32 Created by Master Student Physics Practice Test #4 1 / 40 The ends of a long, taut tightrope are attached to two platforms. A tightrope artist walks along the tightrope and, upon reaching the middle, stops. Someone standing on one of the platforms grabs the rope near one end and sends a transverse wave pulse down the rope. When the pulse reaches the tightrope walker, he briefly rises upward, and the wave passes. This illustrates the fact that the wave transports momentum mass weight wavelength density While the tightrope artist is just standing there in the middle of the rope, he has no momentum. But, as the wave passes, he moves upward, meaning he now has vertical momentum. Therefore, the wave pulse transmitted vertical momentum (and energy). 2 / 40 The half-life of a radioisotope is one week. How much of the substance is left after a 28-day period? 02-Jan 1/4 1/8 1/16 1/32 The correct answer is (D). During a period of 28 days, the sample has 4 half-lives. This means (1/2)4 = .0625, or 1/16 of the substance is left. 3 / 40 An atom with one electron has an ionization energy of 25 eV. How much energy will be released when the electron makes the transition from an excited energy level, where E = -16 eV, to the ground state? 9 eV 11 eV 16 eV 25 eV 41 eV If the atom's ionization energy is 25 eV, then the electron's ground-state energy must be -25 eV. Making a transition from the -16 eV energy level to the ground state will cause the emission of a photon of energy. 4 / 40 Under the influence of a force, an object of mass 4 kg accelerates from 3 m/s to 6 m/s in 8 s. How much work was done on the object during this time? 27 J 54 J 72 J 96 J Cannot be determined from the information given By the work-energy theorem 5 / 40 A student measures the maximum speed of a block undergoing simple harmonic oscillations of amplitude A on the end of an ideal spring. If the block is replaced by one with twice the mass but the amplitude of its oscillations remains the same, then the maximum speed of the block will decrease by a factor of 4 decrease by a factor of 2 decrease by a factor of remain the same increase by a factor of 2 As we derived in Question 2, the maximum speed of the block is given by the equation vmax = . Therefore, vmax is inversely proportional to . If m is increased by a factor of 2, then vmax will decrease by a factor of . 6 / 40 How long would it take a car, starting from rest and accelerating uniformly in a straight line at 5 m/s2, to cover a distance of 200 m ? 9.0 s 10.5 s 12.0 s 15.5 s 20.0 s Use Big Five #2 with v0 = 0. 7 / 40 A simple pendulum of length L and mass m aswings about the vertical equilibrium position (θ = 0) with a maximum angular displacement of θmax. What is the tension in the connecting rod when the pendulum's angular displacement is θ = θmax ? mg sinθmax mg cosθmax mgL sinθmax mgL cosθmax mgL(1 - cosθmax) First draw a free-body diagram. 8 / 40 A battery whose emf is 40 V has an internal resistance of 5 ohm. If this battery is connected to a 15 ohm resistor R, what will be the voltage drop across R ? 10 V 30 V 40 V 50 V 70 V The current through the circuit is 9 / 40 A sky diver jumps from an airplane. After free falling for a while, she opens her parachute and her descent speed begins to decrease. While her descent speed decreases, let F denote the magnitude of the gravitational force on the sky diver and let D denote the magnitude of the upward force of air resistance (drag). Which of the following is then true? F > D F < D F + D < weight of the sky diver F - D > weight of the sky diver F - D > 0 A and E are identical, so both may be eliminated. Since F is equal to the weight of the sky diver, C and D don't make sense. If the sky diver's downward velocity is decreasing, she must be experiencing an upward acceleration. Therefore, the net force on the sky diver must be upward. Since D is an upward force and F is a downward force, D must be greater than F to give a net force, D - F, that's upward. 10 / 40 An object at rest is placed into free fall at a height of 20 m. What is the velocity of the object when the PE equals the KE? 8 m/s 11 m/s 14 m/s 17 m/s Velocity cannot be determined. The correct answer is (C). The problem may be solved in several ways. Perhaps the best solution is a little thought. The potential energy and the kinetic energy are equal to one another when the object has fallen half the distance to the ground, or 10 m. All we have to do now is treat the problem as a free fall problem by restating the situation a little. The velocity of an object that starts from rest and free falls 10 m? The solution is found by . 11 / 40 A light ray is moving parallel to the principal axis of a concave mirror, which it strikes. How will the light ray be reflected? Back upon itself Through the focal point Through the radius of curvature Through a point equal to 0.5f Through a point equal to 2r The correct answer is (B). A ray parallel to the central radius is reflected through the focal point. 12 / 40 The normal force between two surfaces is increased by four times. The coefficient of static friction will be cut in half. be doubled. remain the same. be quadrupled. be undetermined. The correct answer is (C). The frictional force between two surfaces will increase when the normal force increases, but the coefficient of friction between two surfaces is constant. 13 / 40 A radioactive substance is observed to have a count rate of 500 counts/sec. Two hours later the count rate is 62.5 counts/sec. What is the half life of the substance? 