Physics Unit 2 Review
In the the doppler effect, does frequency change? Does wavelength change? Does wave speed change?
Frequency and wavelength both change, but wave speed does not
Define refraction?
Is the bending that occurs when a wave travels into a new medium at an angle
What is the equation for impulse?
Impulse = force x time
What is it that determines the pitch of a note?
The frequency of the fundamental tone in the waveform
A skipper on a boat notices wave crests passing his anchor chain every 5 s? He estimates the distance between wave crests to be 15 m? He also correctly estimates the speed of the waves? What is this speed?
v = fx, so v = 1/5 s^-1 15 m = 3 m/s
On a slide a child has potential energy that decreases by 1000 J while her kinetic energy increases by 900 J. What other form of energy is involved, and how much?
100 J of energy were loss in conversion, probably wasted as heat because of friction work
This question typical on some driver's license exams: A car moving at 50 Km/h skids 15 m with locked brakes. How far will the car skid with locked brakes at 150 km/h?
135 m.
A moving car has kinetic energy. If it speeds up until it is going four times as fast, how much kinetic energy does it have in comparison?
16 x
What is the average range of a young person's hearing?
20 Hz - 20 kHz
How much does it cost to run a 200 W television for 100 hours, if energy cost $0.10/kWh?
200 W = 0.2 kW. The energy used is 0.2 kW x 100 hours = 20 kWh. 20 kWh x 0.10/kWh = $2.00
What is the angle for maximum range of a projectile?
45
How many seconds does it take sound to travel one mile?
About 5 seconds
Another friend says the moon's gravity would prevent rocks dropping from the moon to the Earth, but that if the moon's gravity somehow vanished to zero, then rocks on the moon would fall to Earth. What is wrong with this assumption?
Any rock on the moon be in orbit around Earth and would thus remain in orbit, even without a gravity force from the Moon to hold it there,
A friend says that astronauts in orbit are weightless because they're beyond the pull of Earth gravity. Correct your friend's ignorance?
Astronauts in orbit are quite close Earth, so the force of gravity acting on their bodies is pretty close to Earth-normal. The force must be there, or the astronauts would not be in orbit. They're weightless because they're falling.
When G was first measured by Henry Cavendish, his experiment was called the "weighing the Earth experiment." Why?
At the time of Cavendish's experiment we knew how big Earth but not its mass. When G was measured, we knew the valuable of every factor in the gravity law except the mass of Earth and thus able to solve for it.
A cat can hear sound frequencies up to 70,000 Hz. Bats send and receive ultrahigh-frequency squeaks up to 120,000 Hz. Which hears sound of shorter wavelength, cats or bats?
Bats
Which requires more work - lifting a 50-kg sacks a vertical distance of 2 m or lifting a 25 kg sacks a vertical distance of 4 m?
Both require an equal amount of work: 100 J
What sense does the moon "fall?"
Earth's gravity is pulling on the moon and causes it to accelerate ("fall") toward the Earth while its sideways motion keeps it orbit. So as it goes forward it "falls" toward Earth.
In one word, what is it that moves from source to receiver in wave motion?
Energy
True or false: When air resistance does not affect the motion of a projectile, its horizontal and vertical components of velocity remain constant?
False. The vertical component will be affected by gravity and accelerated at 9.8 m/s^2. The horizontal component is the constant one.
Why will a projectile that moves horizontally at 8 km/s follow a curve that matches the curvature of the Earth?
For that speed, the force of gravity is just right to provide a centripetal force that corresponds to circle equal to the circumference of the Earth.
If a skater who is spinning pulls her arm in so as to reduce her rotational inertia to half, by how much will her angular momentum increase? By how much will her rate of spin increase? (Why are your answers different?)
Her angular momentum will not change - it is a conserved quantity and will remain constant as long as she experiences no outside torque. Her rate of spin will double since her rotational inertia has become half as large
What is the relationship between frequency and pitch?
Higher pitch corresponds to higher frequency
give an everyday example of momentum conservation?
If a moving box car collides and couples with an identical box car at rest, they will move together at half the speed of the moving box car, since momentum is conserved in the collisions.
Cite an example where a force is exerted on an object without doing work on the object?
If the object doesn't move, there's no work. Hold up a wall and no work will be done on the wall
A fully dressed person is at rest in the middle of a pond on perfectly frictionless ice and must get to shore. How can this be accomplished
If the surface is perfectly frictionless, you can't push against it to make yourself move. You're going to take off your clothes, piece by piece, and throw them away from the direction you want to move. Each piece you throw will increase you velocity.
Give an everyday example where you observe the Doppler Effect?
If you are stopped at a railroad crossing and a train passes while sounding its whistle, it sounds higher on approach and lower as it passes by.
Why is it incorrect to say that impulse equals momentum?
