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university physics

Questions and Answers of University Physics

The forces on a dancer can be measured directly when a dancer performs a jump on a force plate that measures the force between her feet and the ground. A graph of force versus time throughout a
While the dancer is in the air and holding a fixed pose, what is the magnitude of the force her neck exerts on her head?(a) 0 N;(b) 60 N;(c) 120 N;(d) 180 N.
Compared with the force her neck exerts on her head during the landing, the force her head exerts on her neck is(a) The same;(b) Greater;(c) Smaller;(d) Greater during the first half of the landing
An 8.00-kg box sits on a level floor. You give the box a sharp push and find that it travels 8.22 m in 2.8 s before coming to rest again.(a) You measure that with a different push the box traveled
The table* gives automobile performance data for a few types of cars:(a) During an acceleration of 0 to 60 mph, which car has the largest average net force acting on it? The smallest?(b) During this
A mysterious rocket-propelled object of mass 45.0 kg is initially at rest in the middle of the horizontal, frictionless surface of an ice-covered lake. Then a force directed east and with magnitude
You have landed on an unknown planet, Newtonia, and want to know what objects weigh there. When you push a certain tool, starting from rest, on a frictionless horizontal surface with a 12.0-N force,
Boxes A and B are connected to each end of a light vertical rope (Fig. P4.49). A constant upward force F = 80.0 N is applied to box A. Starting from rest, box B descends 12.0 m in 4.00 s. The tension
After an annual checkup, you leave your physician’s office, where you weighed 683 N. You then get into an elevator that, conveniently, has a scale. Find the magnitude and direction of the
In a head-on collision between a compact 1000-kg car and a large 2500-kg car, which one experiences the greater force? Explain. Which one experiences the greater acceleration? Explain why. Why are
In a head-on auto collision, passengers who are not wearing seat belts may be thrown through the windshield. Use Newton’s laws of motion to explain why this happens.
When a car is hit from behind, the occupants may experience whiplash. Use Newton’s laws of motion to explain what causes this result.
If you squat down (such as when you examine the books on a bottom shelf) and then suddenly get up, you may temporarily feel light-headed. What do Newton’s laws of motion have to say about why this
If your hands are wet and no towel is handy, you can remove some of the excess water by shaking them. Why does this work?
A manual for student pilots contains this passage: “When an airplane flies at a steady altitude, neither climbing nor descending, the upward lift force from the wings equals the plane’s weight.
Boxes A and B are in contact on a horizontal, frictionless surface. You push on box A with a horizontal 100-N force (Fig. Q4.31). Box A weighs 150 N, and box B weighs 50 N. Is the force that box A
Consider a tug-of-war between two people who pull in opposite directions on the ends of a rope. By Newton’s third law, the force that A exerts on B is just as great as the force that B exerts on A.
A small compact car is pushing a large van that has broken down, and they travel along the road with equal velocities and accelerations. While the car is speeding up, is the force it exerts on the
When a car comes to a stop on a level highway, what force causes it to slow down? When the car increases its speed on the same highway, what force causes it to speed up? Explain.
A large truck and a small compact car have a head-on collision. During the collision, the truck exerts a force F̅T on C on the car, and the car exerts a force F̅C on T on the truck. Which force has
True or false? You exert a push P on an object and it pushes back on you with a force F. If the object is moving at constant velocity, then F is equal to P, but if the object is being accelerated,
A horse is hitched to a wagon. Since the wagon pulls back on the horse just as hard as the horse pulls on the wagon, why doesn’t the wagon remain in equilibrium, no matter how hard the horse pulls?
Why is it incorrect to say that 1.0 kg equals 2.2 lb?
Which feels a greater pull due to the earth’s gravity: a 10-kg stone or a 20-kg stone? If you drop the two stones, why doesn’t the 20-kg stone fall with twice the acceleration of the 10-kg stone?
