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PHY 101 Study Guide - Comprehensive Final Guide: Inelastic Collision, Elastic Collision, Momentum


Department
Physics
Course Code
PHY 101
Professor
Dr.sharon Zane
Study Guide
Final

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UM
PHY 101
FINAL EXAM
STUDY GUIDE

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Chapter 6
6-1 Frictional Forces
friction: the force required to overcome the resistance of microscopic hills and
valleys and bumping together
even smooth surfaces are jagged at the atomic level which causes resistance to
an objects motion; the resistance is the force of friction
friction is thought of as something that should be reduced or eliminated if
possible
friction is helpful in some situations (starting to walk, turning the corner while
driving)
Kinetic Friction
Kinetic friction- the friction encountered when surfaces slide against one
another with a finite relative speed
fk acts to oppose the sliding motion at the point of contact between the
surfaces
When the normal force (N) is doubled fk is also doubled; fk is proportional
to the magnitude of N.
fk= kN
k is referred to as the coefficient of kinetic friction, it is the
constant of proportionality
N in special cases equals the weight of the object overcoming friction but if
someone pushes down on the object N becomes greater than the
objects weight which would increase friction
N is less than the weight of the object if it is on an incline
The force of friction opposes motion and is thus not a vector equation b/c N
is perpendicular to the direction of motion
When an object is pulled at speed v and then later 2v, the force of kinetic
friction is about the same in each case; it does not double fk is
independent of the relative speed of surfaces
If the area of contact of an object is reduced, the force of friction remains
the same regardless of the area of contact; fk is independent of the
area of contact b/t the surfaces
Static Friction
Static friction- the friction that keeps two surfaces from moving relative to
one another due to the microscopic irregularities of surfaces that are in
contact
Typically stronger than kinetic friction b/c the hills and valleys of each
surface can nestle into one another
When an object is motionless, the fs is zero
When a force f1 attempts to pull an object and it does not move, f1=fs; if f1
increases and the object still does not move, fs has also increased to the
same value
When the object starts moving, f s is overcome and fk takes over; the upper
limit that fs reaches is called fs,max > fs > 0
fs,max= sN
s is the coefficient of static friction, the constant of
proportionality; it is usually greater thank k showing that fs > fk
fs is independent of the area of contact between the two surfaces
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fs is parallel to the surface of contact and opposite the direction of
motion
If a car skids, the friction acting on it is kinetic, if it breaks and its wheels are
still rolling the friction is static and can be stopped in less a distance than if its
wheels are locked up
6-2 Strings and Springs
Strings and Tensions
Tension- the force in a spring that causes it to become taut; the force
pulling the ends of a string apart; at any given point tension pulls equally
to the right and left
For the problem of a rope that holds a box from the ceiling
the rope holds the box at rest so the tension where the rope
attaches to the box is the weight of the box
at the midpoint of the rope the tension is the weight of the box +
half the weight of the rope
at the ceiling the tension is the weight of the box and the rope
pulleys are used to redirect a force exerted in a strong, and an ideal pulley
has no mass or friction in its bearings; it changes the direction of the
tension in a strong without change its magnitude
multiple pulleys can magnify a force
Spring’s and Hooke’s Law
If you stretch or compress a string of length L by L + x; the spring
pushes/pulls back by a force F; if x is increased to 2x the force becomes
2F
A spring exerts a force that is proportional to the amount, x, by which it is
stretched or compressed
F= kx (magnitude only) F= -kx (magnitude and direction)
k- spring force constant; the constant of proportionality; has units of N/m
if the spring is stretched, the magnitude of kx is in the negative x
direction
if the spring is compressed, the value of x is negative and the magnitude of
kx is also negative
if a spring is stretched too far from equilibrium it will permanently deform
ideal springs- springs that are massless, and are assumed to obey hooke’s
law
the forces that hold atoms together are modeled by hooke’s law
6-3 Translational Equilibrium
translational equilibrium- the net force acting on an object is zero; the sum
of the forces = 0
when the system is in two dimensions the sum of forces in both the x and y
direction must be at zero
Translational Equilibrium in Tension
if the acceleration of the system is zero, the upward tension in the rope is
equal to the weight of the mass hanging from the rope (mg); this is also
the force the person must exert downward on the rope to pull it; same as
tension downward
The tension in the chain holding the pulley tension up tension down from
the weight tension down from the person pulling; thus the tension in
the change is twice the downward tension or 2mg
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