Phys Sci Midterm 2

Why can't you see infrared light or hear pitch of sound about 20,000 cycles per second?

Because your sensory apparati (eyes and ears) don't resonate with those frequencies.

A Baltimore Raven football player of mass 250 pounds runs at 20 miles per hour down the field. A 5000 pound trailer carrying Beyonce's half-time show equipment is moving at 1 mile per hour. The two collide, black out the stadium. Which one has more momentum? Which one would be harder to stop? What is the most likely outcome for the player and the trailer? Pick the option with the most true statements.

Both the trailer and the player have exactly the same momentum. However, the player has TWENTY times more energy, and would thus be 20 times harder to stop. This is because the kinetic energy depends on the square of the speed, and the momentum does not. Despite the players larger energy, he would sustaing the greater injury, since the trailer has so much mass, it would not accelerate as much for a given force, while the football player, which his lessor mass would. The player could be seriously damaged, but the half-time show would likely go on despite the best efforst of anyone to prevent it.

Suppose you twist the tuning peg on a guitar so as to increase the tension in the string. What happens to the wavelength of the wave on the string as opposed to the wavelength of the wave in the air that comes from that string?

By increasing the tension in the string, the frequency of the note increases, but the since the length of the string is the same, the wavelength on the string does not change. However, the wave in the air must be shorter, since the frequency of the tone is now higher. The governing idea is that the product of the wavelength times the frequency is equal to the velocity of the wave. For the guitar string, the speed of the wave is increased by increasing the tension. Since the wavelength cannot change on the string, then the frequency must. Since ultimately the air vibrates at the same rate as the string, and the speed of the sound wave in air has not changed, then the wavelength of the wave in air shrinks.

In video on Energy, What did Prof. Goodstein call the "true" energy crisis?

Each time energy is changed from one form to another, the quality of that energy degrades, making energy eventually useless to us.

A musical instrument is an example of something that is designed not to resonate well over a very broad band of frequencies.

False

As a siren on an ambulance approaches you hear the amplitude of the siren increase while the frequency of the sound decreases.

False

Prof. Goodstein credited with the invention of the idea of conservation of momentum to Galileo.

False

The echos that one ears in a box canyon are examples of wave diffraction.

False

The force of buoyancy on an object is fluid is equal to the weight of the object minus the weight of the fluid it displaces.

False

The principle of conservation of momentum only applies to objects that stay intact. If an object were to dissociate or explode, then the total momentum of that object as mesaured from the momentum of the sum of its parts would have had a net change in momentum owing to the explosion or dissociative forces.

False

Suppose that the speed of sound in air is 340 meters per second, and that you whistle a note at a frequency of 340 cycles per second, then the wavelength of the whistle would be 6.8 meters.

False speed = wavelength x Frequency 340 = ( wavelength ) x 340 wavelength = 1 meter

The reason that you cannot hear the high pitches of a dog whistle is that they are too soft for your ears to detect.

False - It has nothing to do with how loud or soft the signal is. It has only to do with the fact that your ear drums do not or cannot resonate with the high frequency signal of the whistle. You must be able to resonate with a signal to detect it.

Felix the Fixer is a CGI character in a video game called "Wreck it Ralph". Felix has a magic hammer with which he can repair anything. For instance, he can pound the pieces of a wrecked building and have all the pieces fly back to their original form. A physic professor would object that

Felix's hammer's doesn't heat anything up as it does its repair work. This violates the idea that all mechanical energy eventually turns into heat. That a wrecked car has been put through several irreversable processes in which the microscopic parts have been thoroughly and irrevocably rearranged, and several forms of energy have been thermalized and exhausted to the environment. This energy cannot be reclaimed into its original configuration

Ironman flies through the air in a straight line, but failing to notice a large boom crane, crashes right into it, setting the crane spinning around its axis. Before the collision, Ironman was flying in a straight line, and the crane had no angular motion at all. After the collision, the crane (and Ironman attached to it) has some angular momentum. How can this be?

Ironman carried angular momentum with him, relative to the central axis of the crane. When he hit the crane, he delivered that momentum to it. --- Angular momentum is measured from the "pivot" around which that angular momentum would be measured.

