Physics 112 Exam Theory Review

Waves waves in the ocean move energy but not objects; objects just move up and down as the waves pass beneath them; they can bend and reflect; rays are not real; rays are just arrows showing the path of waves; rays are drawn so they cross wave crests at 90⁰; mechanical waves (e.g., sound) need some material through which to travel but electromagnetic waves can travel through matter and space; refraction is like bending; a prism (and raindrops) uses refraction to separate white light into its hidden components (the rainbow colors); because each color bends a different amount and takes a different path as the light exits the prism or raindrop, we can see the rainbow spread out across a room opposite a suncatcher; at the back of our eye is the light sensitive retina that contains cells that change light into electrical signals that our brain uses to "see"; during a solar eclipse, the lineup of heavenly bodies is SEM (sun, moon, earth); during a lunar eclipse the lineup is SME.

Sound resonance occurs when the energy from a sound source goes out, touches another material and causes it to vibrate at the same frequency as the original sound; a string vibrating in its fundamental pattern produces its lowest note; the string vibrating in this way move to and fro in one segment (just like when you make a skipping rope move up and down); the zone where the string is vibrating most is called the antinode, where there is no vibration and nodes are where there is no vibration; an antinode has a node at each end; two nodes are at if a sound source and a listener are closing or retreating, the Doppler Effect changes the frequency of the sound heard by the listener; the pinna collects sound waves and directs it to the tympanum (ear drum), the middle ear bones (the ossicles) transmit it from the tympanum to the inner ear, in the inner ear, the sound sensitive cells inside the cochlea changes the vibrations into electrical signals, the auditory nerve carries (not makes) these signals to the brain; sound frequencies below 20 Hz are infrasonic, sound frequencies between 20 and 20 000 Hz are the extreme limits of our hearing, sound frequencies above 20 000 Hz are ultrasonic; overexposure to loud sounds causes a constant ringing (more noticeable to the person at night) called tintinitis; Interference occurs when waves meet at some spot; constructive interference occurs if two of the same meet, two crests or two troughs meeting make an extra large crest or trough, destructive interference occurs if opposites meet, a crest and a trough meeting will make a flat spot; dropping a pebble into a pond sends out a series of concentric ripple rings that get smaller in height as they spread out (naturally, the circumference gets larger), their speed is unchanged as they move along.

Motion A scalar tells just how far something has moved, e.g., 10 km; a vector tells us the distance and direction, e.g., 10 km E; you walk in the snow: your first step is the point of reference (the start) and your path is the frame of reference; your steps are like time-position marks in the snow; a "+" motion is E or speeding up; a "-" motion is to the W or slowing down; the plot of a t-p graph tells us distance and direction something has moved in a certain time; the area under the plot (curve) has no meaning to us; the slope of the t-p plot tells us the velocity of the object; a flat t-p plot means no motion; the plot of a t-v graph tells us the speed and direction something has moved in a certain time; the area under the plot (curve) tells us the distance the object has moved; a flat t-v plot means constant speed; when the light turns green, your speed increases only with enough force, time, acceleration, and distance.

Forces In an elevator, your weight lessens for a second or two but only as the elevator starts down; when the elevator is moving at a constant speed, your weight is normal; going up a mountain, gravity lessens a bit; going to the extreme, in space your weight is zero but your mass is still the same as it is on Earth; the tension on the cables also lessens for the tiny bit of time until the elevator speed is constant; starting (static) friction is the strongest type; you notice friction only when you try to move an object; the friction force is parallel to the supporting surface (if the surface is sloping, we need trig to find the force that is the one parallel to the slope); super smooth surfaces touching in space can become "glued" together in the process of cold welding; on Earth, a dust layer between the surfaces prevents this from happening; the only factor controlling the µ is the nature of the two surfaces in contact, i.e., the forces, the pulling angles, etc. do not affect the µ; of course, pulling a crate over various surfaces means the µ (and the forces, distances, and speeds) will change; as an object is pulled into motion, the larger static friction becomes the lesser sliding or rolling friction; the friction seems less because the µ becomes less; once the object is moving, the µ and friction do not lessen again;

Momentum Momentum is greatest for a heavy, fast moving object and the least for a light, slow moving object (it is zero for a motionless object); impulses cause objects or change their speed and perhaps direction; objects that reverse their direction have received a large impulse; air bags protect us because they allow our bodies to slow down more slowly and so we do not experience a large sudden deceleration force (from the dashboard or windshield) that would harm us; in collisions, both objects have the same momentum change; the object that slows loses momentum while the object that speeds up gains an equal momentum; the total momentum is equal; heavy objects show the least speed change while lighter objects show a larger speed change.

Work, Power and Energy Work is done on an object when it is moved up (not just being held); when the object is carried down, it does work on us.