physics concept questions

Force F1 acts on a particle and does work W1. Force F2 acts simultaneously on the particle and does work W2. The speed of the particle does not change. Which one of the following must be true?

W1 = - W2

Two ice skaters, Paul and Tom, are each holding on to opposite ends of the same rope. Each pulls the other toward him. The magnitude of Paul's acceleration is 1.25 times greater than the magnitude of Tom's acceleration. What is the ratio of Paul's mass to Tom's mass?

0.8

A hoop is rolling (without slipping) on a horizontal surface so it has two types of kinetic energy: translational kinetic energy and rotational kinetic energy. The entire mass M of the hoop is concentrated at its rim, so its moment of inertia is I = MR2, where R is the radius. What is the ratio of the translational kinetic energy to the rotational kinetic energy?

1

The velocity of a train is 80.0 km/h, due west. One and a half hours later its velocity is 65.0 km/h, due west. What is the train's average acceleration?

10.0 km/h2, due east

An airplane starts from rest and reaches a takeoff velocity of 60.0 m/s due north in 4.0 s. How far has it traveled before takeoff?

120 m, due north

A ball is thrown straight upward with a velocity v0−→v0→ and in a time t reaches the top of its flight path, which is a displacement y→y→ above the launch point. With a launch velocity of 2v0−→2v0→ , what would be the time required to reach the top of its flight path and what would be the displacement of the top point above the launch point?

2t and 4y→

A boat moves 3.0 km due north and then moves 1.0 km due south. The time for this trip is half an hour. What is the average velocity of the boat?

4 km/h, due north

A powerboat, starting from rest, maintains a constant acceleration. After a certain time t, its displacement and velocity are r→r→ and v→v→. At time 2t, what would be its displacement and velocity, assuming the acceleration remains the same?

4r→ and 2v→

What constant acceleration would a car, starting from rest, have to sustain in order to reach a velocity of 27 m/s due east in 3.0 s?

9.0 m/s2, due east

A certain object is not in equilibrium. Three of the following statements could possibly be true, and one must be true. Which one of the following must be true?

A net force acts on the object.

Which one of the following descriptions violates Newton's first law of motion?

A rabbit's foot is hanging on a chain from the rearview mirror of a car. As the car accelerates, the rabbit's foot swings toward the windshield.

An object is moving at a constant velocity. All but one of the following statements could be true. Which one cannot be true?

A single force acts on the object.

Which one of the following objects is in equilibrium?

A space probe with its engines shut down, moving through a vast empty region of outer space.

The drawing shows three containers filled to the same height with the same fluid. In which container, if any, is the pressure at the bottom greatest?

All three contains have the same pressure at the bottom.

Two balls are thrown from the top of a building, as in the drawing. Ball 1 is thrown straight down, and ball 2 is thrown with the same speed, but upward at an angle θ with respect to the horizontal. Consider the motion of the balls after they are released. Which one of the following statements is true?

Both balls have the same acceleration at all times.

Two objects, A and B, have the same volume and are completely submerged in a liquid, although A is deeper than B. Which object, if either, experiences the greater buoyant force?

Both objects experience the same buoyant force.

What is the difference between distance and displacement?

Displacement is a vector, while distance is not a vector.

Under what condition(s) is the angular momentum of a rotating body, such as a spinning ice skater, conserved?

Each external torque may be non-zero, but the sum of the torques must be zero.

A jogger runs along a straight and level road for a distance of 8.0 km and then runs back to her starting point. The time for this roundtrip is 2.0 h. Which one of the following is true?

Her average speed is 8.0 km/h, and her average velocity is 0 km/h.

Two cars are moving in the same direction (the positive direction) on a straight road. The acceleration of each car also points in the positive direction. Car 1 has a greater acceleration than car 2 has. Which one of the following statements is true?

In the same time interval, the velocity of car 1 changes by a greater amount than car 2 does.

An object is oscillating in simple harmonic motion with an amplitude A and an angular frequency ω. What should you do to increase the maximum speed of the motion?

Increase both A and ω by 10%.

Your car is traveling behind a jeep. Both are moving at the same speed, so the velocity of the jeep relative to you is zero. A spare tire is strapped to the back of the jeep. Suddenly the strap breaks, and the tire falls off the jeep. Will your car hit the spare tire before the tire hits the road? Assume that air resistance is absent.

No. The car will not hit the tire before the tire hits the ground, no matter how close you are to the jeep.

A rocket is sitting on the launch pad. The engines ignite, and the rocket begins to rise straight upward, picking up speed as it goes. At about 1000 m above the ground the engines shut down, but the rocket continues straight upward, losing speed as it goes. It reaches the top of its flight path and then falls back to earth. Ignoring air resistance, decide which one of the following statements is true.

