15.7 Resonance

Question 1

What process is resonance?

Answer 1

one that occurs when an oscillating body is forced to oscillate by an outside source at a frequency that is equal to its own natural frequency.

Question 2

What is the driving force then equal to?

Answer 2

Driving force = natural frequency

Question 3

How can we show Resonance?

Answer 3

By using an experiment involving Barton’s Pendulums.

Question 4

How do we use Barton’s Pendulums to show resonance?

Answer 4

We have a punch of pendulums of different lenghs. We make one oscillate. This makes all of them oscillate to different amplitudes. The one with the same length gives the same amplitude as the one that was shaken.

Question 5

Why is one of the same length able to abosrb the most energy ?

Answer 5

As it has the same natural frequency.

Question 6

When we approach the natural frequency what happens to the amplitudes.

Answer 6

They increase every half an oscillation greatly

Question 7

When the energy transferred is at maximum (to the bob of the same natural frequency) what happens? as long as system isnt damped

Answer 7

The system is said to be resonating. The frequency is now called resonating frequency.

Question 8

What set up can we use to investigate resonance?

Answer 8

An arrangement of a spring with masses attatched by a string to a variable frequency signal generator.

Question 9

How can we use this setup to determine the resonance?

Answer 9

1 - we determine the natural frequency (free oscillations over 10 seconds)
2 - We set the signal generator to about half the natural frequency and then switch it on.
3 - The frequency is gradually increased until the mass vibrates with maximum amplitude - which should happen when set frequency is equal to natural frequency.
4 - The experiment should be repeated but with signal generator set to twice the natural frequency which should be decreased over time.
5 - We find the average value for resonant frequency.
— This experiment can be repeated with different masses

Question 10

If the system is damped is the resonating frequency when driving frequency is equal natural frequency.

Answer 10

The driving frequency would be lower, this becomes lower and lower as damping increases

Question 11

If from the practical above we were to plot a graph of amplitude over driving frequency, what shape graph would we get and where would resonating frequency be on it?

Answer 11

A mountain like graph with resonating frequency at peak amplitude

Question 12

If there is damping would amplitude of the oscillation decrease?

Answer 12

YES

Question 13

If damping is involved in the system what would happen to the mountain like graph ?

Answer 13

The peak would decrease and the graph broaden.

Question 14

Where is resonance often used?

Answer 14

musical instruments

e. g. recorder to change note
e. g.tune radios

Question 15

Example of system where damping takes place and we can see a frequency decrese?

Answer 15

A pendulum in water, it is definitely going to travel slower and hence longer to swing

Question 16

Do heavier damped systems have a flatter response to oscillations?

Answer 16

Yes, amplitude increases at a lower rate near natural frequency and in general are not as sensitive as less damped systems

Question 17

Near natural frequncy what happens to little damped systems?

Answer 17

Their amplitude of oscillations increases greatly.

Question 18

Why is damping important in machine and building designs?

Answer 18

Unwanted vibrations could build up to large amplitudes an hence damages

Question 19

What type of materials are often used to absorb a lot of energy from vibrations making them die off.

Answer 19

Ductile materials, ones which can deformed plastically without fracture. These work by hysterisis

Question 20

How do we use resonance tuning the radio?

Answer 20

We alter the capacitance of a variable capacitor in the tuning circuit. When the natural oscillations in the tuning circuit matches the frequnecy of incoming radio signals, resonance occurs absorbing energy strongly from the EM waves.

Question 21

Is resonance used in microwaves?

Answer 21

Yes, the oscillating electric field must have a frequency close to the natrual frequency of the vibrations of the water molecules. This would mean energy is absorbed strongly from the electric field.

Question 22

What process does the microwave essentially use to heat up food?

Answer 22

Frictional heating

Question 23

What does MRI stand for?

Answer 23

magnetic resonance imaging

Question 24

How does an MRI scanner work?

Answer 24

A strong magnetic field is used to to exert a force on the protons within the hydrogen atoms of patient’s body. Normally the protons move in random directions, but now they’re aligned with the magnetic field. Now radio waves are sent and energy from these waves are absorbed by the protons (by resonance) and are consequently knocked out of alignment. When the emission of radio waves is stopped the protons return to original alignment by magnetic field and release radio signals, which can be detected outside the body.`

Question 25

How can we make really strong magnetic fields?

Answer 25

superconductivity