Physics 218: Mechanics

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Physics 218 Mechanics
Instructor Dr. Tatiana Erukhimova
Lecture 17
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Quiz
A block of mass m is placed at rest on a plane
inclined at angle ?. After moving down L meters,
it has velocity V1. Find the coefficient of
friction between the block and the plane.
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Second exam Tuesday, October 26
105 Heldenfels
Section 807 and half of 802 start at 7 pm
Section 809 and half of 808 start at 805 pm
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Work Energy Theorem
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If a force can be written as the gradient (slope)
of some scalar function, that force is
conservative.
1D case
U(x) is called the potential energy function for
the force
If such a function exists, then the force is
conservative
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does NOT depend on path!
1D case
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If Fx(x) is known, you can find the potential
energy function as
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or
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Newtons 2nd Law
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Work does not depend on the choice of
coordinates! You can calculate work of each force
in the most convenient system of coordinates.
Do not forget to work with components!
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Usually there are several ways to solve the
problem. Indicate the law or the theorem you use!
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Center of Mass
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Find the position of the center of mass of the
system of the sun and Jupiter. (Since Jupiter is
more massive than the rest of the planets put
together, this is essentially the position of the
center of mass of the solar system.) Does the
center of mass lie inside or outside the sun?
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Motion of the Center of Mass
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The center of mass of a system moves as if all of
the mass of the system were concentrated at that
point and as if all of the forces were acting at
that point
There is only the external forces that affect the
motion of the center of mass ( )
Only external forces affect the motion of the
center of mass
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Momentum is a vector!
Vector equation!
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If
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Conservation of Momentum
If there is no external force on a system, then
the total momentum of the system is a constant
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True in X and Y directions separately!