Only part of the rocket's motion, from just after the engines shut down until just before it lands, is free-fall.

Three runners start at the same place. Shaun runs 4.0 km due east and then runs 1.0 km due west. Mark runs 3.0 km due east. Jeff runs 2.0 km due west and then runs 5.0 km due east. Which of the following is true concerning the displacement of each runner?

Shaun, Mark, and Jeff have the same displacements.

The drawing shows identical stones that are being whirled on a tabletop at the same speed. The radius of the larger circle is twice that of the smaller circle. How is the tension T1 in the longer cord related to the tension T2 in the shorter cord?

T1 = (1/2)T2

A high-jumper, having just cleared the bar, lands on an air mattress and comes to rest. Had she landed directly on the hard ground, her stopping time would have been much shorter. Using the impulse-momentum theorem as your guide, determine which one of the following statements is correct.

The air mattress exerts the same impulse, but a smaller net average force, on the high-jumper than does the hard ground.

A block is attached to a horizontal spring. On top of this block rests another block. The two-block system slides back and forth in simple harmonic motion on a frictionless horizontal surface. At one extreme end of the oscillation cycle, when the blocks come to a momentary halt, the top block is lifted vertically upward, without disturbing the bottom block. What happens to the amplitude and the angular frequency of the ensuing motion?

The amplitude remains the same, and the angular frequency increases.

Suppose you are driving due east, traveling a distance of 1500 m in 2 minutes. You then turn due north and travel the same distance in the same time. What can be said about the average speeds and the average velocities for the two segments of the trip?

The average speeds are the same, but the average velocities are different.

Two beakers are filled to the same level with water. One of them has a plastic ball floating in the water. If the beakers are placed on a scale, one at a time, which weighs more?

The beakers have the same weight.

The wheels on a moving bicycle wheel have both translational (or linear) and rotational motions. What is meant by the phrase "a rigid body, such as a bicycle wheel, is in equilibrium?"

The body can have translational and rotational motions, as long as its translational acceleration and angular acceleration are zero.

The apparent weight of a passenger in an elevator is greater than his true weight. Which one of the following is true?

The elevator is either moving upward with an increasing speed or moving downward with a decreasing speed.

A force F acts on an object undergoing a displacement s, the force being oriented at an angle θ with respect to the displacement. Which one of the following correctly describes ALL of the circumstances under which the work done by the force is zero?

The force is zero, or the displacement is zero, or the angle is 90°.

A nonzero net force acts on a particle and does work. Which one of the following statements is true?

The kinetic energy, speed, and velocity of the particle change.

Two cars are driving at the same constant speed v around a racetrack. However, they are traveling through turns that have different radii, as shown in the drawing. Which statement is true about the magnitude of the centripetal acceleration of each car?

The magnitude of the centripetal acceleration of the car at A is greater than that of the car at B, since the radius of the circular track is smaller at A.

A tennis ball is hit upward into the air and moves along an arc. Neglecting air resistance, where along the arc do the minimum and maximum speeds occur?

The maximum speed occurs at the lowest point and the minimum occurs at the highest.

You have two springs. One has a greater spring constant than the other. You also have two objects, one with a greater mass than the other. Which object should be attached to which spring, so that the resulting spring-object system has the greatest possible period of oscillation?

The object with the greater mass should be attached to the spring with the smaller spring constant.

Consider two people, one on the earth's surface at the equator and the other at the north pole. Which has the larger centripetal acceleration?

The person at the equator

The drawing shows a side view of a swimming pool. The pressure at the surface of the water is atmospheric pressure. The pressure at the bottom of the pool is greater because of the weight of water above it. As a storm approaches, the atmospheric pressure drops. What happens to the pressure at the bottom of the pool?

The pressure at the bottom of the pool decreases by exactly the same amount as the atmospheric pressure decreases.

The muzzle velocity of a gun is the velocity of the bullet when it leaves the barrel. The muzzle velocity of one rifle with a short barrel is greater than the muzzle velocity of another rifle that has a longer barrel. In which rifle is the acceleration of the bullet larger?

The rifle with the short barrel

The same force F is applied to the edge of two hoops. The hoops have the same mass, although the radius of the larger hoop is twice that of the smaller one. The entire mass of each hoop is concentrated at its rim, so the moment of inertia is I = Mr2, where M is the mass and r is the radius. Which hoop has the greater angular acceleration, and how many times as great is it compared to that of the other hoop?

The smaller hoop; two times as great.

A bicyclist races around a circular track and covers the same number of meters per second everywhere. Which one of the following is true?

The speed of the bicyclist is constant.

Which one of the following statements describes the condition(s) for which a rigid body is in equilibrium?

