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college physics reasoning

Questions and Answers of College Physics Reasoning

Give two examples in which the work done on an object by friction is negative.
A snowboarder of mass 80 kg slides down the trail shown in Figure P6.10. The first part of the trail is a ramp that makes an angle of 20° with the horizontal, and the final portion of the trail is
Give two examples in which a nonzero force acts on an object, but the work done by that force is zero.
Two railroad cars, each of mass 2.0 ?? 104kg, are connected by a cable to each other, and the car in front is connected by a cable to the engine as shown in Figure P6.11. The cars start from rest and
Explain how a mosquito can have a greater kinetic energy than a baseball.
A person pushes a broom at an angle of 60° with respect to the floor (Fig. P6.12). If the person exerts a force of 30 N directed along the broom handle, what is the work done by the person on the
A hockey puck slides along a level surface, eventually coming to rest. Is the energy of the hockey puck conserved? Is the total energy of the universe conserved? Discuss where the initial kinetic
A car is pushed along a long road that is straight, horizontal, and parallel to the x direction.? (a) The horizontal force on the car varies with x as shown in Figure P6.13. What is the work done on
A spring is initially compressed. It is then released, sending an object flying off the spring. Discuss this process in terms of conservation of energy. What and “where” is the initial energy,
The force on an object varies with position as shown in Figure P6.14. Estimate the work done on the object as it moves from the origin to x = 5.0 m. Assume the motion is one dimensional and the force
Tarzan starts at rest near the top of a tall tree. He then swings on a vine and reaches the base of the tree, where he jumps off the vine and comes to rest on the ground. Discuss and compare
A tomato of mass 0.22 kg is dropped from a tall bridge. If the tomato has a speed of 12 m/s just before it hits the ground, what is the kinetic energy of the tomato?
During the course of each day, you move from place to place and do work on various objects. Where does the energy for these everyday activities come from? Explain.
Two objects have the same kinetic energy. One has a speed that is 2.5 times the speed of the other. What is the ratio of their masses?
When a rubber ball is dropped onto a concrete floor, it bounces to a height that is slightly lower than its initial height. Compare the ball’s (a) Initial mechanical energy to (b) Its
A car of mass 1500 kg is initially traveling at a speed of 10 m/s. The driver then accelerates to a speed of 25 m/s over a distance of 200 m. Calculate the change in the kinetic energy of the car.
Construct bar charts showing the kinetic energy and potential energy of the projectile in Figure 6.16. Show these energies when the projectile has just left the ground, when it is at the highest
A rock of mass 0.050 kg is thrown upward with an initial speed of 25 m/s. Find the work done by gravity on the rock from the time it leaves the thrower’s hand until it reaches the highest point on
Construct bar charts showing the kinetic energy and potential energy for the bungee jumper in Example 6.9 at various points during his jump. Explain how these charts illustrate the principle of
A horizontal force of 15 N pulls a block of mass 3.9 kg across a level floor. The coefficient of kinetic friction between the block and the floor is μK = 0.25. If the block begins with a speed of
You are pushing a refrigerator across the floor of your kitchen. You exert a horizontal force of 300 N for 7.0 s, during which time the refrigerator moves a distance of 3.0 m at constant
An archer is able to fire an arrow (mass 0.020 kg) at a speed of 250 m/s. If a baseball (mass 0.14 kg) is given the same kinetic energy, what is its speed?
