physics exam 2

By what transfer mechanisms does energy enter and leave for a tv

(a) For the television set, energy enters by electrical transmission (through the power cord) and electromagnetic radiation (the television signal). Energy leaves by heat (from hot surfaces into the air), mechanical waves (sound from the speaker), and electromagnetic

By what transfer mechanisms does energy enter and leave for a gas powered lawn mower

(b) For the gasoline-powered lawn mower, energy enters by matter transfer (gasoline). Energy leaves by work (on the blades of grass), mechanical waves (sound), and heat (from hot surfaces into the air).

You are shipwrecked and floating in the middle of the ocean on a raft. Your cargo on the raft includes a treasure chest full of gold that you found before your ship sank and the raft is just barely afloat. To keep you floating as high as possible in the water, should you (a) leave the treasure chest on top of the raft, (b) secure the treasure chest to the underside of the raft, (c) hang the treasure chest in the water with a rope attached to the raft? (Assume that throwing the treasure chest overboard is not an option you wish to consider!)

(b) or (c). In all three cases, the weight of the treasure chest causes a downward force on the raft that makes it sink into the water. In (b) and (c), however, the treasure chest also displaces water, which provides a buoyant force in the upward direction, reducing the effect of the weight of the chest on the raft.

By what transfer mechanisms does energy enter and leave for a hand cranked pencil sharpener

(c) For the hand-cranked pencil sharpener, energy enters by work (from your hand turning the crank). Energy leaves by work (done on the pencil), mechanical waves (sound), and heat resulting from the temperature increase from friction.

What is the gravitational acceleration close to the surface of a planet with twice the mass and twice the radius of Earth? Answer as a multiple of g, the gravitational acceleration near Earth's surface.

0.5 g

A block of mass m is dropped from the fourth floor of an office building, subsequently hitting the sidewalk at speed v. From what floor should the mass be dropped to double that impact speed?

16th floor

If the radius of the output piston is doubled, the output force increases by a factor of:

4

a bow is drawn so it has 40 J of potential energy. when fired how much kinetic energy does it have

40 J

a tv is pushed a distance of 2 m and a force of 20N . how much work is done

40 J

Suppose an asteroid has a semimajor axis of 4 AU. How long does it take the asteroid to go around the sun?

8 years because T2=a3 T=43=8

Suppose you are on a cart, initially at rest on a track with very little friction. You throw balls at a partition that is rigidly mounted on the cart. If the balls bounce straight back as shown in the figure, is the cart put in motion? a) Yes, it moves to the right. b) Yes, it moves to the left. c) No, it remains in place. 5

ANS: B Before any balls are thrown, the momentum of the balls-cart system is 0 N∙s. While the balls are moving towards the partition they have a non-zero momentum directed left. Since momentum is conserved, the cart must then have a non-zero momentum directed right, and the cart moves to the right. But once the balls hit the partition, their momentum changes direction, so so too must the cart to maintain a total momentum of 0 N∙s. Since the rebounded balls no longer interact with the cart, the cart will keep its left- directed momentum and continue to move left, passed its original position.

Suppose a ping-pong ball and a bowling ball are rolling toward you. Both have the same momentum, and you exert the same force to stop each. How do the time intervals to stop them compare? a) It takes less time to stop the ping-pong ball. b) Both take the same time. c) It takes more time to stop the ping-pong ball.

ANS: B The change in momentum will be the same for both the golf ball and the bowling ball (same initial to zero), so if the same force is applied on both, the time required to stop them will be the same.

Consider two carts, of masses m and 2m, at rest on an air track. If you push first one cart for 3 s and then the other for the same length of time, exerting equal force on each, the momentum of the light cart is a) four times b) twice c) equal to d) one-half e) one-quarter

ANS: C The change in momentum will be the same for both carts since the force and the time interval over which the force was applied was the same for both objects. Since they both had the same initial momentum, they will both have the same final momentum.

A comet moves in an elliptical orbit around the Sun. Which point in its orbit represents the highest value of the total energy of the comet-Sun system?

All points. The total energy of the system is the same regardless of where the comet is in its orbit

A cylinder is rolling without slipping down an inclined plane. The friction at the contact point P is 1. Static and points up the inclined plane. 2. Static and points down the inclined plane. 3. Kinetic and points up the inclined plane. 4. Kinetic and points down the inclined plane. 5. Zero because it is rolling without slipping. 15

Answer 1. The friction at the contact point P is static and points up the inclined plane. This friction produces a torque about the center of mass that points into the plane of the figure. This torque produces an angular acceleration into the plane, increasing the angular speed as the cylinder rolls down.

