phy201_4

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Physics 201
4 Classical Mechanics - Newtons Laws

• Force and Equilibrium
• Hookes Law
• Newtons Laws
• Weight
• Friction
• Free Body Diagrams
• Force Problems

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How do we know if a Force is present ? Newton
discovered that it corresponds to changes in
velocity

• Forces are vector quantities
• add together like vectors

The sum of all forces acting on an object Net
force Total force Resultant force
If net force is zero body will stay at rest or
travel at same uniform velocity
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• Equilibrium
• If the velocity of an object is constant, then
  the object is said to be in
• EQUILIBRIUM

• Known types of basic forces
• Gravitational (long range)
• Electromagnetic (long range)
• Strong Nuclear (short range)
• Weak Nuclear (short range)

All known forces can be described in terms of
these forces
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Measuring force
Hookes Law for elastic materials F-kd Restoring
force constant (k) times displacement from
equilibrium position
The magnitude of the force exerted on an elastic
object is equal to the restoring force (when
equilibrium is reached) thus proportional to the
displacement d
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Newtons Laws

• First Law of Motion
• An object at rest will remain at rest and an
  object in motion will remain in motion with
  constant velocity unless it experiences a net
  external force

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• Inertial reference frame
• An inertial frame of reference is one in which an
  object, subject to no force, moves with constant
  velocity.
• A reference frame in which Newtons first law is
  valid is an inertial frame of reference

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• can identify inertial frames of reference with
  coordinate systems moving with constant velocity
• if one finds one such frame, then all frames
  moving with constant velocity with respect to
  this one are also inertial frames of reference
• non inertial frames are ones that are
  accelerating
• in such frames we observe fictitious forces

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• Inertial mass
• If you attempt to change state of motion of an
  object, the object will resist this change
• This is a consequence of inertia
• mass is a measure of inertia
• inertial mass

• Gravitational mass
• Objects with mass exert gravitational forces on
  each other
• The gravitational force exerted by a celestial
  body (earth, moon, sun etc) on an object on (or
  near) its surface is called the WEIGHT of the
  object

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• the magnitude of the weight force is proportional
  to the (gravitational) mass of the object and the
  celestial body
• mass measured through the effects of gravity
  agrees with
• mass measured through the effects of other forces

mass of an object does not depend on its
position or relation to other objects weight of
an object does
The force of gravity acts at the center of mass
of an object
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• Newtons Second Law
• the acceleration of an object is directly
  proportional to the resultant force acting on it
  and inversely proportional to its mass.

å
F

F

m
a
net
i
kg m




SI units


F

m
a


s
2

N
(Newton
)

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The MOMENTUM P of an object is defined to be the
mass of the object times its velocity
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• Newtons third law
• If two objects interact, the force exerted on the
  first object by the second is equal but opposite
  to the force exerted by the second object on the
  first

-
F
F
12
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(
)
(
)
action force

reaction force
Þ
a single isolated force cannot exist
action force and reaction force act on
DIFFERENT
objects

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Friction

• Force exerted by a surface on an object on the
  surface that resists motion of the object is
  called FRICTION
• Friction is a CONTACT force
• it is the result of electromagnetic forces
• If the force of friction is zero the surface is
  said to be smooth.
• Friction opposes motion along the surface
• STATIC friction force exerted when object is
  stationary
• DYNAMIC friction force exerted when object is
  moving

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N
Ffriction
Fexternal
W
magnitude of force of friction (N)
object moves
magnitude of external force (N)
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F

m
F
stat.
fric
ext
stat
F

m
N
kin.
fric
kin
N

constant NORMAL
force
m

coefficients of friction

Normal force is the reaction force from the
surface to the force exerted by the object on
the surface. It is NOT the reaction force to the
weight of the object
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If an object has zero component of acceleration
in a certain direction then there is NET FORCE
of ZERO acting on the object in that direction

• Newtons Laws and circular motion
• acceleration associated with uniform circular
  motion must be produced a force

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Free Body Diagrams
Draw all forces acting on the object of interest
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If the object is stationary then the total net
force on the object MUST be zero Thus the
components of the net force in any direction
must be zero If the objects velocity in one
direction does not change then the net force in
that direction is zero