Impulse is equal to the change in momentum
Describe the monkey gun experiment, and what it tells us about projectile motion?
In the monkey gun demonstration, an object is dropped at the same time a projectile is fired toward it. Regardless of how fast the projectile is launched, it will hit the object. This demonstrates that the horizontal and vertical motions of the projectile are independent of each other.
Distinguish between infrasonic and ultrasonic sound waves?
Infrasonic waves have frequencies too low to be heard by human ears
What does a torque tend to do to an object?
Is a twisting force - it produces rotation.
Define wave?
Is energy that travels through a medium. ER, however, needs no medium since it is made of oscillating electric and magnetic fields.
Define diffraction?
Is the bending that occurs when a wave moves past an edge or around an obstacle
Why does a horizontally moving projectile have to have a large speed to become an Earth satellite?
It has to move sideways so fast that it misses the edge of the Earth as it falls, like Isaac Newton's magic cannon.
An apple hanging from a limb has potential energy because of its height. If falls, what becomes of this energy just before it hits the ground? When it hits the ground?
It will be kinetic just before hitting the ground. After hitting the ground, the energy has becomes heat, potential energy due to the arrangement of apple molecules, sound energy, and perhaps light energy if the limb is REALLY high.
How many joules of kinetic energy does 1 -Kg book have when it is tossed across the room at a speed of 2 m/s?
K.E. = 0.5 x 1 Kg x (2 m/s)^2 = 2 J
Consider a ball thrown straight up in the air. At what position is its kinetic energy a maximum? where is its gravitational potential energy a maximum?
KE is maximum when leaving the hand, potential is maximum at the top.
If a bell is ringing inside bell jar, we can no longer hear it when the air is pumped out, but we can still see it. What differences in the properties of sound and light does this indicate?
Light can travel through vacuum but sound can't, ergo a silent Death Star explosive in Star Wars IV
What is the equation for momentum?
Momentum = Mass x Velocity
When the mass of a moving object is doubled with no change in speed, by what factor is its momentum changed? Its kinetic energy?
Momentum is 2x, Kinetic energy is 2x.
A 60 - kg skydiver moving at terminal speed falls 50 m in 1 s. What power is the skydiver expending on the air?
P = PE/t = mgh/t = 60 kg x 9.8 m/s^2 x 50 m/1 s = 29400 w
What is rotational inertia, and how does it compare to inertia as studied in previous chapters?
Rotational inertia is a measure of how hard it is to spin something; to get it to rotate. It depends on the mass of the object but also on the distribution of the mass around the rotation axis. Farther away from the axis means it's harder to spin.
What kind of motion should you impart to a stretched coiled spring (or slinky) to provide a transverse wave? a longitudinal wave?
Shake the slinky sideways for transverse; gather the coils together and release for longitudinal. You can also create longitudinal waves by pushing and pulling along the length of the spring
Distinguish between sound intensity and loudness?
Sound intensity is a physical measure of the energy of a sound wave, while loudness is a subjective quite different for different frequencies, and changes in intensity are not identical to changes in loudness
When you whirl a can at the end of a string in a circular path, what is the direction of the force that is exerted on the can?
The can experiences a centripetal force, toward the center.
If a ball is projected upward from the ground with 10 kg m/s of momentum. What is the momentum of recoil of the world? Why do we not feel this?
The change of momentum of the Earth will be equal and opposite: 10 kg m/s. The Earth is what we scientists call "freaking huge," however, so this change in momentum will produce an incredibly tiny change in the speed of the Earth.
When a bullet is fired, its momentum indeed changes! Also the momentum of the recoiling rifle changes. So momentum is conserved for the bullet, and momentum is not conserved for the rifle. Why can we nevertheless say that when a rifle fires a bullet, momentum is conserved?
The changes for bullet and gun are equal and opposite to each other, so when you add a positive to an equal negative the net zero, which is what you start with.
How does the paper come of a radio loudspeaker emit sound?
The cone vibrates and pushes back and forth against air, creating longitudinal airwaves?
How many times louder is a 110 dB sound than an 80 dB sound?
The difference is 30 dB. Every 10 dB is a 10 x increase in loudness. So 10x10x10 = 1000x louder
The wavelength of a transverse wave is the distance between successive crests (or troughs). What is the wavelength of a longitudinal wave?
The distance between successive regions of compression (or regions or rarefaction).
How does the V shape of a bow wave depend on the speed of the source?
The faster the source travels, the smaller the angle between the edges of the wave
When produces the greater change in kinetic energy: exerting a 10-N force for a distance of 5 m, or exerting a 20-N force over a distance of 2 m? (Assume that all of the work goes in KE)
The first one is a work of 50 J, which is larger than the second (40 J)
Why doesn't the force of gravity change the speed of a satellite in circular orbit?
The force of gravity acts at right angles to the motion of the satellite, pulling it neither faster or slower but just keeping it in orbit like a mass on a string.
