Transcript Physics Chapter 6

Physics Chapter 6
Forces
Newton’s Laws of Motion
1st Law (Law of inertia)
– An object moving at constant velocity
keeps moving at that velocity unless a
net force acts on it
2nd Law (F=ma)
– The net force acting on an object causes
the object to accelerate in the direction
of the net force
3rd Law
– For every action there is an equal but
opposite reaction
Force
–A push or a pull
System
–The object and what is
around it
Environment
–World around object that
exerts forces on it
Contact force
–Acts on an object only by touching it
Long-range force
–A force exerted without contact
Force of gravity
–An attractive force that exists
between all objects
Agent
–A specific, identifiable, immediate
cause of a force
Newton’s 1st Law of Motion
Inertia (deals with 1st law)
–The tendency of an object to resist
any change in motion
Equilibrium
–When an object is moving at
constant velocity
An object at rest is moving at a
constant velocity of 0 and is in
equilibrium
Free body diagram
Uses dots to represent objects
Uses lines to represent forces
–Lines are always drawn in the
direction of the force which acts
upon the object
–The tail of the line representing the
force is always placed on the dot of
the object that it is acting on.
FORCES
Net Force (Fnet)
– The vector sum of two or more forces on an
object
Friction (Ff )
– The contact force that acts to oppose sliding
motion between surfaces
– Parallel to the surface and opposite the
direction of sliding friction
Normal Force (FN)
– contact force exerted by surface on an object
– Perpendicular to and away from the surface
FORCES con’t
Spring Force (Fsp)
– Restoring force, that is, the push or pull a
spring exerts on an object
– Opposite the displacement of the object at
the end of the spring
Tension (FT)
– The pull exerted by a string, rope, or cable
when attached to a body and pulled taut
– Away from the object and parallel to the
string, rope, or cable at the point of
attachment
FORCES con’t again
Thrust (Fthrust)
– A general term for the forces that move
objects such as rockets, planes, and cars
– In the same direction as the acceleration of
the object barring any resistive forces
Weight (Fg)
– A long-range force due to gravitational
attraction between two objects, generally
Earth and an object
– Straight down toward the center of the earth
W = mg
Common Misconceptions
When a ball has been thrown,
the force of the hand that
threw it remains on it
A force is needed to keep an
object moving
Inertia is a force
Air does not exert a force
The quantity ma is a force
Apparent weight
–The force exerted by the scale
Weightlessness
–Doesn’t mean your weight is zero
–It means your APPARENT weight is zero
Free fall
–When an object is influenced only by
the acceleration due to gravity
Terminal velocity
–When the drag force is equal to the
acceleration due to gravity
FRICTION
Static friction force
– Force exerted on one surface by the
other when there is no relative motion
between the two surfaces
0 ≤ Ff, static ≤ μsFN
Kinetic friction force
– Force exerted on one surface by the
other when the surfaces are in relative
motion
Ff, kinetic = μkFN
Periodic Motion
Simple harmonic motion
–When the force that restores an object
to its equilibrium position is directly
proportional to the displacement of the
object
Period (T)
–Amount of time needed to repeat one
complete cycle of motion
Amplitude
–The maximum distance the object
moves from equilibrium
Mechanical resonance
– The increase vibration of the amplitude
due to small forces being applied at
regular intervals to a vibrating or
oscillating object
Period of a Pendulum
T = 2
l
g
If an object is on a flat surface, then
the Force due to Weight will equal
the Normal Force
FW = FN
The force of friction (Ff ) is always
opposing the applied force (Fapp)
If the net force is zero (means no
acceleration or constant velocity) the
force due to friction will equal the
applied force.
Ff = Fapp