Transcript Lecture powerpoint

PHY 2048C
General Physics I with lab
Spring 2011
CRNs 11154, 11161 & 11165
Dr. Derrick Boucher
Assoc. Prof. of Physics
Sessions 11-12, Chapter 10
Chapter 9 Homework
Due Friday 2/18 @ midnight
Chapter 10 Homework
Due Tuesday 2/22 @ midnight
Chapter 10
Practice Problems
5, 7, 29, 31, 33, 35, 47, 53
Unless otherwise indicated, all practice
material is from the “Exercises and Problems”
section at the end of the chapter. (Not
“Questions.”)
Money-Energy Analogy
From the Parable of the Lost Penny
Money—Energy Analogy
From the law of conservation of energy
Kinetic and Potential Energy
There are two basic forms of energy. Kinetic energy
is an energy of motion
Gravitational potential energy is an energy of
position
The sum K + Ug is not changed when an object is
in freefall. Its initial and final values are equal
Energy Units
SI: joule (J)
1 kg  m
1J 
 1N  m
2
s
2
Other popular units: calorie, Calorie, BTU
(British Thermal Unit), foot-pound, erg,
electron-volt, kilowatt-hour (kWh), therm,
Hartree, Rydberg…
Kinetic and Potential Energy
IMPORTANT: The potential energy ONLY
depends on the height, even if the object
moves along a ramp, hill, etc.
The Zero of Potential Energy
• You can place the origin of your coordinate system,
and thus the “zero of potential energy,” wherever
you choose and be assured of getting the correct
answer to a problem.
• The reason is that only ΔU has physical significance,
not Ug itself.
The Zero of Potential Energy
The Zero of Potential Energy
Conservation of Mechanical
Energy
The sum of the kinetic energy and the potential
energy of a system is called the mechanical energy.
Here, K is the total kinetic energy of all the particles in
the system and U is the potential energy stored in the
system. The kinetic energy and the potential energy
can change, as they are transformed back and forth
into each other, but their sum remains constant.
Problem-Solving Strategy: Conservation of Mechanical
Energy
Problem-Solving Strategy: Conservation of Mechanical
Energy
Example, Problem 10-13, p. 296
Chapter 10. Clicker Questions
A. half as much kinetic
energy as the first.
B. as much kinetic energy
as the first.
C. twice as much kinetic
energy as the first.
D. Four times as much
kinetic energy as the first.
© 1997 Lewis Carroll Epstein
A. half as much kinetic
energy as the first.
B. as much kinetic energy
as the first.
C. twice as much kinetic
energy as the first.
D. four times as much
kinetic energy as the first.
© 1997 Lewis Carroll Epstein
Preparing his case for trial, a
lawyer pondered this question.
A 1-N flowerpot fell one meter
from a shelf and struck his client
squarely on the head. How
much force did the pot exert on
his client’s head?
A. 1 N
B. 4.9 N
C. 9.8 N
D. 19.6 N
E. The lawyer’s question
cannot be answered from
the given information
© 1997 Lewis Carroll Epstein
Hooke’s Law
If you stretch a rubber band, a force appears that tries
to pull the rubber band back to its equilibrium, or
unstretched, length. A force that restores a system to an
equilibrium position is called a restoring force. If s is the
position of the end of a spring, and se is the equilibrium
position, we define Δs = s – se. If (Fsp)s is the scomponent of the restoring force, and k is the spring
constant of the spring, then Hooke’s Law states that
The minus sign is the mathematical indication of a
restoring force.
Hooke’s Law
Example, Problem 10-16, p. 296
Elastic Potential Energy
Consider a before-and-after
situation in which a spring
launches a ball. The
compressed spring has
“stored energy,” which is then
transferred to the kinetic
energy of the ball. We define
the elastic potential energy Us
of a spring to be
Example, Problem 10-45, p. 298
Chapter 9. Quiz
Get your clickers ready
Impulse is
A. a force that is applied at a
random time.
B. a force that is applied very
suddenly.
C. the area under the force curve in
a force-versus-time graph.
D. the time interval that a force lasts.
The total momentum of a system is conserve
A. always.
B. if the system is isolated.
C. if the forces are conservative.
D. never; it’s just an approximation.
In an inelastic collision,
A. impulse is conserved.
B. momentum is conserved.
C. force is conserved.
D. energy is conserved.
E. elasticity is conserved.
A 0.24 kg blob of clay is thrown at a wall with an
initial velocity of 23 m/s. If the clay comes to
a stop in 91 ms, what is the average force
experienced by the clay?
A) 0.5 N
B) 2.1 N
C) 61 N
D) 122 N
A 1200 kg ore cart is rolling at 10.8 m/s across a
flat surface. A crane dumps 691 kg of ore
(vertically) into the cart. How fast does the
cart move after being loaded with ore?
Assume that frictional forces may be
neglected.
A) 6.2 m/s
B) 4.1 m/s
C) 10.8 m/s
D) 6.8 m/s
Elastic Collisions
Elastic Collisions
Consider a head-on, perfectly elastic collision of a ball
of mass m1 having initial velocity (vix)1, with a ball of
mass m2 that is initially at rest. The balls’ velocities after
the collision are (vfx)1 and (vfx)2.These are velocities, not
speeds, and have signs. Ball 1, in particular, might
bounce backward and have a negative value for (vfx)1.
Elastic Collisions
Consider a head-on, perfectly elastic collision of a ball
of mass m1 having initial velocity (vix)1, with a ball of
mass m2 that is initially at rest.
The solution, worked out in the text, is
The Equations
for
Elastic collisions
These do not appear on the
equation sheet but should be
used for homework. I will
provide these separately for
the next exam (on the board).
Example, Problem 10-28, p. 297
Energy Diagrams
A graph showing a system’s potential energy and total
energy as a function of position is called an energy
diagram.
Energy Diagrams
The kinetic
energy is the
difference
between the
total energy
(TE) and the
potential energy
(PE).
Energy diagrams are commonly
used in chemistry. E.g. molecular
vibrations.
The different vibrational
energy levels (v=
0,1,2…) are different
average kinetic energy
levels of the molecule.
Most molecular bonds
are approximately
“springs” for the lowest
vibrational energy
levels.
Another chemistry example
Example, Problem 10-32, p. 297
Chapter 10. Clicker Questions
A runner starts from rest. She puts a
certain amount of momentum into herself
and
A. more momentum into the ground.
B. less momentum into the ground.
C. the same amount of momentum into the
ground.
© 1997 Lewis Carroll Epstein
A runner starts from rest. She puts a
certain amount of kinetic energy into
herself and
A. more kinetic energy into the ground.
B. less kinetic energy into the ground.
C. the same amount of kinetic energy into
the ground.
© 1997 Lewis Carroll Epstein