A large crate is suspended from the end of a vertical rope. Is the tension in the rope greater when the crate is at rest or when it is moving upward at constant speed? If the crate is traveling
When a string barely strong enough lifts a heavy weight, it can lift the weight by a steady pull; but if you jerk the string, it will break. Explain in terms of Newton’s laws of motion.
Why are cars designed to crumple in front and back for safety? Why not for side collisions and rollovers?
A small car of mass 380 kg is pushing a large truck of mass 900 kg due east on a level road. The car exerts a horizontal force of 1600 N on the truck. What is the magnitude of the force that the
A person can dive into water from a height of 10 m without injury, but a person who jumps off the roof of a 10-m-tall building and lands on a concrete street is likely to be seriously injured. Why is
(a) An ordinary flea has a mass of 210 µg. How many newtons does it weigh?(b) The mass of a typical froghopper is 12.3 mg. How many newtons does it weigh? (c) A house cat typically weighs 45 N.
Why can it hurt your foot more to kick a big rock than a small pebble? Must the big rock hurt more? Explain.
Students sometimes say that the force of gravity on an object is 9.8 m/s2. What is wrong with this view?
You can play catch with a softball in a bus moving with constant speed on a straight road, just as though the bus were at rest. Is this still possible when the bus is making a turn at constant speed
The acceleration of a falling body is measured in an elevator that is traveling upward at a constant speed of 9.8 m/s. What value is obtained?
Some students refer to the quantity ma̅(vector) as “the force of acceleration.” Is it correct to refer to this quantity as a force? If so, what exerts this force? If not, what is a better
Does Newton’s second law hold true for an observer in a van as it speeds up, slows down, or rounds a corner? Explain.
Why is the earth only approximately an inertial reference frame?
Suppose you chose the fundamental physical quantities to be force, length, and time instead of mass, length, and time. What would be the units of mass in terms of those fundamental quantities?
A passenger in a moving bus with no windows notices that a ball that has been at rest in the aisle suddenly starts to move toward the rear of the bus. Think of two possible explanations, and devise a
Some people say that the “force of inertia” (or “force of momentum”) throws the passengers forward when a car brakes sharply. What is wrong with this explanation?
You walk into an elevator, step onto a scale, and push the “up” button. You recall that your normal weight is 625 N. Draw a free-body diagram.(a) When the elevator has an upward acceleration of
When a car stops suddenly, the passengers tend to move forward relative to their seats. Why? When a car makes a sharp turn, the passengers tend to slide to one side of the car. Why?
A 68.5-kg skater moving initially at 2.40 m/s on rough horizontal ice comes to rest uniformly in 3.52 s due to friction from the ice. What force does friction exert on the skater?
You tie a brick to the end of a rope and whirl the brick around you in a horizontal circle. Describe the path of the brick after you suddenly let go of the rope.
If the two ends of a rope in equilibrium are pulled with forces of equal magnitude and opposite directions, why isn’t the total tension in the rope zero?
When you fly in an airplane at night in smooth air, you have no sensation of motion, even though the plane may be moving at 800 km/h (500 mi/h). Why?
A helium balloon hovers in midair, neither ascending nor descending. Is it in equilibrium? What forces act on it?
A ball thrown straight up has zero velocity at its highest point. Is the ball in equilibrium at this point? Why or why not?
If a seed is launched at an angle of 0° with the maximum initial speed, how far from the plant will it land? Ignore air resistance, and assume that the ground is flat.(a) 20 cm;(b) 93 cm;(c) 2.2
About how long does it take a seed launched at 90° at the highest possible initial speed to reach its maximum height? Ignore air resistance.(a) 0.23 s;(b) 0.47 s;(c) 1.0 s;(d) 2.3 s.
