PHY 309K Final Exam

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conceptual physics final exam

A ball is rolling across the top of a billiard table and slowly rolls to a stop. How would Aristotle interpret this observation? How would Galileo interpret it?

Aristotle would say that the ball was striving to reach its "natural state" of rest. Galileo would say that the friction between the ball and the table stopped it.

How would you find velocity given free fall from rest? How would you find distance fallen?

velocity = 10 x time
distance fallen = 1/2(10t^2)

What is the instantaneous velocity of a freely falling object 10s after it is released? What is its average velocity over the interval? How far will it go during this time?

instantaneous velocity = g x t = 98 m/s
avg velocity = (Vo+Vf)/2 = 49 m/s
distance traveled = 1/2 gt^2 = 490 m

Suppose you drop an object from an airplane traveling at a constant velocity and air resistance does not affect it.

1. What will be its falling path by someone at rest on the ground with a clear view?

2. What will be the falling path by you looking downward from the airplane?

3. Where will the object strike the ground relative to you in the airplane?

4. Where will it strike in the more realistic case where air resistance is involved?

1. It will be falling in a parabolic trajectory.

2. It will look like the object was falling straight down.

3. It will look like the object would strike the ground directly below the moving airplane.

4. With air resistance, horizontal component will decrease, so object will fall BEHIND the moving airplane.

A park ranger shoots a monkey hanging from a branch of a tree. The ranger aims directly at the monkey, not realizing that the dart will follow a parabolic path and fall below the monkey. The monkey sees the dart and lets go of the branch.

Will the monkey be hit anyway? Does the velocity of the dart affect your answer, assuming it is great enough to travel the horizontal distance to the tree before hitting the ground?

The monkey WILL be hit. This will occur because the dart and the monkey are both falling at the same rate (gravity). As long as the velocity will cover the horizontal distance, the specific magnitude does not matter.

Drop a sheet of paper and a coin at the same time. Which reaches the ground first? Why?

Now crumple the paper into a small, tight wad and again drop it with the coin. Explain the difference observed.

Will they fall together if dropped from higher storied windows?

The coin reaches the ground first because it is affected less by air resistance than the paper, meaning the coin has greater inertia.

After crumpling the paper, they both hit the ground at the same time. This also occurs from higher-storied windows.

Your weight is the result of a gravitational force of the earth on your body. What is the corresponding reaction force?

It is the gravitational force of your body on the earth.

A horse pulls a heavy wagon with a certain force. The wagon, in turn, pulls back with an opposite, but equal force on the horse. Doesn't this mean the forces cancel each other, making acceleration impossible?

NO. Acceleration is still possible because each force acts on a different object, so the action and reaction do not cancel out.

In the absence of air resistance, if a ball is thrown vertically upward with a certain initial speed, on returning to its original level it will have the same speed.

When air resistance is a factor affecting the ball, will the ball fall to the ground faster, the same, or slower?

The ball will fall slower because as it falls, the ball will gain speed until air resistance equals its weight. Then, acceleration will cease and the ball will fall at a constant speed (at terminal velocity).

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?

The person can take off an article of clothing and throw it in the opposite direction of which they want to go. By Newton's 3rd law, the force exerted on the clothing is equal and opposite to the force exerted on the person throwing it. Because these are both internal forces, momentum is conserved.

Your friend says that the law of momentum conservation is violated when a ball rolls down a hill and gains momentum. What do you say?

The law of momentum conservation says that IF there are no external forces acting on the system, momentum will not change. Since, gravity, an external force, affects the ball, it is not a violation of the law when the ball gains momentum.

Before rockets in space were commonplace, a misconception was that a rocket needs air to push against.

How is a rocket propelled in a region completely devoid of an atmosphere?

The rocket exerts a force equal and opposite to the direction it needs to move in. Because gravity is not an issue in outer space, there are no external forces, and momentum is conserved,

If a Mack track and a Volkswagen have a head-on collision, which vehicle will experience the greater force of impact? The greater impulse? The greater change in momentum? The greater acceleration?