Bowing to OSHA pressure, Tony Stark modifies his Ironman suit to incorporate collision airbags. He quickly becomes annoyed at how frequently these deploy as he is flung from one explosive situation to another, and his face plate airbag blows out. However, he finds that he now comes away with fewer bruises. The reason an air bag works is because

It extends the time of the collision, meaning that for a given change in momentum, there is less average force exerted on the body.

Suppose you are in space, far from any star or planet. You see an asteroid move in a straight line past you. What statement is true?

It is impossible to know which, of either of you is at rest. You can only state that neither one of you is experiencing accelerated motion.

As air is heated by the ground, it can expand and become buoyant. As it rises it cools. Why?

It takes work to lift air up against the pull of gravity. The gravitational potential energy of the air is therefore increased. The only place that energy can come from is from the energy that is already in the air, namely in the thermal motions of the atoms and molecules themselves. These slow down as the air rises, transferring their energy to gravitational potential energy and lowering the temperature of the air

A submarine near the surface of the ocean takes on just enough water so that it becomes denser than the water surrounding it. It then begins to sink. If all the hatches are sealed and no more water is taken onto the submarine, and no water or air leaves the submarine, what will happen to the sub?

It will sink all the way to the bottom of the ocean, because once denser than water it remains denser than water. Besides, buoyancy is determined by the pressure difference across an object, not by the absolute value of the pressure. Yes - The later part of the answer is critical. Even though the pressure at the bottom of the ocean is much higher, it is higher on all parts of the submarine. It is only the pressure difference that counts, not the particular value of the pressure at any one spot.

This may test some of your ancient movie knowledge about Superman. In the movie of the same name 33 years ago, Lois Lane dies in an Earthquake. Superman is so upset that he zooms around the Earth to the west so fast that he makes the Earth stop spinning to the East, and reverse its direction, somehow undoing the damage to Lois. Suppose that through some cosmic mishap, Superman finds that he must suddenly obey the law of conservation of momentum as he flies faster and faster westward, what would the Earth's correct response be?

It would necessarily turn faster and faster to the East

Regarding the principle of conservation of energy and the conservation of kinetic and potential energy it is general true that

No one form of energy is "conserved" by itself, and that invariably kinetic energy is converted to thermal or heat energy.

Suppose a helicopter is hovering high up in the air where it is cold and that you are down on the ground where it is hot. You are not directly under the helicopter, but are some distance away. Does the sound of the helicopter come at you in a straight line like the light from the helicopter does?

No. Soundwaves refract, and sound moves slower in cold air than it does in warm air, so the sound will refract, or bend, as it comes to you from the helicopter. It will not travel in a straight line, and if you listen carefully, your brain will tell you that the sound is coming from a different position than that of the visual position of the helicopter.

What does "resonance" mean? What happens to a system that "resonates" when something vibrates it at that resonant frequency?

Resonance occurs when a system is able to store and easily transfer energy between two or more different storage modes (such as kinetic energy and potential energy in the case of a pendulum). However, there are some losses from cycle to cycle, called damping. When damping is small, the resonant frequency is approximately equal to the natural frequency of the system, which is a frequency of unforced vibrations. Some systems have multiple, distinct, resonant frequencies. Resonance is also the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Frequencies at which the response amplitude is a relative maximum are known as the system's resonant frequencies, or resonance frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.

Which statement has the most true statements about sound?

Sound is a mechnical wave. It is a longitudal wave in which the force that produces the wave moves back and forth along the direction the wave travels in. Sound waves can propagate through any material object made of atoms, including air, because the mode of propagation is basically for one atom to hit its neighbor. In air, the speed of sound only depends on how hot it is. The hotter it is the faster sound most. In some other liquid or solid substance it depends more on how elastic and dense that material is.

Two different guitar strings are the same length, but one is more than twice the mass of the other. Which statement below has the most correct statments? Both strings are tightened with the same tension force.