You are a member of a geological team in Central Africa. Your team comes upon a wide river that is flowing east. You must determine the width of the river and the current speed (the speed of the
In the middle of the night you are standing a horizontal distance of 14.0 m from the high fence that surrounds the estate of your rich uncle. The top of the fence is 5.00 m above the ground. You have
A firefighting crew uses a water cannon that shoots water at 25.0 m/s at a fixed angle of 53.0° above the horizontal. The firefighters want to direct the water at a blaze that is 10.0 m above ground
A boy 12.0 m above the ground in a tree throws a ball for his dog, who is standing right below the tree and starts running the instant the ball is thrown. If the boy throws the ball horizontally at
A baseball thrown at an angle of 60.0° above the horizontal strikes a building 18.0 m away at a point 8.00 m above the point from which it is thrown. Ignore air resistance.(a) Find the magnitude of
You are on the west bank of a river that is flowing north with a speed of 1.2 m/s. Your swimming speed relative to the water is 1.5 m/s, and the river is 60 m wide. What is your path relative to the
In a rainstorm with a strong wind, what determines the best position in which to hold an umbrella?
Raindrops hitting the side windows of a car in motion often leave diagonal streaks even if there is no wind. Why? Is the explanation the same or different for diagonal streaks on the windshield?
In uniform circular motion, how does the acceleration change when the speed is increased by a factor of 3? When the radius is decreased by a factor of 2?
A projectile is fired upward at an angle θ above the horizontal with an initial speed v0. At its maximum height, what are its velocity vector, its speed, and its acceleration vector?
If a jumping frog can give itself the same initial speed regardless of the direction in which it jumps (forward or straight up), how is the maximum vertical height to which it can jump related to its
A projectile moves in a parabolic path without air resistance. Is there any point at which a̅(vector) is parallel to v̅(vector)? Perpendicular to v̅(vector)? Explain.
A simple pendulum (a mass swinging at the end of a string) swings back and forth in a circular arc. What is the direction of the acceleration of the mass when it is at the ends of the swing? At the
If the contraction of the left ventricle lasts 250 ms and the speed of blood flow in the aorta (the large artery leaving the heart) is 0.80 m/s at the end of the contraction, what is the average
A small rock is thrown vertically upward with a speed of 22.0 m/s from the edge of the roof of a 30.0-m-tall building. The rock doesn’t hit the building on its way back down and lands on the street
A small block has constant acceleration as it slides down a frictionless incline. The block is released from rest at the top of the incline, and its speed after it has traveled 6.80 m to the bottom
When you drop an object from a certain height, it takes time T to reach the ground with no air resistance. If you dropped it from three times that height, how long (in terms of T) would it take to
An object is thrown straight up into the air and feels no air resistance. How can the object have an acceleration when it has stopped moving at its highest point?
You run due east at a constant speed of 3.00 m/s for a distance of 120.0 m and then continue running east at a constant speed of 5.00 m/s for another 120.0 m. For the total 240.0-m run, is your
Consider the circuit shown in Fig. E25.26. The terminal voltage of the 24.0-V battery is 21.2 V. What are(a) The internal resistance r of the battery and(b) The resistance R of the circuit
The battery for a certain cell phone is rated at 3.70 V. According to the manufacturer it can produce 3.15 × 104 J of electrical energy, enough for 5.25 h of operation, before needing to be
In an ionic solution, a current consists of Ca2+ ions (of charge +2e) and Cl- ions (of charge -e) traveling in opposite directions. If 5.11 × 1018 Cl- ions go from A to B every 0.50 min, while 3.24
An overhead transmission cable for electrical power is 2000 m long and consists of two parallel copper wires, each encased in insulating material. A short circuit has developed somewhere along the
Unlike the idealized ammeter described in Section 25.4, any real ammeter has a nonzero resistance.(a) An ammeter with resistance RA is connected in series with a resistor R and a battery of emf ε
Compact fluorescent bulbs are much more efficient at producing light than are ordinary incandescent bulbs. They initially cost much more, but they last far longer and use much less electricity.
