Transcript Physics 151 Week 12 Day 3

Physics 151 Week 12 Day 3
Topics:
Hooke’s Law and Oscillations (Chs. 8 & 14)
Oscillations
 Period & Frequency
 Angular Frequency
 Forces
 Acceleration
 Energy
Resonance
Mathematical Models
 Mass and Spring
 Pendulum
Exam 2 Scores
(mean = 60 / Std. Dev. = 20)
Number of Students
14
12
10
8
6
4
2
0
20
30
40
50
60
70
80
90
100
Score
Graded out of 125 points - score divided by 97
Slide 8-29
Equilibrium and Oscillation
Slide 14-12
Linear Restoring Forces and Simple Harmonic
Motion
Slide 14-13
Sinusoidal Relationships
Slide 14-21
Frequency and Period
The frequency of oscillation depends on physical properties of the
oscillator; it does not depend on the amplitude of the oscillation.
Slide 14-14
Mathematical Description of Simple Harmonic Motion
Slide 14-11
Energy in Simple Harmonic Motion
As a mass on a spring goes
through its cycle of oscillation,
energy is transformed from
potential to kinetic and back to
potential.
Slide 14-12
Frequency and Period
The frequency of oscillation depends on physical properties of
the oscillator; it does not depend on the amplitude of the
oscillation.
Slide 14-13
Resonant Frequence and Resonance
Slide 14-23
Solving Problems
Slide 14-14
Checking Understanding
A set of springs all have initial length 10 cm. Each spring now
has a mass suspended from its end, and the different springs
stretch as shown below.
Now, each mass is pulled down by an additional 1 cm and
released, so that it oscillates up and down. Rank the
frequencies of the oscillating systems A, B, C and D, from
highest to lowest.
Slide 14-15
A series of pendulums with different length strings and different
masses is shown below. Each pendulum is pulled to the side by
the same (small) angle, the pendulums are released, and they
begin to swing from side to side.
Rank the frequencies of the five pendulums, from highest to
lowest.
Slide 14-16
A ball on a spring is pulled down and then released. Its
subsequent motion appears as follows:
1)
2)
3)
4)
5)
6)
At which of the above times is the displacement zero?
At which of the above times is the velocity zero?
At which of the above times is the acceleration zero?
At which of the above times is the kinetic energy a maximum?
At which of the above times is the potential energy a maximum?
At which of the above times is kinetic energy being transformed to
potential energy?
7) At which of the above times is potential energy being transformed
to kinetic energy?
Slide 14-17
A pendulum is pulled to the side and released. Its subsequent
motion appears as follows:
1)
2)
3)
4)
5)
6)
At which of the above times is the displacement zero?
At which of the above times is the velocity zero?
At which of the above times is the acceleration zero?
At which of the above times is the kinetic energy a maximum?
At which of the above times is the potential energy a maximum?
At which of the above times is kinetic energy being transformed to
potential energy?
7) At which of the above times is potential energy being transformed
to kinetic energy?
Slide 14-18
Examples
The first astronauts to visit Mars are each allowed to take along
some personal items to remind them of home. One astronaut
takes along a grandfather clock, which, on earth, has a
pendulum that takes 1 second per swing, each swing
corresponding to one tick of the clock. When the clock is set up
on Mars, will it run fast or slow?
A 5.0 kg mass is suspended from a spring. Pulling the mass
down by an additional 10 cm takes a force of 20 N. If the mass
is then released, it will rise up and then come back down. How
long will it take for the mass to return to its starting point 10 cm
below its equilibrium position?
Slide 14-19
Example
We think of butterflies and moths as gently fluttering their wings,
but this is not always the case. Tomato hornworms turn into
remarkable moths called hawkmoths whose flight resembles that
of a hummingbird. To a good approximation, the wings move with
simple harmonic motion with a very high frequency—about 26 Hz,
a high enough frequency to generate an audible tone. The tips of
the wings move up and down by about 5.0 cm from their central
position during one cycle. Given these numbers,
A. What is the maximum velocity of the tip of a hawkmoth wing?
B. What is the maximum acceleration of the tip of a hawkmoth
wing?
Slide 14-20
Example
The deflection of the end of a diving board produces a linear
restoring force, as we saw in Chapter 8. A diving board dips by 15
cm when a 65 kg person stands on its end. Now, this person
jumps and lands on the end of the board, depressing the end by
another 10 cm, after which they move up and down with the
oscillations of the end of the board.
A. Treating the person on the end of the diving board as a mass
on a spring, what is the spring constant?
B. For a 65 kg diver, what will be the oscillation period?
C. For the noted oscillation, what will be the maximum speed?
D. What amplitude would lead to an acceleration greater than that
of gravity—meaning the person would leave the board at some
point during the cycle?
Slide 14-21
Example
In Chapter 10, we saw that the Achilles tendon will stretch and then rebound, storing and
returning energy during a step. We can model this motion as that of a mass on a spring. It’s
far from a perfect model, but it does give some insight. Suppose a 60 kg person stands on a
low wall with her full weight on the balls of one foot and the heel free to move. The stretch of
the Achilles tendon will cause her center of mass to lower by about 2.5 mm.
A. What is the value of k for this system?
B. Given the mass and the spring constant, what would you expect for the period of this
system were it to undergo an oscillation?
C. When the balls of the feet take the weight of a stride, the tendon spring begins to stretch
as the body moves down; kinetic energy is being converted into elastic potential energy.
Ideally, when the foot is leaving the ground, the cycle of the motion will have advanced
so that potential energy is being converted to kinetic energy. What fraction of an
oscillation period should the time between landing and lift off correspond to? Given the
period you calculated above, what is this time?
D. Sprinters running a short race keep their foot in contact with the ground for about 0.10 s,
some of which corresponds to the heel strike and subsequent rolling forward of the foot.
Given this, does the final number you have calculated above make sense?
Slide 14-22
Example
A 204 g block is suspended from a vertical spring, causing the
spring to stretch by 20 cm. The block is then pulled down an
additional 10 cm and released. What is the speed of the block
when it is 5.0 cm above the equilibrium position?
Slide 14-23