The force of impact, impulse, and momentum are all the same.

The Volkswagen has a greater acceleration than the Mack truck.
V: a = f/small mass = BIG A
M: a = f/large mass = small a

At what point in its motion is the KE of a pendulum bob a maximum?

A what point is PE a maximum?

When KE is 1/2 its max, how much PE does it have?

KE is at its max at the bottom of the swing.

PE is at its max at the endpoints (top).

When KE is 1/2, PE is 1/2.

Ask a friend to stand facing a wall. With toes against the wall, ask your friend to stand on the balls of her feet without toppling backward. Explain why this can't be done.

This occurs because when my toes are against the wall, I can't counterbalance myself. My center of gravity is not above a point of support.

Using the ideas of torque and center of gravity, explain why a ball rolls down a hill.

The center of gravity of the ball is perpendicular to the earth. If it is on a level surface, the center of gravity coincides with where the ball touched the surface. Thus, there is no lever arm and 0 torque to start rolling.

If the surface is at a slope, the point of contact of the ball on the surface is NOT in line with the center of gravity, The distance between the two lines is the lever arm, through which the mass of the ball is able to exert torque --> THE BALL CAN ROLL!

If you should buy a quantity of gold in Mexico and weigh it on a spring balance, how would the weight be different if you weighed it in Alaska?

The gold would weigh MORE in Alaska. Centrifugal force is less in Alaska than Mexico, since Alaska is closer to the Earth's axis of rotation.

Does a centripetal force that acts on an object rotating on a circular path do work on the object?

NO. The direction of the force is perpendicular to the direction of velocity, so 0 work is done.

A sizable quantity of earth is washed down the Mississippi River and deposited in the Gulf of Mexico. What effect does this have on the length of a day?

The radius is larger in the Gulf of Mexico than at the mouth of the Mississippi. This means the earth at the Gulf is like the man with his arms spread, who is moving at a lesser rotational velocity. Thus, rotational speed of the Earth is decreased and the length of a day is INCREASED.

The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater, smaller, or the same size as the force with which the moon attracts the earth?

The earth and the moon have the SAME force on each other (Newton's 3rd law)

Why do the passengers in a high-altitude jet planes feel the sensation of weight while passengers in an orbiting space vehicle such as the space shuttle do not?

Passengers in an orbiting space vehicle are essentially falling towards the Earth. However, the pull of gravity is balanced by the centrifugal force of the vehicle (caused by its rotational speed while in orbit). Thus, they feel weightless.

Passengers in a jet plane do not feel a net force, only the force of gravity, because it moves at a constant velocity and is not in free fall.

You weigh a tiny bit less when you are in the lobby of a massive skyscraper. Why is this so?

When you move further away from the center of the earth, the gravitational pull you feel is decreased.

Consider two bridges that are exact replicas of each other except that every dimension in the larger bridge is exactly twice of the other. Which bridge is stronger?

The smaller bridge is stronger. While the larger bridge can hold 4 times as much capacity, it weighs 8 times as much as the old bridge (giving only 50% efficiency)

Punch a couple of holes in the bottom of a water filled container, and water will spurt out because of water pressure. Now drop the container, and as it freely falls, note that the water no longer spurts out. Why is this the case?

This occurs because both the water and the container and falling together, in the same direction by the force of gravity.

A barge filled with scrap iron is in a canal lock. If the iron is thrown overboard, does the water level at the side of the lock rise, fall, or remain unchanged?

The water level will FALL because when the iron is thrown overboard, the barge will rise. When the scrap is dropped into the canal, the water level rises, but not as much as the original since iron is more dense than water.

Fold the ends of a filing card down so you make a little bridge. Stand it on the table and blow through the arch. Why will the card not blow off the table?

Bernoulli's Principle. The moving air pushes against the arch of the paper with less pressure than the air pushes the other surfaces. Thus, it remains at rest.

Why do bubbles of gas rising in a liquid become larger as they approach the surface?

There is less pressure at the top, so they expand.