Two cylinders of the same size and mass roll down an incline, starting from rest. Cylinder A has most of its mass concentrated at the rim, while cylinder B has most of its mass concentrated at the center. Which reaches the bottom first? 1)A 2) B 3) Both at the same time.

Answer 2: Because the moment of inertia of cylinder B is smaller, more of the mechanical energy will go into the translational kinetic energy hence B will have a greater center of mass speed and hence reach the bottom first.

A bowling ball is initially thrown down an alley with an initial speed v0, and it slides without rolling but due to friction it begins to roll until it rolls without slipping. Which quantity is constant before it rolls without slipping? 1. Energy. 2. Angular momentum about the center of mass. 3. Angular momentum about a fixed point on the ground

Answer 3. Energy is not conserved because there are energy losses due to kinetic friction. Angular momentum about the center of mass is not constant because the friction exerts a torque about the center of mass. Angular momentum about a fixed point on the ground is constant because the sum of the torques about that point is zero. The friction force will always be parallel to the line of contact between the bowling bowl and the surface. So, if we pick any fixed point along the line of contact between the bowling bowl and the surface then the vector from the point to the contact point where friction acts is either parallel or anti- parallel and hence the torque is zero. The torque about a fixed point on the ground due to the gravitational force and the normal forces are in opposite directions, have the same moment arms, and because mg = N have the same magnitudes hence add to zero.

wo cylinders of the same size but different masses roll down an incline, starting from rest. Cylinder A has a greater mass. Which reaches the bottom first? 1)A 2) B 3) Both at the same time. Concept Q. Ans.: Cylinder Race Different Masses

Answer 3. The initial mechanical energy is all potential energy and hence proportional to mass. When the cylinders reach the bottom of the incline, both the mechanical energy consists of translational and rotational kinetic energy and both are proportional to mass. So as long as mechanical energy is constant, the final velocity is independent of mass. So both arrive at the bottom at the same time.

If 3 balls are thrown with the same initial speed, but in different directions as shown in the figure. Rank the speeds on impact with the ground from fast to slow. Neglect air resistance.

Answer is D, all have the same speed the moment they hit the ground because all start with the same kinetic energy and undergo the same total change in gravitational potential energy.

A comet moves in an elliptical orbit around the Sun. Which point in its orbit represents the highest value of the potential energy of the comet-sun system

Aphelion. The potential energy of the comet-Sun system is highest when the comet is at its farthest distance from the Sun.

A disk rotates about a fixed axis that is perpendicular to the disk and passes through its center. Which of the following statements is true?

At any instant, every point on the disk has the same angular velocity.

A car accelerates from rest. In doing so the absolute value of the car's momentum changes by a certain amount and that of the Earth changes by a) a larger amount. b) the same amount. c) a smaller amount. d) The answer depends on the interaction. 1

B Since momentum is conserved during the interaction between the car and the Earth, and initially the total momentum of the Earth-car system was 0 N∙s, the final momentum must also be 0 N∙s. The only way to achieve this is for the Earth to have the same magnitude of momentum as the car (but in the opposite direction).

Consider these situations: (i) a ball moving at speed v is brought to rest; (ii) the same ball is projected from rest so that it moves at speed v ; (iii) the same ball moving at speed v is brought to rest and then projected backward to its original speed. In which case(s) does the ball undergo the largest change in momentum? a) (i) b) (i) and (ii) c) (i), (ii), and (iii) d) (ii) e) (ii) and (iii) f) (iii)

F Since momentum is a vector quantity, slowing it down and then speeding it up to the same speed in the opposite direction is double the change in momentum compared to slowing it down or speeding it up separately.

A rigid body is rotating counterclockwise about a fixed axis. Each of the following pairs of quantities represents an initial angular position and a final angular position of the rigid body. Which of the sets can occur only if the rigid body rotates through more than 180°? 6 rad -3 rad =3 rad 1 rad -(-1rad)= 2 rad 5 rad -1 rad =4 rad

For a rotation of more than 180º, the angular displacement must be larger than p=3.14 radians. so the last one

You tape two different sodas straws together end to end to make a longer straw with no leaks. The two straws have radii of 3 mm and 5 mm. You drink a soda through your combination straw. In which straw is the speed of the liquid higher?

