Download Students` questions

Ph170A Spring 2004
Prof. Pui Lam
Reading Assignment #6 :Ch.6-ALL (don’t worry about the math in 6-3) , Ch 7-1, 2,3 & 5
Homework #6: Momentum and Conservation of Momentum
Students’ questions
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6-1 Collision
"Before and after the collision, we assume that the objects are far enough apart that they do not
exert any forces on each other." How do we determine how far is "far enough apart?"
When two objects, for example, bat and ball, collide, do they deform because the bonds of the
atoms break/adjust?
"we have two objects with different initial motions that are originally so far apart that neither
exerts a measurable force on the other. Eventually they approach one another, so that each exerts
a force on the other and alters its motion." So an object was originally set to motion by some
force and eventually make a switch over to be affected by a different force?
The slingshot effect increases speed in which direction?
Quantitiatively, what is a small time compared to the time of observation?
In reference to figure 6-3, what does it mean by two galaxies colliding?
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6-2 Linear Momentum
What exactly does linear momentum mean, is it only in the x-y
plane? Since momentum equals mass times velocity and force equals mass times acceleration, does that
mean that force is the derivative of momentum with respect to time?
In non- ideal situations, is there more to momentum than mass times
velocity?
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6-3 Impulse amd Momentum
What is the difference between impulse and momentum?
what are the units for momentum and impulse?
How do you know when to use linear momentum or impulsive momentum in an
example?
does the "dt" stand for the change in time?
is momentum simply mass multiplied by the velocity, and is the momentum added from an
impulse the average force multiplied by the duration of time of the impulse?
Impulse is the change in momentum in respect to time what happens to the impulse as time
approaches zero? what happens to the object? (p121)
Is their only one main force that is accounted for when calculating impulse? (p.121-122)
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6-4 Conservation of Momentum
When two objects get closer together is the attractive force between them ever enough to
significantly alter their momentum?
Is the equation m1v1 + m2v2 = m1v1f + m2v2f the principal equations for collisions? (p126)
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6-5 Two-body Collision
I've heard of inelastic collisions, what are they and are there
any formulas for those types of collisions? Why are there special equations for elastic collisions? For the work kinetic energy theorm one
equation K1+ P1 - unconservative forces = K2 + P2 is used to solve all the cases we used.
Why are there special equations for elastic collions? (p129)
Can you explain the center-of-mass reference frame?
How are "inertial reference frames" different form "laboratory
reference frames" or "center of mass reference frames"?
Does the motion of the Earth around the sun in a "center-of-mass"
manner affect us in detectable ways? (such as change in temperature or
length of day)
How do you determine which is the center-of-mass reference frame?
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7-1 Motion of a Complex Object
So if there is friction, or any other external force, we can find the
movement of the center of mass by pretending there is a giant particle with
the mass of the object at the center of mass?
Can we do more center of mass problems in class?
So basically, when you're dealing with a system of particles and momentum,
you can just act like the entire system is one big object?
When an object that doesn't have any type of symmetry undergoes rotation does the center mass
undergo rotation also or does it travel onlt tranlationally?
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7-2 Two-Particle Systems
Can you explain many and two particle systems? How exactly do we find the center of mass and is the process different
for a two-particle system or a many-particled system?
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7-3 Many -Particles Systems
When dealing with the center mass, is it like treating the whole object as a point where the center
mass exists? Do we follow the center mass to determine the path the entire object travels?
How much do external forces affect the path of the center of mass?
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7-5 Conservation of Momentum in a s System of Particles
In problem 7-7, what happens if the gun does not fire all 8 bullets before the first one makes
impact? Are all the bullets assumed to hit the block at the same time?