Would it be more difficult to draw soda through a straw at sea level of on top of a very high mountain?

Mountain, because there is less atmospheric pressure, therefore less force to push the soda up through the straw

The force outside of a 10m2 store window is about 1 million N. Why does this not shatter the window? Why might the window shatter in a strong wind?

This does not shatter the window because there is equal pressure inside and outside the window. In a strong wind, the pressure is only on one side of the window, so the force does not cancel out any more.

Why is it that when passing an oncoming truck on the highway, your car tends to lurch toward the truck?

Bernoulli's Principle. When my car passes the truck, the air around is compressed in between, increasing velocity and decreasing pressure. The greater pressure on the other side of my car to sway me towards the truck.

Which has greater internal energy, an iceberg or a cup of hot coffee?

Iceberg because it has a greater mass.

When a mercury thermometer is warmed, the level goes down before it rises. Why?

The outside glass of thermometer is heated first, and thus expands first, causing the mercury level to sink before it is heated. When it is heated, it rises faster than the glass expands.

If cooling occurred at the bottom of a pond instead of at the surface, would a lake freeze from the bottom up?

No. This is because when ice forms at the bottom, it will float up to the top of the lake because it is less dense than water.

Why would no harm result if a piece of wood were licked on a very cold day?

Metal is a better conductor than wood. Wood is a poor conductor, a better insulator.

You can bring water in a paper cup to a boil by placing it in a hot flame. Why doesn't the paper cup burn?

Water boils at a lower temperature that paper burns. Energy from the flame is transferred through the paper to the water by conduction. The water absorbs the energy that would have heated the cup.

In a mixture of H and O gases at the same temperature, which move faster?

Hydrogen molecules. Because they're at the same temp, KE is equal. Because its mass is so small, its velocity must be that much larger to equal O.

The temperature of the mixture may initially be well above freezing temp, yet crystals of snow are formed as the mixture is ejected from the nozzle. HOW?

The mixture of compressed air and water expands when it is ejected and COOLS. As it hits the freezing temp, ice forms.

Would thermal equilibrium be possible if good absorbers were poor emitters and poor absorbers were good emitters?

No. If good absorbers were poor emitters, their temps would rise above the poor absorbers. Also the poor absorbers temps would fall below the others.

On a very cold sunny day you were a black coat and a transparent plastic coat. Which should be worn on the outside?

The plastic coat. The sun's rays will pass through it and be trapped in the layer between the black coat and the plastic.

Porous canvas bags filed with water are slung on the outside of a fast-moving car. The water inside is cooled. Why?

The canvas bag is kept cool because of evaporation. When the car moves extremely fast, the water molecules leave the cloth faster and the temp of the bag and water decrease.

Why will spraying fruit trees with water before a frost help to protect the fruit from freezing?

The water that is sprayed releases heat when it freezes. This heat raises the temperature of the tree and the air around it, and prevents the fruit from freezing.

Water evaporates from a salt solution and leaves behind salt crystals. Has the entropy principle been violated?

No. When water evaporates, it take energy from its surroundings. This would normally reduce entropy, but since it is an open system and the salt crystals were left behind, net entropy is increased.

Suppose one wishes to cool a kitchen by leaving the refrigerator door open and closing the kitchen door and windows. What will happen to the room temperature? Why?

It will increase. The heat from inside the refrigerator will transfer to the kitchen.

If a pendulum is shortened, how do frequency and period change?

frequency increases, period decreases

What kind of motion should you impart to a slinky for a transverse or longitudinal wave?

t: up and down/side to side
l: to and fro

If the handle of a tuning fork is held solidly against a table, the sound from the tuning fork becomes louder. Why? How will this affect the length of the time the fork keeps vibrating?

It becomes louder because more surface vibrates and more air is pushed. It will decrease the time because more sounds energy is released so the tuning fork loses energy more quickly.

Explain how you can produce a low-pitched note on a guitar by altering length, tension, or thickness of string.

increase length, decrease tension, increase thickness

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