It is higher in the one of radius 3 mm. The liquid moves at the highest speed in the straw with the smallest cross sectional area

A comet moves in an elliptical orbit around the Sun. Which point in its orbit represents the highest value of the speed of the comet,

Perihelion. Because of conservation of angular momentum, the speed of the comet is highest at its closest position to the Sun.

A comet moves in an elliptical orbit around the Sun. Which point in its orbit represents the highest value of the kinetic energy of the comet

Perihelion. The kinetic energy is highest at the point at which the speed of the comet is highest.

Two spheres, one hollow and one solid, are rotating with the same angular speed around an axis through their centers. Both spheres have the same mass and radius. Which sphere, if either, has the higher rotational kinetic energy?

The hollow sphere has the larger moment of inertia, so it will have the higher rotational kinetic energy. I=MR2

Two forces are acting on an object. Which of the following statements is correct? The object is in equilibrium if the net torque on the object is zero. The object is in equilibrium if the forces are equal in magnitude and opposite in direction. The object cannot be in equilibrium because more than one force acts on it. The object is in equilibrium if the forces act at the same point on the object. The object is in equilibrium if the net force and the net torque on the object are both zero.

The object is in equilibrium if the net force and the net torque on the object are both zero.

In a perfectly inelastic one-dimensional collision between two objects, what condition alone is necessary so that all of the original kinetic energy of the system is gone after the collision?

The objects must have momenta with the same magnitude but opposite directions.

A solid disk and a hollow disk are released together (from rest) to roll down an incline. They have the same diameter and mass.

The solid disk will reach the bottom first. because it has a lower moment of inertia

an ice cube is placed in a glass of water. What happens to the level of the water as the ice melts?

The water level remains the same when the ice melts. This is because volume=mass/density. When the ice melts, it turns into water of volume. The added volume is the same and therefore the level of water will not change. A floating object like an ice cube displaces an amount of water equal to its own weight.

Andrea and Chuck are riding on a merry-go-round. Andrea rides on a horse at the outer rim of the circular platform, twice as far from the center of the circular platform as Chuck, who rides on an inner horse. When the merry-go-round is rotating at a constant angular speed, Andrea's tangential speed is which of the following?

a) twice Chuck's, Andrea and Chuck have the same angular speed, but Andrea moves in a circle with twice the radius of the circle followed by chuck. Thus from vt=rw, it is seen that Andrea's tangential speed is twice Chucks.

Andrea and Chuck are riding on a merry-go-round. Andrea rides on a horse at the outer rim of the circular platform, twice as far from the center of the circular platform as Chuck, who rides on an inner horse. When the merry-go-round is rotating at a constant angular speed, Andrea's angular speed is which of the following?

b) the same as Chuck's, All points on a rotating body have the same angular speed

Suppose you are standing directly behind someone who steps back and accidentally stomps on your foot with the heel of one shoe. Would you be better off if that person were (a) a large male professional basketball player wearing sneakers or (b) a petite woman wearing spike-heeled shoes?

basketball player Because the basketball player's weight is distributed over the larger surface area of the shoe, the pressure (F/A) that he applies is relatively small. The woman's lesser weight is distributed over the very small cross-sectional area of the spiked heel, so the pressure is high.

An object moves in a circular path with constant speed v. Which of the following statements is true concerning the object?

c) Both the velocity and the acceleration are changing in direction so neither of these vector quantities is constant

perigee

close to earth

perihelion

close to sun

Cars used to be built as rigid as possible to withstand collisions. Today, though, cars are designed to have "crumple zones" that collapse upon impact. What is the advantage of this new design? Explain. 7

crumple zones are like air bags in that they increase the time of interaction during a collision and therefore lower the average force required for the change in momentum that the car undergoes in the collision

If the raft is placed in salt water, which has a density greater than fresh water, the value of h would: (Select all that apply.) decrease because each cubic meter of salt water displaced weighs more than fresh water. decrease because less volume of salt water must be displaced to have its weight equal that of the raft. not change. increase because the saltwater displaced is heavier. increase because the raft must displace more water.

decrease because each cubic meter of salt water displaced weighs more than fresh water. decrease because less volume of salt water must be displaced to have its weight equal that of the raft.