A baseball pitcher can throw a baseball at a speed of 50 m/s. If the mass of the ball is 0.14 kg and the pitcher has it in his hand over a distance of 2.0 m, what is the average force exerted by the
A softball pitcher can exert a force of 100 N on a softball. If the mass of the ball is 0.19 kg and the pitcher has it in her hand over a distance of 1.5 m, what is the speed of the ball when it
A truck of mass 9000 kg is traveling along a level road at an initial speed of 30 m/s. The driver then shifts into neutral so that the truck is coasting, and the truck gradually slows to a final
Consider a skydiver of mass 70 kg who jumps from an airplane flying at an altitude of 1500 m. With her parachute open, her terminal velocity is 8.0 m/s.(a) What is the work done by gravity
You use an elevator to travel from the first floor (h = 0) to the fourth floor of a building (h = 12 m). If you start from rest and you end up at rest on the fourth floor, what is the work done by
Consider a small car of mass 1200 kg and a large sport utility vehicle (SUV) of mass 4000 kg. The SUV is traveling at the speed limit (v = 35 m/s). The driver of the small car travels so as to have
The force on an object varies with position as shown in Figure P6.29. The object begins at rest from the origin and has a speed of 5.0 m/s at x = 4.0 m. What is the mass of the object? Assume one-
Suppose the object in Figure P6.29 has a mass of 2.5 kg and a speed of 9.0 m/s at x = 0. What is its speed at x = 5.0 m?? Figure P6.29 ? F(N) 50'+ 40 30 20 10- x (m)
A skier of mass 110 kg travels down a friction less ski trail. (a) If the top of the trail is a height 200 m above the bottom, what is the work done by gravity on the skier? (b) Find the
Consider a roller coaster that moves along the track shown in Figure P6.32. Assume all friction is negligible and ignore the kinetic energy of the roller coaster???s wheels.? (a) Is the mechanical
Suppose the ski trail in Problem 31 is not friction less. (a) Find the work done by gravity on the skier in this case. (b) If the skier has a speed of 30 m/s at the bottom of the hill, what
Consider again the roller coaster in Figure P6.32, but now assume the roller coaster starts with a speed of 12 m/s at point A. Find the speed of the roller coaster when it reaches locations B and C.
Mount McKinley (also called Denali) is the tallest mountain in North America, with a height of 6200 m above sea level. If a person of mass 120 kg walks from sea level to the top of Mount McKinley,
The roller coaster in Figure P6.36 starts with a velocity of 15 m/s. One of the riders is a small girl of mass 30 kg. Find her apparent weight when the roller coaster is at locations B and C. At
A skateboarder starts at point A on the track (Fig. P6.38) with a speed of 15 m/s. Will he reach point B? Assume friction is negligible and ignore the kinetic energy of the skateboard???s
A roller coaster (Fig. P6.37) starts at the top of its track (point A) with a speed of 12 m/s. If it reaches point B traveling at 16 m/s, what is the vertical distance (h) between A and B? Assume
Front side air. A skateboarder is practicing on the ???half-pipe??? shown in Figure P6.39, using a special friction less skateboard. (You can also ignore the kinetic energy of the skateboard???s
A hockey puck of mass 0.25 kg starts from rest and slides down the friction less ramp shown in Figure P6.41. The bottom end of the ramp is horizontal. After it leaves the ramp, the puck travels as a
A rock of mass 12 kg is tied to a string of length 2.4 m, with the other end of the string fastened to the ceiling of a tall room (Fig. P6.42). While hanging vertically, the rock is given an initial
Consider the rock and string in Figure P6.43. The string is fastened to a hinge that allows it to swing completely around in a vertical circle. If the rock starts at the lowest point on this circle
A rock climber (m = 90 kg with gear included) starts at the base of a cliff and climbs to the top (h = 35 m). He then walks along the plateau at the top for a distance L = 40 m. Find the work done by
Two crates of mass m1= 40 kg and m2= 15 kg are connected by a mass less rope that passes over a mass less, friction less pulley as shown in Figure P6.45. The crates start from rest. (a) Add a
Consider again the crates in Problem 45, but now assume the rope has a mass of 2 kg. If the tops of the crates start at the same initial height and the rope has a length of 10 m, what is the final
Sketch how the potential energy, kinetic energy, and total mechanical energy vary with time for an apple that drops from a tree.