What does it mean to say that a radio station is "at 101.1 on your FM radial"?
The frequency for FM stations are given in units of MHz, so that station broadcasts a radio wavelength twice as long as source A.
Why is it easier to stop a lightly loaded truck than a heavier one that has equal speed?
The heavier one will have more kinetic energy, thus requiring more work to stop it
A rock is thrown upward at an angle. What happens to the horizontal component of its velocity as it rises? As it falls?
The horizontal component is unaffected during both the rise and fall.
For the same force. Which cannon imparts the greater speed to cannonball - a long cannon or a short one? explain
The long cannon can apply force for a longer time, thus a larger impulse, thus a greater change in momentum. Long barrels look scarier too.
To bring a supertanker to a stop, its engines are typically cut off about 25 km from port. Why is it so difficult to stop or turn a supertanker?
The momentum of the huge boat is commensurately large. You need to apply a force for a LONG time in order to change the momentum enough to get the boat to stop.
How does the force of gravity between two bodies change when the distance between them is doubled.
The new force will be 1/4 as much as the original.
What are the three principal classes of musical instruments?
The shape of the sound wave (its waveform), which is composed of the fundamental and overtones for a given instrument.
In olympic competition, a microphone picks up the sound of the starter's gun and sends it electrically to the speakers at every runner's starting block? Why?
The sound from the gun travels from the gun to the runners' ears. The nearest runner will hear the sound before anyone else, which isn't fair.
Why is lightning seen before thunder is heard?
The speed of light is much faster than the speed of sound
If the frequency of sound is doubled, what change will occur in its speed? in its wavelength?
The speed will not change but the wavelength will be half as long.
If a gas tap is turned on for a few seconds, someone a couple meters away will hear the gas escaping long before she smells it. What does this indicate about the speed of the sound and the motion of molecules in the sound-carrying medium?
The wave motion through the molecules travels much faster than the molecules themselves.
You dip your finger repeatedly into a puddle of water and make waves. What happens to the wavelength if you dip your finger more frequently?
The wavelength gets shorter
Why do flying bees buzz?
Their wings beat at an audible frequency, producing a sound wave
Why is the moon described as a "silent planet'?
There is no air on the moon, thus no sound. In space no one can hear you scream.
Why do gymnasts use floor-mats that are very thick?
Thicker mats apply forces that last longer to change to the momentum of the jumpers, thus smaller forces.
A car is lifted a certain distance in a service station and therefore has potential energy relative to the floor. If it were lifted twice as high. How much potential energy would it have?
Twice as much energy
What is the source of all sounds?
Vibrations that push against air, creating longitudinal airwaves.
You are in front of a floating canoe near a dock. You jump, expecting easily to land on the dock. Instead you land in water explain?
When jumping from a boat, you apply a force to the boat so that it can apply force to you. The change of momentum of the boat will be equal and opposite to that of your body. If the boat is small, you will have to really push it hard and make it go away from you quickly in order to generate enough impulse to get you to the dock. Otherwise splash.
Does a sonic boom occur at the moment when an air-craft exceeds the speed of sound? Explain?
Yes, but not just at that moment as many people think. The bow wave is powerful when the plan is supersonic, traveling away from the plane and audible wherever it crosses the path of a listener
When a sound wave moves past a point in air, are there changes in the density of air point? Explain?
Yes- the compression part of a sound wave represents an increase in air pressure. The air particles are closer together, thus at a higher pressure. The rarefied parts of the wave have a lower air pressure
What is the magnitude of the gravitational force between the Earth and a 1 kilogram body?
You don't need to use W = mg = 1 Kg x 9.8 m/s^2 = 9.8 N.
Why is it a good idea to have your hand extended forward when you are getting ready to catch a fast-moving baseball with your bare hand?
You need to let your hand travel backwards after impact so that the impact lasts longer. For a given p, longer time means less force.
When do you weigh more; at the bottom of Death Valley, or atop the high sierras? Why?
You weigh more in a valley because you are closer to the center of Earth and thus the force of gravity is larger
What kind of wave is each of the following: (a) an ocean wave rolling toward Waikiki beach? (b) The sound of the whale calling another whale under water? (c) A pulse sent down a stretched rope by snapping one end of it?
a) Transverse b) longitudinal c) transverse
two important regions of a sound wave?
are areas of compression and rarefaction
What causes a standing wave?
are formed when waves of the same wavelength travel in opposite directions in the medium
What is decibel, and how many correspond to the lowest intensity sound we can hear?
is a relative measure of the loudness of the sound. It is a logarithmic scale created by comparing all sound intensities to a sound intensity barely audible by humans, 10^12 W/m^2, which is the threshold of hearing (0 db)
What are the three principals characteristics of musical tones?
pitch, loudness, and quality (timbre)