10 min 20 min 30 min 40 min 50 min The correct answer is (D). Take the count rate of 62.5 counts/ sec and divide by 500 counts/sec. The result is the decimal number of .125, which is (1/2)3. The 3 half-lives occur during the two-hour period. We have 120 min ÷ 3 = 40 min per half-life. 14 / 40 Two large glass containers of equal volume each hold 1 mole of gas. Container 1 is filled with hydrogen gas (2 g/mol), and Container 2 holds helium (4 g/mol). If the pressure of the gas in Container 1 equals the pressure of the gas in Container 2, which of the following is true? The temperature of the gas in Container 1 is lower than the temperature of the gas in Container 2. The temperature of the gas in Container 1 is greater than the temperature of the gas in Container 2. The value of R for the gas in Container 1 is the value of R for the gas in Container 2. The rms speed of the gas molecules in Container 1 is lower than the rms speed of the gas molecules in Container 2. The rms speed of the gas molecules in Container 1 is greater than the rms speed of the gas molecules in Container 2. Neither A nor B can be correct. Using PV = nRT, both containers have the same V, n is the same, P is the same, and R is a universal constant. Therefore, T must be the same for both samples. C is also wrong, since R is a universal constant. The kinetic theory of gases predicts that the rms speed of the gas molecules in a sample of molar mass M and temperature T is 15 / 40 A pile driver is raised to a height of 25 m in 10 seconds. It is released and allowed to fall onto a piling. Although guided by a set of rails, the pile driver essentially is in free fall after its release. Which of the following is/ are correct? I. The power input equals the power output. II. The PE in equals the KE out. III. The KE in equals the work out. I only II only I and III only II and III only I, II, and III only The correct answer is (D). The potential energy transforms into kinetic energy, or the pile driver falls. At the end of the fall, all the kinetic energy the pile driver has is converted into frictional work to drive the post into the ground. 16 / 40 A nonconducting sphere is given a nonzero net electric charge, +Q, and then brought close to a neutral conducting sphere of the same radius. Which of the following will be true? An electric field will be induced within the conducting sphere. The conducting sphere will develop a net electric charge of -Q. The spheres will experience an electrostatic attraction. The spheres will experience an electrostatic repulsion. The spheres will experience no electrostatic interaction. The proximity of the charged sphere will induce negative charge to move to the side of the uncharged sphere closer to the charged sphere. Since the induced negative charge is closer than the induced positive charge to the charged sphere, there will be a net electrostatic attraction between the spheres. 17 / 40 If 360 g of water at 95 degrees celsius is mixed with 275 g of water at 10 degrees celsius, what is the resulting temperature of the water? 37 degrees celsius 49 degrees celsius 58 degrees celsius 70 degrees celsius 82 degrees celsius The correct answer is (C). This is a calorimetric problem. Since the ratio of C degrees to K is 1 to 1, we will not change temperature scales. 18 / 40 If a particle of charge -0.2 mC were placed at a certain location within an electric field, the magnitude of the electric force it would feel is 1 N. What is the magnitude of the electric field at this location? (1 mC = 10-3 C) 2,000 N/C 5,000 N/C 20,000 N/C 50,000 N/C 500,000 N/C From the equation F = qE, where q is the magnitude of the charge in the electric field, we find that 19 / 40 A 24 V battery supplies a total current of .75 amperes to a circuit. How much power does the battery supply to the circuit? .04 watts 13.5 watts 18 watts 32 watts 32 watts The correct answer is (C). The power developed in a circuit element is equal to (V)(I). Solve by substituting the correct values into the equation: (24V)(.75A) = 18 watts. 20 / 40 An electron that accelerates from a point near a collection of negative source charges toward a point near a collection of positive source charges experiences a decrease in electrical potential energy as it moves toward a region at a lower electric potential a decrease in electrical potential energy as it moves toward a region at a higher electric potential an increase in electrical potential energy as it moves toward a region at a lower electric potential an increase in electrical potential energy as it moves toward a region at a higher electric potential no change in electrical potential energy Since the electron is naturally being accelerated toward a region containing positive source charges, its potential energy decreases. (It's like a ball dropping to the ground; it is accelerated downward by the gravitational field, and it loses gravitational potential energy.) Therefore, the answer is either A or B. Now since the potential due to a positive charge is higher than the potential due to a negative charge (because positive numbers are greater than negative numbers), the electron is accelerating toward a region of higher electric potential choice B. 21 / 40 Which of the following occurs when light passes into a clear glass cube? The light's wavelength changes. The light's frequency changes. The light's speed changes. The light is polarized. Both A and C The correct answer is (E). Light slows down when it enters a material that is more optically dense. Since the velocity is related to both the frequency and the wavelength by V = λf it follows that one of these quantities will change, too. Since the distance between the waves changes at the optical boundary, the wavelength also changes. The frequency remains unchanged. 