Speed of the waves along the strings is proportional to the tension or stiffness of the string divided by its mass. So the heavier string will have the slower waves. Since both strings are the same length, each will have the same fundamental wavelengths for waves that set up along them. Since the wavelengths are the same, but the speeds are different, then the frequencies must be different too. In this case, the heavier string, with the slower wave will have the lower frequency. Waves that set up along the guitar string are examples of wave interference. Speed = frequency x wavelength, AND Speed is proportional to Tension/mass So to satisfy both statements, the heavier string has the lower frequency or lower pitch The wave patterns on the string are the result of waves reflecting from both ends of the string and interfering with one another to make the complete pattern

Spiderman finds himself swing back and forth at the end of a long strand of webbing. There is very little friction in the web or with the air. As a result the swinging motion continues for some time. Which option has the most true statements in it?

Spiderman is swinging back and forth because he has some excess energy compared to an equilibrium rest position. As he swings, his kinetic energy is exchanged for gravitional potential energy and back again in such a way that the total energy is a fixed constant number. The period of the swing depends only on the length of the web, but not on Spiderman's mass. Not only that, but for even reasonably large angles, the period of the swing does not depend on how big the angle or amplitude of the swing is. It is close to being true that for angles of less that about 30 degrees, the period of the swing depends only on the length of the web, and not on the angle through which it swings. All the other stuff is true too.

The Incredible Hulk holds up two large, fully loaded M-1 tanks. He holds these for several minutes in order to prevent them from falling on Susie, a 10 year old 5th grade girl who happened to pass through the midst of an Avengers battle. While Hulk is holding up the tanks, Susie stoops down and plucks a nearby flower to offer it to the Hulk for his services. Who does more work in this interval of time?

Susie does. She did work in lifting the flower. Hulk, despite whatever must be made to maintain the tension in his muscles is doing no work whatsoever, because he is not transferring any energy to the tanks he is holding. (Although he must have done work in the past to get them where they are being held.)

Sound travels faster through water than through air because water is "stiffer" than air.

TRUE

A child's swing is a naturally resonant system. What would happen if you tried to push a child on a swing at a frequency with which is does not naturally swing?

The amplitude of the oscillation would damp out, to the point where very little energy was being put into the swing.

Which of the following is not determined by the tension and length of a plucked guitar string?

The amplitude of the wave- the amplitude is determined by how hard you pluck the string. The pitch is determined by the length and tension of the string.

Dog is laying on the ground minding its own business when a cat walks by. The dog bounds up and gives chase. Before that cat came, there dog had no motion. How does the principle of conservation of momentum apply?

The dog's motion came from the Earth, and the Earth itself lost and equal amount of motion to what the dog gained. You can say the Earth recoiled from the bounding dog!

The principle of Special Relativity is simply the statement that

The laws of physics are the same in all inertial reference frames, where no apparent external forces are in effect.... If there are forces producing accelerations, then all bets are off. It requires "General Relativity" to account for the General case of accelerated motion and Special Relativity to account for the Special case of no acceleration.

You are zipping along to the next BYU away game, and get going a little to fast. So fast, in fact, that you zoom right past the football stadium. As you do so, one of your passengers measures the length of the football field and finds that

The length of the football field appears shorter than 100 yards owing fundamentally to the arrival times of photons from different ends of the field. This is called the Lorentz contraction. Not only is time slowed, but lengths are shortened. ... Not in YOUR frame, where things are at rest, but in the rest of the moving universe.

What is an Aeolian Harp?

The musical sounds of an Aeolian Harp are the results are of vortex "swirls" flipping around in the back of the strings... left-right.... left-right... You can watch these vortexes in the left-right ripples moving along a flag during a medium breeze. It is this very same alternating push-pull (at resonance frequency) that can cause a flexible suspension bridge to twist and sway larger and larger..... until destruction. It was this over stress from excessive twisting, that destroyed the Tacoma Narrows Bridge.

As the tension on a guitar string is increased, the pitch of the note it plays goes up. Why?

The speed of any wave is proportional to the "stiffness" of the thing oscillating. Tightnening the string makes it stiffer, increasing the stiffness, and increasing the resonant frequency. Since the length is not changed, the speed of the wave must increase too.