The sum of the externally applied forces must be zero, and the sum of the externally applied torques must be zero.

An object is thrown upward at an angle Ө above the ground, eventually returning to earth. Which statement is true?

The velocity and acceleration are perpendicular only at the top of the trajectory.

Only one of the following is possible for a moving object. Which one?

The velocity is zero at one instant, and the acceleration is not zero at that instant.

Two forces F1 and F2 act on a particle. As a result the speed of the particle increases. Which one of the following is NOT possible?

The work done by each force is negative.

In preparation for this question, review Conceptual Example 7. Vectors A→A→ and B→B→ satisfy the vector equation A→+B→=0A→+B→=0. How does the magnitude and direction of B→B→ compare to the magnitude and direction of A→A→?

Their magnitudes are the same, but their directions aren't.

Two identical cars are traveling at the same speed. One is heading due east and the other due north, as the drawing shows. Which statement is true regarding the kinetic energies and momenta of the cars?

They have the same kinetic energies, but different momenta.

An experimental vehicle slows down and comes to a halt with an acceleration whose magnitude is 9.80 m/s2. After reversing direction in a negligible amount of time, the vehicle speeds up with an acceleration of 9.80 m/s2. Except for being horizontal, is this motion (a) the same as or (b) different from the motion of a ball that is thrown straight upward, comes to a halt, and falls back to earth? Ignore air resistance.

a

The kinetic energy of an object attached to a horizontal ideal spring is denoted by KE and the elastic potential energy by PE. For the simple harmonic motion of this object the maximum kinetic energy and the maximum elastic potential energy during an oscillation cycle are KEmax and PEmax, respectively. In the absence of friction, air resistance, and any other nonconservative forces, which of the following equations applies to the object-spring system? A. KE + PE = constant B. KEmax = PEmax

a and b

A rotating object has an angular acceleration α that is not zero. Which one or more of the following three statements is consistent with a non-zero angular acceleration? A. The angular velocity ω increases as time passes. B. The angular velocity ω has the same value at all times. C. The angular velocity ω decreases as time passes

a and c, NOT b

A cup of coffee is sitting on a table in a recreational vehicle (RV). The cup slides toward the rear of the RV. According to Newton's first law, which one or more of the following statements could describe the motion of the RV? (A) The RV is at rest, and the driver suddenly accelerates. (B) The RV is moving forward, and the driver suddenly accelerates. (C) The RV is moving backward, and the driver suddenly hits the brakes.

a, b, and c

In a race two horses start from rest and each maintains a constant acceleration. In the same elapsed time Silver Bullet runs 1.20 times farther than Shotgun. According to the equations of kinematics, which one of the following is true concerning the accelerations of the horses?

aSilver Bullet = 1.20 aShotgun

A race finishes at the same place as it started. The contestants start from rest and cross the finish line running. Which one of the following is true about the average speed, the average velocity, and the average acceleration? Note that the symbol ≠ means "not equal to".

average speed ≠ 0 m/s, average velocity = 0 m/s, average acceleration ≠ 0 m/s2

A ball is dropped from rest from the top of a building and strikes the ground with a speed vf. From ground level, a second ball is thrown straight upward at the same instant that the first ball is dropped. The initial speed of the second ball is v0 = vf, the same speed with which the first ball eventually strikes the ground. Ignoring air resistance, decide whether the balls cross paths (a) at half the height of the building, (b) above the halfway point, or (c) below the halfway point.

b

Two objects are thrown vertically upward, first one, and then, a bit later, the other. Is it (a) possible or (b) impossible that both objects reach the same maximum height at the same instant of time?

b

A rotating object has an angular acceleration of α = 0 rad/s2. Which one or more of the following three statements is consistent with a zero angular acceleration? A. The angular velocity is ω = 0 rad/s at all times. B. The angular velocity is ω = 10 rad/s at all times. C. The angular displacement θ has the same value at all times.

b only

Two balls are launched upward from the same spot at different angles with respect to the ground. Both balls rise to the same maximum height. Ball A, however, follows a trajectory that has a greater range that that of ball B. Ignoring air resistance, decide which ball, if either, has the greater launch speed.

ball a

A rifle, at a height H above the ground, fires a bullet parallel to the ground. At the same instant and at the same height, a second bullet is dropped from rest. In the absence of air resistance, which bullet, if either, strikes the ground first?

both

The average acceleration is zero when __________

both the magnitudes and the directions of the initial and final velocities are the same.