22 / 40 Two objects move toward each other, collide, and separate. If there was no net external force acting on the objects, but some kinetic energy was lost, then the collision was elastic and total linear momentum was conserved the collision was elastic and total linear momentum was not conserved the collision was not elastic and total linear momentum was conserved the collision was not elastic and total linear momentum was not conserved None of the above Total linear momentum is conserved in a collision during which the net external force is zero. If kinetic energy is lost, then by definition, the collision is not elastic. v = λf = (8.75e4−3 m)(40e3 Hz) = 350 m/s 23 / 40 An object is placed 100 cm from a plane mirror. How far is the image from the object? 50 cm 100 cm 200 cm 300 cm 400 cm The distance from the mirror to the image is equal to the distance from the mirror to the object. Therefore, the distance from the object to the image is 100 cm + 100 cm = 200 cm. 24 / 40 Sir Isaac Newton's third law of motion is called the action-reaction law. Which of the following statements appropriately describes the action-reaction forces? They act on different objects. They act on the same object. They are unequal in magnitude. They act at right angles to one another. They act in the same direction. The correct answer is (A). The force pairs discussed in the third law are equal in magnitude, oppositely directed, and operate on differing bodies. 25 / 40 Which type of decay would cause the number of neutrons in the nucleus to decrease by 1 ? Alpha decay alpha decay delta V+ decay Electron capture Gamma decay When a nucleus undergoes alpha decay, a neutron is converted into a proton and an electron, and the electron is ejected. Because of this, the number of neutrons in the nucleus is decreased by 1. 26 / 40 Consider a double-slit interference experiment using yellow light of wavelength λ, with the slits labeled S1 and S2. If P is the center of a dark fringe on the screen on which the resulting diffraction pattern is projected, which of the following equations relating S1P and S2P, the distances from slits S1 and S2, respectively, to the point P could be true? S1P - S2P = λ S1P - S2P = λ S1P - S2P = 2λ S1P - S2P = 3λ S1P = S2P Since P is the center of a dark fringe, it is a location of completely destructive interference. In order for this to occur, the waves must be exactly out of phase when they reach P. This will happen if the difference between their path lengths from the slits is an odd number of half wavelengths. Only the equation in A satisfies this requirement. 27 / 40 During a lacrosse game, an attacking player shoots a low hard shot into the goal. The ball leaves the lacrosse stick and travels into the net very quickly. What happens as the ball flies toward the goal? The horizontal acceleration of the ball increases. The vertical acceleration of the ball increases. The horizontal velocity of the ball is constant. The vertical velocity of the ball is constant. All the listed quantities are constant. The correct answer is (C). When the player shoots the ball, there is no force affecting the ball except gravity. Nothing can slow or speed the ball's x velocity, which is the horizontal. It is a constant velocity. 28 / 40 A car skids a distance of 54 m on a dry road. What was the velocity of the car when the skid began if the coefficient of friction between the tires and the road is .46? 16 m/s 22 m/s 28 m/s 34 m/s Velocity cannot be determined with the information given. The correct answer is (B). The problem is a work energy conversion. Set the kinetic energy of the car equal to the frictional work to be done by the tires. 1/2 mv2 = Fs. The mass of the car is not given, but if we expand the equation a little more we find it is not necessary. 1/2 mv2 = μNs, which leads to 1/2 mv2 = mgs. At this point the mass cancels out on both sides, which leaves 1/2 v2 = μgs. The working equation becomes . The solution to the problem is 29 / 40 The distribution of the charge density on the surface of a conducting solid depends upon the density of the conductor. the shape of the conductor. the size of the conductor. the age of the conductor. the substance of the conductor. The correct answer is (B). Charge will distribute itself equally over the surface of a body because of the repulsion the electrons have for one another. ∆E = (-16 eV) - (-25eV) = 9 eV 30 / 40 Rutherford's results in his famous gold foil experiment proved that atoms are mostly space. are in continuous motion. have negative orbitals. have diffuse charge distribution. have dense crystalline structure. The correct answer is (A). Most of the alpha particles passed through the gold foil, leading Rutherford to conclude that atoms were composed mostly of space with a dense core. -10 eV - (-40 eV) = 30 eV 31 / 40 When a woman pushes on her grocery cart, the woman moves because of the force the woman exerts on the grocery cart. the force the grocery cart exerts on the woman. the force the woman exerts on the ground. the force the ground exerts on the woman. the force the grocery cart exerts on the ground. The correct answer is (D). The woman pushing on the ground is one part of a force pair. The second part of the pair is the ground pushing on the woman. The force the woman exerts cannot move the ground, but the force the ground exerts does move the woman. 