A skater is using very low friction rollerblades. A friend throws a Frisbee at her, on the straight line along which she is coasting. Describe each of the following events as an elastic, an inelastic, or a perfectly inelastic collision between the skater and the Frisbee. she tries to catch the frisbee but it bounces off her hands and falls

elastic

apogee

far from earth

apohelion

far from sun

We claim that momentum is conserved, yet most moving objects eventually slow down and stop. Explain. 6

for momentum to be conserved, the system must be closed

A dart is loaded into a spring-loaded toy dart gun by pushing the spring in by a distance d. For the next loading, the spring is compressed a distance 2d. How much work is required to load the second dart compared to that required to load the first?

four times as much

the ways that mechanical energy is lost from the system in this experiment include

friction to bring the bullet to a stop relative to the block deformation of the bullet as it enters the wood work in deforming the block

If global warming continues, it's likely that some ice from the polar ice caps of the Earth will melt and the water will be distributed closer to the Equator. If this occurs, the length of the day (one rotation) would do which of the followin

increase

If global warming continues, it's likely that some ice from the polar ice caps of the Earth will melt and the water will be distributed closer to the Equator. If this occurs, the length of the day (one revolution) would do which of the following?

increase Earth already bulges slightly at the Equator, and is slightly flat at the poles. If more mass moved toward the Equator, it would essentially move the mass to a greater distance from the axis of rotation, and increase the amount of inertia. Because conservation of angular moment requires that wzIz = constant, an increase in the amount of inertia would cause a decrease the angular velocity , and slow down the spinning earth. Thus, the length of the each day would increase.

What happens to your angular velocity as you pull in your arms on a free rotating bar seat?

increases

A competitive diver leaves the diving board and falls toward the water with her body straight and rotating slowly. She pulls her arms and legs into a tight tuck position. What happens to her angular speed?

increases The diver is an isolated system, so the product Iω remains constant. As the moment of inertia of the diver decreases, the angular speed increases by the same factor. For example, if I goes down by a factor of 2, ω goes up by a factor of 2.

A competitive diver leaves the diving board and falls toward the water with her body straight and rotating slowly. She pulls her arms and legs into a tight tuck position. (ii) From the same list of choices, what happens to the rotational kinetic energy of her body

increases The rotational kinetic energy varies as the square of ω. If I is halved, ω2 increases by a factor of 4 and the energy increases by a factor of 2.

What happens to your kinetic energy as you pull in your arms?

increases Your angular velocity increases and moment of inertia decreases, but angular velocity is squared, so KE will increase with increasing angular velocity

A skater is using very low friction rollerblades. A friend throws a Frisbee at her, on the straight line along which she is coasting. Describe each of the following events as an elastic, an inelastic, or a perfectly inelastic collision between the skater and the Frisbee. she catches the frisbee and immediately throws it back with the same speed

inelastic

A merry-go-round rotates with constant angular speed. As a rider moves from the rim of the merry-go-round toward the center, what happens to the magnitude of total centripetal force that must be exerted on him?

it decreases

Alex and John are loading identical cabinets onto a truck. Alex lifts his cabinet straight up from the ground to the bed of the truck, whereas John slides his cabinet up a rough ramp to the truck. Which statement is correct?

john does more work

A boy standing at one end of a floating raft that is stationary relative to the shore walks to the opposite end of the raft, away from the shore. As a consequence, the raft does which of the following? [Hint: Use conservation of momentum.]

moves towards shore Because the momentum of the system (boy+raft) remains constant with zero magnitude, the raft moves toward the shore as the boy walks away from the shore.

an object that has kinetic energy must be

moving

During inhalation, the pressure in the lungs is slightly less than external pressure and the muscles controlling exhalation are relaxed. Under water, the body equalizes internal and external pressures. Discuss the condition of the muscles if a person under water is breathing through a snorkel. (a) Would a snorkel work in deep water?

no The external pressure exerted on the chest by the water makes it difficult to expand the chest cavity and take a breath while under water so it would not be possible to use a snorkel in deep water.

