope away from the object.
T
mg
Friction
 Friction is the force that opposes motion.
 There is friction present everywhere, however, many
times we consider the case where there is NO friction.
The best we can do is
outer space.
 Types of friction:
 Static friction
 Kinetic friction
 Air resistance
 Rolling friction
 Water resistance
Static Friction vs. Kinetic Friction
 Static Friction
 The force that
opposes an object
before it is moved.
 Static friction
increases with the
amount of applied
force until the object
begins it’s motion
 It’s maximum value
is:
 Fs ≤ sFN
 Kinetic Friction
 The force that
opposes an object’s
motion while it is
moving.
 Kinetic friction is equal
to:
 Fk = kFN
 Friction is dependent
only on , coefficient
of friction, and the
normal force.
Coefficients of Friction
 The coefficient of

friction (static or
kinetic), is a unit-less
number that depends
on the types of
surfaces that are in
contact.
Values are found on reference table
Terminal Velocity
Ff
When the air
resistance equals the
weight of the
parachute and
jumper, the
acceleration stops,
and he reaches
terminal velocity, or
constant speed.
mg
The Elevator Problem
Constant
velocity
Upward
acceleration
Downward
acceleration
Free-fall
In Summary
 All forces are vectors
 The 4 basic forces are gravitational,






electromagnetic, and the strong and weak
nuclear
Newton’s 1st law- Law of Inertia.
Newton’s 2nd Law- F=ma
Newton’s 3rd Law- Action-Reaction
Weight is the force of gravity.
Normal force is always  to the surface.
Friction is a force that opposes motion.