32 / 40 The coefficient of static friction between a box and a ramp is 0.5. The ramp's incline angle is 30 degrees. If the box is placed at rest on the ramp, the box will accelerate down the ramp accelerate briefly down the ramp but then slow down and stop move with constant velocity down the ramp not move Cannot be determined from the information given The force pulling the block down the ramp is mg sinθ, and the maximum force of static friction is μsFN = μsmg cosθ. If mg sin θ is greater than μsmg cosθ, then there is a net force down the ramp, and the block will accelerate down. So, the question becomes, Is sinθ greater than μs cosθ? Since θ = 30 degrees and μs = 0.5, the answer is yes. 33 / 40 Two concurrent forces act at right angles to one another.The resultant force is 65N and one of the component forces is 35N. What is the force of the other component? 40N 45N 50N 55N 60N The correct answer is (D). The missing vector component is found by using the Pythagorean theorem.. 34 / 40 A golf ball is placed inside an unmounted tire, which is then rolled down a long hill. Identify the statement(s) that best describe the situation while the tire and golf ball are rolling down the hill. I. The centripetal force on the tire operates in the same direction as the centripetal force on the golf ball. II. The centripetal force acting on the tire is opposite the centripetal force acting on the golf ball. III. The centripetal force acting on the golf ball is equal to the centripetal force on the tire. I only II only I and III only II and III only I, II, and III The correct answer is (A). Centripetal force is also called the center-seeking force. Both the tire and the golf ball roll about their centers of mass, which are located in the center of the objects. The centripetal force for the two cannot be equal because the radii of the two objects are different, yet they roll with the same tangential velocity. 35 / 40 A beam of light traveling in medium 1 strikes the interface to another transparent medium, medium 2. If the speed of light is less in medium 2 than in medium 1, the beam will refract toward the normal refract away from the normal undergo total internal reflection have an angle of reflection smaller than the angle of incidence have an angle of reflection greater than the angle of incidence If the speed of light is less in medium 2 than in medium 1, then medium 2 must have the higher index of refraction; that is, n2 > n1. Snell's law then implies that θ2 < θ1: The beam will refract toward the normal upon transmission into medium 2. 36 / 40 As a bat flies at a constant speed of 0.04 V toward a large tree trunk (where V denotes the speed of sound), the bat emits an ultrasonic pulse. The pulse is reflected off the tree and returns to the bat, which can detect and analyze the returning signal. If the returning signal has a frequency of 61 kHz, at approximately what frequency did the bat emit the original ultrasonic pulse? 56 kHz 62 kHz 68 kHz 74 kHz 78 kHz Because the bat flies toward the tree, the frequency of the waves as they hit the tree is higher than the frequency with which they were emitted by the bat. Then as these waves reflect off the tree, they are detected by the bat, which is still flying toward the tree, so the frequency gets shifted higher again. Therefore, if the returning signal is detected by the bat as having a frequency of 61 kHz, the original pulse must have been emitted at a frequency lower than 61 kHz. Only A is lower than 61 kHz. 37 / 40 A moon of mass m orbits a planet of mass 100m. Let the strength of the gravitational force exerted by the planet on the moon be denoted by F1, and let the strength of the gravitational force exerted by the moon on the planet be F2. Which of the following is true? F1 = 100F2 F1 = 10F2 F1 = F2 F2 = 10F1 F2 = 100F1 The gravitational force that the moon exerts on the planet is equal in magnitude to the gravitational force that the planet exerts on the moon (Newton's third law). 38 / 40 Of the following types of waves, which type travels at the greatest speed through vacuum? Radio waves Microwaves Ultraviolet light X-rays None of the above; all these waves would travel at the same speed. All electromagnetic waves ”regardless of frequency ”travel at speed c (the speed of light) through vacuum. 39 / 40 A sound wave travels through a metal rod with wavelength λ and frequency f. Which of the following best describes the wave when it passes into the surrounding air? Wavelength Frequency Less than λ Equal to f Less than λ Less than f Greater than λ Equal to f Greater than λ Less than f Greater than λ Greater than f In general, sound travels faster through solids than through gases. Therefore, when the wave enters the air from the metal rod, its speed will decrease. The frequency, however, will not change. Since v = λf must always be satisfied, a decrease in v implies a decrease in λ. 40 / 40 The velocity of a moving object is doubled. Which of the following statements about the object is correct? The kinetic energy of the object increases by four. The displacement of the object increases by four. The momentum of the object increases by four. The frictional force increases by four. None of these quantities increase by four. The correct answer is (A). The kinetic energy equation is KE = . mv2 . Every time the velocity of an object doubles, its energy increases by a factor of 4. The momentum is directly related to the velocity, so a doubling of an object's velocity doubles the momentum of the object. The frictional force is a constant force no matter what happens to the velocity. 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