Consider the circuit shown in Fig. P25.72. The battery has emf 72.0 V and negligible internal resistance. R2= 2.00 Î©, C1= 3.00 µF, and C2= 6.00 µF. After the capacitors have
Consider the circuit shown in Fig. P25.73. The emf source has negligible internal resistance. The resistors have resistances R1= 6.00 Î© and R2= 4.00 Î©. The capacitor has
An external resistor R is connected between the terminals of a battery. The value of R varies. For each R value, the current I in the circuit and the terminal voltage Vabof the battery are measured.
In which 120-V light bulb does the filament have greater resistance: a 60-W bulb or a 120-W bulb? If the two bulbs are connected to a 120-V line in series, through which bulb will there be the
The voltage drop Vabacross each of resistors A and B was measured as a function of the current I in the resistor. The results are shown in the table:(a) For each resistor, graph Vab as a function of
What is the best explanation for the behavior exhibited in the data? (a) Longer threads can carry more current than shorter threads do and so make better electrical conductors. (b) The thread stops
If the conductivity of the thread results from the aqueous coating only, how does the cross-sectional area A of the coating compare when the thread is 13 mm long versus the starting length of 5 mm?
What is the maximum current that flows in the thread during this experiment if the voltage source is a 9-V battery?(a) About 1 A;(b) About 0.1 A;(c) About 1 µA;(d) about 1 nA.Some types of
In another experiment, a piece of the web is suspended so that it can move freely. When either a positively charged object or a negatively charged object is brought near the web, the thread is
From the top of a tall building, you throw one ball straight up with speed v0 and one ball straight down with speed v0.(a) Which ball has the greater speed when it reaches the ground?(b) Which ball
If you know the initial position and initial velocity of a vehicle and have a record of the acceleration at each instant, can you compute the vehicle’s position after a certain time? If so, explain
A dripping water faucet steadily releases drops 1.0 s apart. As these drops fall, does the distance between them increase, decrease, or remain the same? Prove your answer.
Prove these statements:(a) As long as you can ignore the effects of the air, if you throw anything vertically upward, it will have the same speed when it returns to the release point as when it was
You throw a baseball straight up in the air so that it rises to a maximum height much greater than your height. Is the magnitude of the ball’s acceleration greater while it is being thrown or after
Under constant acceleration the average velocity of a particle is half the sum of its initial and final velocities. Is this still true if the acceleration is not constant? Explain.
The official€™s truck in Fig. 2.2 is at x1= 277 m at t1= 16.0 s and is at x2= 19 m at t2= 25.0 s.(a) Sketch two different possible x-t graphs for the motion of the truck.(b) Does the
An automobile is traveling west. Can it have a velocity toward the west and at the same time have an acceleration toward the east? Under what circumstances?
Can you have zero velocity and nonzero average acceleration? Zero velocity and nonzero acceleration? Use a vx-t graph to explain, and give an example of such motion.
Can you have zero acceleration and nonzero velocity? Use a vx-t graph to explain.
Can you have zero displacement and nonzero average velocity? Zero displacement and nonzero velocity? Illustrate your answers on an x-t graph.
A driver in Massachusetts was sent to traffic court for speeding. The evidence against the driver was that a policewoman observed the driver’s car alongside a second car at a certain moment, and
When a Dodge Viper is at Elwood’s Car Wash, a BMW Z3 is at Elm and Main. Later, when the Dodge reaches Elm and Main, the BMW reaches Elwood’s Car Wash. How are the cars’ average velocities
Individuals vary considerably in total lung volume. Figure P1.94 shows the results of measuring the total lung volume and average alveolar volume of six individuals. From these data, what can you
If we assume that alveoli are spherical, what is the diameter of a typical alveoulus?(a) 0.20 mm;(b) 2 mm(c) 20mm(d) 200 mm
What is total volume of the gas-exchanging region of the lungs? (a) 2000 µm3(b) 2m3(c) 2.0 L(d) 120 L

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