If two particles have equal momenta, are their kinetic energies equal?

no except when their masses are equal

A skater is using very low friction rollerblades. A friend throws a Frisbee at her, on the straight line along which she is coasting. Describe each of the following events as an elastic, an inelastic, or a perfectly inelastic collision between the skater and the Frisbee. she catches the frisbee and holds it

perfectly inelastic

a job is done slowly while an identical job is done quickly. both jobs require the same amount of work but different amounts of a. energy b. power c. neither d. both

power

In a game of billiards, a red billiard ball is traveling in the positive x-direction with speed v and the cue ball is traveling in the negative x-direction with speed 3v when the two balls collide head on. Which statement is true concerning their velocities subsequent to the collision? Neglect any effects of spin.

red ball: -3v; cue ball: v

If the angle of the slope is increased, the change of gravitational potential energy between the two heights:

remains the same and depends only on the difference between the two heights

A student sits on a barstool holding a bike wheel. The wheel is initially spinning CCW in the horizontal plane (as viewed from above) L= 25 kg m2/s She now turns the bike wheel over. What happens?

she starts to spin clockwise Start w/ angular momentum L pointing up from wheel. When wheel is flipped, no more angular momentum from it pointing up, so need to spin person/stool to conserve L!

Give two reasons why skiers typically assume a crouching position when going down a slope.

ski poles push more effectively when they are nearly horizontal air resistance is lower in the crouched position

The pressure at the bottom of a filled glass of water (ρ = 1 000 kg/m3) is P. The water is poured out and the glass is filled with ethyl alcohol (ρ = 806 kg/m3). What is the pressure at the bottom of the glass?

smaller than p Because both fluids have the same depth, the one with the smaller density (alcohol) will exert the smaller pressure

What happens to the angular momentum as you pull in your arms on a free rotating bar seat?

stays the same Because angular momentum is conserved in this case, it must stay the same, so when the skater decreases his/her radius, the velocity of rotation must increase since they can't increase their mass, so the skater spins faster.

A car and a large truck traveling at the same speed collide head-on and stick together. Which vehicle experiences the larger change in the magnitude of its momentum?

the change in magnitude of momentum is the same for both

When a person is involved in a car accident, why is the likelihood of injury greater in a head-on collision as opposed to being hit from behind? Answer using the concepts of relative velocity, momentum, and average force.

the change in momentum is greater in the head-on collision. The average force on the driver is greater in the head-on collision.

a real spring will continue to vibrate slightly after the mass has left it. How would this affect the results

the departing block leaves at a lower speed it results in less energy being transferred to the block it decreases the max height of the block

A solid disk and a hoop are simultaneously released from rest at the top of an incline and roll down without slipping. Which object reaches the bottom first?

the disk arrives first

An apple is held completely submerged just below the surface of a container of water. The apple is then moved to a deeper point in the water. Compared with the force needed to hold the apple just below the surface, what is the force needed to hold it at a deeper point?

the same For a totally submerged object, the buoyant force does not depend on the depth in an incompressible fluid.

A satellite moves in a circular orbit at a constant speed around Earth. Which of the following statements is true?

the satellite has an acceleration directed toward earth

A rigid body is rotating counterclockwise about a fixed axis. Each of the following pairs of quantities represents an initial angular position and a final angular position of the rigid body. Suppose that the change in angular position for each of the pairs of values below occurred in 1 s. Which choice represents the lowest average angular speed? [6 rad -3 rad]/1s = 3 rad/s [1 rad -(-1rad)]/1s= 2 rad/s [5 rad -1 rad]/1s = 4 rads/s

the second b) Because all angular displacement occurred in the same time interval, the displacement with the lowest value will be associated with the lowest average speed

If the net work done on a particle is zero, which of the following statements must be true?

the speed is unchanged

You observe two helium balloons floating next to each other at the ends of strings secured to a table. The facing surfaces of the balloons are separated by 1 to 2 cm. You blow through the opening between the balloons. What happens to the balloons?

they move toward each other The high-speed air between the balloons results in low pressure in this region. The higher pressure on the outer surfaces of the balloons pushes them toward each other.

if you push an object twice as far while applying the same force you a. do twice as much work b. four times as much work c. same amount of work

twice amount

when an object is lifted 10 meters it gains a certain amount of potential energy . if the same object is lifted 20 meters the potential energy gain is

twice as much

A car of mass m traveling at speed v crashes into the rear of a truck of mass 2m that is at rest and in neutral at an intersection. If the collision is perfectly inelastic, what is the speed of the combined car and truck after the collision?

v/3