(a) Sketch how the potential energy, kinetic energy, and total mechanical energy vary with time for a cannon shell that is fired from a large cannon. Ignore air drag and assume the cannon is aimed
A ball of mass 1.5 kg is tied to a string of length 6.0 m as shown in Figure P6.50. The ball is initially hanging vertically and is given an initial velocity of 5.0 m/s in the horizontal direction.
Calculate the escape speed for an object on the Moon.
An engineer at NASA decides to save money on fuel by launching satellites from a very tall mountain. (a) Calculate the escape speed for a satellite launched from an altitude of 8000 m above sea
Calculate the velocity needed for an object starting at the Earth’s surface to just barely reach a satellite in a geosynchronous orbit. Ignore air drag and assume the object has a speed of zero
Sketch how the potential energy, kinetic energy, and total mechanical energy vary with position for a projectile that is fired from the Earth’s surface. Assume the projectile’s initial speed is
Consider a spacecraft that is to be launched from the Earth to the Moon. Calculate the minimum velocity needed for the spacecraft to just make it to the Moon’s surface. Ignore air drag from the
space station is orbiting the Earth. It moves in a circular orbit with a radius equal to twice the Earth’s radius. A supply satellite is designed to travel to the station and dock smoothly when it
A mass and spring are arranged on a horizontal, friction less table as shown in Figure P6.58. The spring constant is k = 500 N/m, and the mass is 4.5 kg. The block is pushed against the spring so
A block is dropped onto a spring with k = 30 N/m. The block has a speed of 3.3 m/s just before it strikes the spring. If the spring compresses an amount 0.12 m before bringing the block to rest, what
A diving board acts like a spring and obeys Hooke???s law (Fig. 6.22A). Estimate the spring constant of a diving board. Figure 6.22A ? Tx spring A
A block of mass 35 kg is sitting on a platform as shown in Figure P6.60. The platform sits on a spring with k = 2000 N/m. The mass is initially at rest.? (a) Add a coordinate system to this sketch.
An archer’s bow can be treated as a spring with k = 3000 N/m. If the bow is pulled back a distance 0.12 m before releasing the arrow, what is the kinetic energy of the arrow when it leaves the bow?
For the arrow in Problem 61, what is the work done on the bow as the bowstring is pulled back into position?Data From Problem 61An archer’s bow can be treated as a spring with k = 3000 N/m. If the
The struts on a car are fancy springs. Because one is attached to each wheel, your car is supported by four of these springs. Estimate the spring constant for one of these springs.
A tennis racket can be treated as a spring. The displacement of the spring (what we commonly call x) is the displacement of the strings at the center of the racket. Estimate the spring constant of
A tennis ball is a flexible, elastic object. If a person of average size stands on a tennis ball, the ball will compress to about half its original (non-compressed) diameter. What is the approximate
A tennis ball (m = 57 g) is projected vertically with an initial speed of 8.8 m/s. (a) If the ball rises to a maximum height of 3.7 m, how much kinetic energy was dissipated by the drag force of
A snowboarder of mass 80 kg travels down the slope of height 150 m shown in Figure P6.68. If she starts from rest at the top and has a velocity of 12 m/s when she reaches the bottom, what is the work
Big bounce. A Super Ball is a toy ball made from the synthetic rubber polymer polybutadiene vulcanized with sulfur. Manufactured by Wham-O since 1965, these toys have a “super” elastic property
Consider the skateboarder in Figure P6.70. If she has a mass of 55 kg, an initial velocity of 20 m/s, and a velocity of 12 m/s at the top of the ramp, what is the work done by friction on the
A skier of mass 100 kg starts from rest at the top of the ski slope of height 100 m shown in Figure P6.71. If the total work done by friction is -3.0 ?? 104J, what is the skier???s speed when he
Every year, there is a foot race to the top of the Empire State Building. The vertical distance traveled (using the stairs!) is about 440 m. The current record for this race is about 9 min 30 s. If
An electric motor is rated to have a maximum power output of 0.75 hp. If this motor is being used to lift a crate of mass 200 kg, how fast (i.e., at what speed) can it lift the crate?