Two cars both start from rest and have the same constant acceleration. When radar is used to measure the velocity of each car, car A is found to be moving twice as fast as car B. At the instant the radar is used, car A has a displacement that is ______ times as great as the displacement of car B.

four

You are in a tall building located near the equator. As you ride an elevator from the ground floor to the top floor, your tangential speed due to the earth's rotation ____________.

increases

Air resistance is a nonconservative force. It always opposes the motion of an object. An airplane flies from New York to Atlanta and then returns to its point of departure. The net work done by air resistance during this round trip ___________.

is negative

A person is riding on a Ferris wheel. When the wheel makes one complete turn, the net work done on the person by the gravitational force ____________.

is zero

The speedometer of your car shows that you are traveling at a constant speed of 35 m/s. Is it possible that your car is accelerating?

its possible

An ice skater is spinning on frictionless ice with her arms extended outward. She then pulls her arms in toward her body, reducing her moment of inertia. Her angular momentum is conserved, so as she reduces her moment of inertia, her angular velocity increases and she spins faster. Compared to her initial rotational kinetic energy, her final rotational kinetic energy is _________

larger, because her angular speed is larger.

A runner runs half the remaining distance to the finish line every ten seconds. She runs in a straight line and does not ever reverse her direction. Does her acceleration have a constant magnitude?

no

An object moving with a constant acceleration slows down if the acceleration points in the direction opposite to the direction of the velocity. But can an object ever come to a permanent halt if its acceleration truly remains constant?

no

At one instant of time, a car and a truck are traveling side by side in adjacent lanes of a highway. The car has a greater velocity than the truck has. Does the car necessarily have the greater acceleration?

no

The car in the drawing is moving clockwise around a circular section of road at a constant speed. What are the directions of its velocity and acceleration at positions 1 and 2?

position 1: velocity south, acceleration west. position 2: velocity west, acceleration north

Is the average speed of a vehicle a vector or a scalar quantity?

scalar

Which one of the following is not a kinematic variable that appears in the equations of kinematics?

speed

One of the following statements is incorrect. (a) The car traveled around the circular track at a constant velocity. (b) The car traveled around the circular track at a constant speed. Which statement is incorrect?

statement a

The top of a cliff is located a distance H above the ground. At a distance H/2 there is a branch that juts out from the side of the cliff, and on this branch a bird's nest is located. A stone is thrown at the nest with the same initial speed, either straight downward from the top of the cliff or straight upward from the ground. In the absence of air resistance, which stone hits the nest in the least amount of time?

stone thrown from the top of the cliff

A certain object is in equilibrium. Which one of the following statements is not true?

the object must be at rest

Vectors A→A→, B→B→, and C→C→ satisfy the vector equation A→+B→=C→A→+B→=C→, and their magnitudes are related by the scalar equation A2+B2=C2A2+B2=C2. How is vector A→A→ oriented with respect to vector B→B→?

the two vectors are perpendicular

Vectors A→A→, B→B→, and C→C→ satisfy the vector equation A→+B→=C→A→+B→=C→, and their magnitudes are related by the scalar equation A+B=CA+B=C. How is vector A→A→ oriented with respect to vector B→B→?

their directions are the same

Of the five kinematic variables, how many must be known before the equations of kinematics can be used to determine the unknown variables in a problem that deals with a single moving object?

three

In the absence of air resistance two balls are thrown upward from the same launch point. Ball A rises to a maximum height above the launch point that is four times greater than that of ball B. The launch speed of ball A is ________ times greater than that of ball B.

two

A person, standing on the ground, measures the velocity of a car to be vCG = velocity of the Car relative to the Ground = +25 m/s The driver of the car looks out the window and sees the ground go past her with a velocity of vGC, where vGC = velocity of the Ground relative to the Car How is vGC related to vCG?

vGC is the negative of vCG, so vGC = - vCG = -25 m/s

At each moment during its flight, a projectile has a velocity v and an acceleration a. The components of these variables are: vx = x component of the velocity vy = y component of the velocity ax = x component of the acceleration ay = y component of the acceleration Which of these components remain constant throughout the motion? Assume that air resistance is absent.

vx, ax and ay are constant.

In which one of the following situations can the equations of kinematics not be used?

when the acceleration changes from moment to moment

The magnitude of a vector has doubled, its direction remaining the same. Can you conclude that the magnitude of each component of the vector has doubled?

yes

A wheel rotates with a constant angular speed ω. Which one of the following is true concerning the angular acceleration α of the wheel, the tangential acceleration aT of a point on the rim of the wheel, and the centripetal acceleration ac of a point on the rim?

α = 0 rad/s2, aT = 0 m/s2, and ac ≠ 0 m/s2

A motorcycle starts from rest and has a constant acceleration. In a time interval t, it undergoes a displacement x and attains a final velocity v. Then t is increased so that the displacement is 3x. In this same increased time interval, what final velocity does the motorcycle attain?

√3v