WHAT DO YOU KNOW ABOUT WAVES What are

WHAT DO YOU KNOW ABOUT WAVES? What are they? Where have you seen them? What do they do?

Waves � Wave: a disturbance that transfers energy from place to place � Remember, in science, energy is the ability to do work � Raft example on p. 510: wave’s energy lifts raft as wave passes under it

What carries waves? � Most waves need something to travel through: called a medium � May be gas (air), liquid (water), or solid (rope) � Mechanical waves: waves that require a medium through which to travel � Electromagnetic waves: waves that can travel without a medium � Ex: sunlight travels through empty space

How do waves transfer energy? � � � Wave enters medium, which is made of tiny particles Transfers energy to medium’s particles Particles bump into each other and pass along wave’s energy After wave has passed, medium is unchanged – it doesn’t move with the wave **think of food being passed around a table

What causes waves? � � � Energy – always required to make a wave Mechanical waves are produced when a source of energy causes a medium to vibrate Vibration: repeated back-and-forth or up-anddown motion Wave results when a vibration moves through a medium Usually, a moving object has energy and will transfer that energy to a medium

Types of waves � 2 types of mechanical waves – classified by how they move: � Transverse waves � Longitudinal waves

Transverse waves

Transverse waves � Waves move the medium at right angles to the direction that the waves travel � Transverse = “across” � As wave moves left to right, medium moves up and down

Transverse waves � � � High point of wave: crest Low point of wave: trough Middle (resting position): equilibrium Distance from one crest to the next: wavelength Distance from equilibrium to crest or trough: amplitude

Longitudinal Waves

Longitudinal Waves � Wave that moves the medium parallel to the direction that the waves travel � Ex: in a spring, the coils move back and forth parallel to the wave motion � After wave passes, each coil returns to its starting point � Example: sound (air particles move back and forth parallel to distance sound travels)

Longitudinal Waves � Compressions: parts where coils are close together � Rarefactions: parts where coils are spread out � Wavelength: distance from one compression to the next

� Waves can vary greatly � short or long � lots of energy or a little � rare of frequent � fast or slow

� Amplitude � Wavelength � Frequency � Speed

� � � Maximum distance that the medium moves away from its resting position Distance from rest to the crest or trough Ex: water Transverse waves: medium moves up and down Longitudinal waves: measured by how compressed or rarefied medium becomes

� Distance between 2 corresponding parts of a wave � Can be measured as distance between 2 crests or between 2 troughs

� Number of complete waves that pass a point in a certain amount of time � Measured in Hertz (Hz) � Hertz = waves that pass per second � Ex: 3 Hz = 3 waves per second

� Distance traveled over time (same as when we talked about motion) � Speed = Wavelength x Frequency � Can rearrange formula to solve for other factors

WHAT HAPPENS WHEN A TENNIS BALL IS BOUNCED INTO THE WALL?

WHAT DO YOU THINK WILL HAPPEN TO A WAVE THAT RUNS INTO A NEW MEDIUM?

� When a wave hits a surface that it cannot pass through, so it bounces back � Ex: mirror reflects light so you can see yourself � Ex: when you shout in an empty gym, your voice echoes back

� All waves obey the law of reflection: angle of incidence = angle of reflection

WHAT HAPPENS IF YOU ARE BIKING OF SKATEBOARDING AND ONE WHEEL GOES OFF THE SIDEWALK INTO THE GRASS?

� When a wave enters a new medium at an angle, one side of the wave changes speed before the other side � The change in speed causes the wave to bend � Refraction: bending of waves due to a chance in speed

� Only occurs when wave enters new medium at an angle � One side enters first, changes speed � Other side continues moving at original speed � Wave bends

CAN YOU THINK OF OTHER EXAMPLES OF REFRACTION?

� Skateboarding – one wheel in the grass � Reaching for an object underwater

� When a waves bends around a barrier or through an opening in a barrier � Wave bends and spreads out

� Interference: the interactions of 2 waves when they meet � Can be constructive or destructive – what do you think the difference is?

� � Waves combine to make a wave with a larger amplitude Constructive = works together

� � Waves combine to make a wave with a smaller amplitude Destructive = destroy

� In both cases, waves will continue in their own directions as if they had not met

� � A wave might travel through a medium then be reflected back (think tying a rope to a doorknob) Reflected wave and incoming wave will interfere If waves have the right frequency, they may appear to stand still Called a standing wave – looks like no wave at all but is really 2 waves passing through each other

� Nodes: points with amplitude of 0 (zero) � Caused by destructive interference � Evenly spaced � Antinodes: points of maximum amplitude � Caused by constructive interference � Maximum energy � Halfway between nodes

� Most objects have a natural frequency of vibrations � Standing waves occur when objects vibrate at their natural frequency � If nearby objects vibrate at that same frequency, it causes resonance � Resonance: increase in amplitude of a vibration that occurs when external vibrations match an object’s natural frequency

� Can be good or bad � Good: musical instruments use resonance to produce stronger, clearer sounds � Bad: earthquake in Mexico City caused more damage because seismic waves had same natural frequency as clay layer of ground