The following experiment illustrates the twin paradox: Of two synchronized atomic clocks, one was left stationary on the ground, while the second was flown around the world in a jet. After the two were together, it was easy to see the the travelling clock had aged less by simply seeing that it now running behind the stationary clock. Ultimately, the reason for the time difference is because:

The traveling clock accelerated more than the stationary one - It's all in the acceleration. Time really does slow down in accelerated conditions. Somehow putting a force on something slows down time.

A large volume of water is sent through a funnel. The exit of the funnel is 100 times smaller than the entrance to the funnel. As a consequence:

The water sprays out 100 times faster than when it entered the funnel. The extra kinetic energy could only come from the thermal energy of the fluid itself. That means the internal pressure and temperature of the water must be less when it exits the funnel. Yeah... that's Bernoulli's law.

A child being pushed on a swing-set in the park is an example of resonance.

True

A resonant response is one in which a maximum effect is observed for a minimum input in effort.

True

Any mechanical usually has very many natural frequencies or "modes" at which it can resonate.

True

At resonance, a mechanical system is vibrating at (at least one of) its natural frequency(ies).

True

Harmonies in western music are based on simple integer ratios of harmonic waves.

True

Momentum is defined to be mass x velocity. In the absence of external forces, the mass x velocity of any object cannot change. This is a consequence of the law of conservation of momentum.

True

Pressure in a fluid depends only on the depth of the fluid and not on the lateral extent. For instance, the pressure at the bottom of a vertical, water-filled pipe that is 100 feet tall is the same as that at the bottom of a large lake that is 100 feet deep.

True

The SS Newtonian, defender of galactic laws, fires a rocket at an angry red bird of prey alien spaceship. As it does so, it recoils because of the force exerted on it by the very projectile that it launches.

True

The continents of the Earth float on its surface in much the same way that icebergs float in the ocean. The higher the continent projects above "sea level" the deeper the continental material is rooted into the mantle of the Earth.

True

The speed of a wave depends on the material properites fo the medium through which it travels. The stiffer the material (the more elastic it is), the higher the speed, while the denser the material, the slower the speed.

True

Two speakers are separated by about a meter or so, and are broadcasting the same audio signal. There will be places in the near vicinity of the speaker where some listeners will hear a stronger signal that others because of constructive interference of the signals from the two speakers.

True

Ultimately, the principle of conservation of momentum can be linked to Newton's third law. Since the forces acting between objects must be equal and opposite, then for a give interaction time, the momentum change in one object must be equal an opposite to the momentum change in the other.

True

Virtually every physical signal that any instrument (including your human senses) is detected and processed as wave signal.

True

Which observation is NOT consistent with the principle of conservation of angular momentum?

Water in a bath-tub flowing down a vortex or an eddy into the drain must slow down as it nears the drain in order to pass into it. The water speeds up as it nears the drain!

The fact, that while swimming, it is almost impossible to splash away a bug floating on the water reflects the fact that

Waves transport energy and force, not mass - Your splashed wave might lift the bug up and down, but it doesn't move it any where on average. You need to create a current to do that!

In terms of momentum what does a force do when it acts on an object? How does the principle of conservation of momentum apply in the collision between two objects? Answer specifically in the case of objects colliding with different masses and different initial speeds.

When two objects interact, a force, acting for a certain time between them, produces an impulse which changes the momentum of both objects. Whatever momentum one object gain, the other must have an equal, but opposite change. It does not matter what their relative speeds are. The change in the momentum of the two must be equal and opposite. And, for a given change in momentum, the longer the time of collision, the smaller the force required to affect the change in momentum.

A space ship is seen approaching Earth at a high rate of speed. It shines a laser at us to let us know that it is coming. What can you say about the relative motion between the moving spacecraft and Earth has on the laser beam?

Whether a light beam travels against Earth's orbital motion or with it the measured value of its speed is the same. Light indeed travels the same speed regardless of the velocity of the people measuring the light.

What makes water waves in lakes and oceans?

Wind and bernoulli's law. The wind blowing over a ripple in the wave lowers the pressure above it, causing it to be pushed up a little more. The wave grows the longer the wind blows.

