Plan for Today (AP Physics I) Lecture/notes on Waves

Plan for Today (AP Physics I)

• Lecture/notes on Waves

Waves

• Motion of a disturbance

• Ex – jump• Do you feel it?• Do you hear it?

When I jump. . .

• Energy is transferred through the floor (vibrates)

• Energy is transferred through air (sound)

Wave motion

• Transmits energy (not particles)• Part of the floor/air stayed in place

Types of Waves

• Mechanical– Require a medium– Types of mechanical waves:

• E-M (electromagnetic)– Doesn’t require– Types of electromagnetic waves:

Other classifications

• Longitudinal– Vibration parallel to motion

• Transverse– Vibration perpendicular to motion

• Examples with slinky• Spring with ribbon attached– Shows motion of energy not particles

• Sound – big longitudinal wave we will look at (next unit)

Wave information

• Laser disks 11-15

Pictures of Waves

• Picture of Height of water striking a dock

Pictures of Waves• Picture of Height of water striking a dock

• T = period (time to repeat)• Lambda = wavelength (distance)• A = amplitude

Picture of Wave Continued

• Looking out over a lake with waves on it

Velocity of a Wave as a function of what?

• Brain teaser– In WWII, a sailor dives off a ship about to explode.

He swims under water until he hears the explosion, then he pops his head out of the water and hears another explosion

– After he is rescued, others claim there was only 1 explosion. But he heard 2 – why?

Speed of Sound

• Air – 350 m/s• Water – 1100 m/s• Steel – 5000 m/s

• Speed of sound is a function of the medium and the medium only

• Velocity of a wave is a function of the medium

Why is there the velocity difference

• Spacing of the molecules

• Air– Motion has to travel all the way over

• Water

• Steel

• How fast does an air molecule vibrate?• Around 350 m/s

Formula

• v = x/t• But since wavelength () is a distance and T is a

time. . .

• v = / T• v = * f

Example Problems

• A tuning fork produces a sound with a frequency of 256 Hz and a wavelength in air of 1.35 m.– What value does this give for the speed of sound

in air?– What would be the wavelength of the same sound

in water?

Example

• The red light emitted by a He-Ne laser has a wavelength of 633 nm in air and travels at 3 * 10 ^8 m/s. Find the frequency of the laser light.

Echo Problem

• A sound wave traveling at 343 m/s is emitted by the foghorn of a tugboat. An echo is heard 2.60 s later. How far away is the reflecting object?