Suppose that the suspension system of your car had no shock absorbers. What would be the likely consequence?

Without shocks, the suspension springs would have little damping or friction, so as the car drove over bumps and things, the resonant modes of vibration would be set up in the springs, and the amplitude of the oscillations could grow to the point of destruction of the whole suspension system - remember that "resonance" is that condition in which energy is added to a system as exactly that systems natural vibration rate. In such a circumstance, energy is absorbed by the oscillator, but not dissipated. So the amplitude of the oscillation can grow out of control

Which is not an example of refraction?

You can hear sound waves around corners - this is an example of diffraction.

Suppose a freezing rain completely coated the campus. You find yourself totally stuck in the middle of the quad on completely frictionless ice. Despite your best efforts, any attempt to walk simply results in your falling down in a less-than-graceful slip. In order to get out of the center of the quad to the edge, you need some momentum. How are you going to get it? Which way would work best?

You take each one of your books one at a time, and fling them successively in the same direction as hard as you can. You recoil in the other direction - technically speaking this would work better that throwing your backback all at once. The reason being is that you should be able to throw each book at a higher velocity, and hence each would carry away more momentum than would be the case if the lot of them were thrown at once.

The kind of wave pattern on the Tacoma narrows bridge is called

a standing wave

How much does the air in your bath tube weigh?

about a pound

Suppose that one person throws a shot put at 10 meters a second, and another puts it 20 meters a second. How many times more kinetic energy does the second put than the first?

four times as much

A weight lifter does work to lift a weight and gives the weight

gravitational potential energy

A space ship appears be approaching Earth at a very high rate of speed. It shines a laser at us to let us know that it is coming. When we measure the speed of the laser light we find that

its speed is exactly "c" - Light can only move in space at light speed.

The video on energy shows how energy is transferred from one form to the another using a

pole vaulter: The kinetic energy of the runner is transformed to the elastic potential energy of the pole which is then turned into the gravitational potential energy of the vaulter as he rises.

The reason that Earthquake waves "curve" as they travel through the Earth is because

the speed of the wave changes as the properties of the Earth change, and this causes continuous refraction of the wave. Refraction doesn't have to occur just at the interface of something like air and water or air and glass. It the speed of the wave changes as it moves, then it must refract.

A standing wave is formed when

two (or more) traveling waves of the same wavelength interfere with each other through the principle of superposition.

Suppose that an angry bird of mass 1 kg, flies through the air at a speed of 100 miles per hour, and that at some point, the bird hits a (presumably happy) pig of mass 99 kilograms, who had, until that time, been stationary. Suppose further, that the bird sticks to the pig. What is the speed of the pig and the bird together immediately after the collision?

1 mile per hour

A boat captain notices a wave takes 2 seconds to pass along the 30 foot length of his boat. What is the frequency of the wave?

1/2 cycle per second Yep - the length of the boat is irrelevant here. It is enough to know that it takes 2 seconds to complete one cycle.

Kinetic energy is defined as

1/2 x mass x velocity squared

If a wave has a period of 0.05 seconds, what is the waves frequency?

20 cycles per second. Sure... 0.05 is 1/20 th of a second. There are 20 repetitions of the wave every second.

An opera singer breaks a wine glass by singing a note that is resonant with the glass. The frequency of the tone is 1000 cycles per second and the wavelength of the wave is 0.33 meters. What is the speed of the soundwave?

330 meters per second Speed equal wavelength x frequency

In order for anything to ocsillate, what must be true?

A stable, elastic, mechanical system (a tree, for instance) have some added extra energy, and that energy must be able to be easily transferred between those forms of energy. Sounds hard - but its really simple. A tree sways in the breeze, because the wind gives it some extra energy. That energy then sloshes back and forth between the elastic energy stored by the bending branches, and the kinetic energy of the swaying limbs.

Define resonance:

A state in which you get the maximum oscillatory response for the minimum amount of energy input. A situation often involving more than one object in which the vibrations of one object occur at the natural vibration rate of another, and the two objects exchange energy.