In Example 6.11, we compared the gravitational potential energy of a 20-W light bulb to the electrical energy consumed when it is turned on. Repeat that calculation, but now assume the light bulb is
The main frictional force on a good bicycle is the force of air drag (Eq. 3.20). Consider a bicyclist coasting at a constant velocity on a road that is sloped downward at 5.08 with respect to the
The rate of energy use in a typical house is about 2.0 kW. If the kinetic energy of a car (mass 1400 kg) is equal to the total energy used by the house in 1 min, what is the car’s speed?
A typical car has a power rating of 150 hp. Estimate the car’s maximum speed on level ground. Assume air drag is much larger than any frictional forces.
Consider a molecular motor that has an efficiency of 50%. Find the maximum force the motor can produce. Assume a step size of 4 nm. Also assume the energy consumed for each step is 5 × 10–20 J (as
Suppose a molecular motor consumes 3 ATP molecules in every step. If this motor has a step size of 8.0 nm, what is the maximum force the motor could exert?
Molecular motors are often studied by attaching them to small plastic spheres. These spheres enable the motor’s motion to be observed with a microscope and are much larger than the motor. Consider
Consider a molecular motor that consumes the energy from 100 ATP molecules per second. What is the power output of this motor?
If the motor in Problem 81 takes a step of length 6.0 nm for each ATP molecule it consumes, what are the speed of the motor and the force it is able to produce?Data From Problem 81Consider a
In Section 6.8, we calculated the force produced by a myosin molecular motor and found that it is approximately 10 pico-newtons. Compare this force to the weight of a typical amino acid.
Conceptualizing units. A small burrito, like those bought at your local fast-food restaurant, has a mass of approximately 100 g and contains about 300 C. (The capital “C” indicates that this unit
Counting calories. The chemical potential energy in foods is measured in units called Calories (notice the capital C). This “food Calorie” is equal to 1000 calories. For example, a typical apple
Repeat Problem 85, but replace the apple with a typical fast-food hamburger (approximately 400 C).Data From Problem 85Counting calories. The chemical potential energy in foods is measured in units
A doughnut contains roughly 350 C (1.5 × 106 J) of potential energy locked in chemical bonds. Find the ratio of the potential energy in the doughnut to that in an equivalent volume of TNT
One gallon of gasoline contains 3.1 × 107 calories of potential energy that are released during combustion. If 1 gal of gasoline can provide the force that moves a car through a displacement of 25
Tarzan (m = 74 kg) commutes to work swinging from vine to vine. He leaves the platform of his tree house and swings on the end of a vine of length L = 8.0 m. (a) If the platform is 1.9 m above
A toy gun shoots spherical plastic projectiles by means of a spring. A typical projectile has mass m = 25 g. The spring used has spring constant k = 15 N/m, and when put under load, it is displaced
A bomb that is initially at rest breaks into several pieces of approximately equal mass, two of which are shown with their velocity vectors in Figure Q7.1. Use conservation of momentum to determine
A pole vaulter of mass 70 kg can run horizontally with a top speed of 10.0 m/s. The current record height for the pole vault is about 6.2 m. (a) For the vaulter to clear this height,
A spring is mounted at angle u = 35° on a friction less incline as illustrated in Figure P6.92. The spring is compressed by 15 cm where it is allowed to propel a mass of 5.5 kg up the incline.? (a)
Consider the system in Problem 92 and assume the coefficient of kinetic friction between the mass and the incline is 0.17. (a) At what position, measured from the starting point, does the
Spring shoes. Tae-Hyuk Yoon of South Korea is the inventor of a novel method of human locomotion: the Poweriser spring boot shown in Figure P6.94. Promotional materials claim that an 80-kg man can
A chairlift rises in elevation 600 m (about 2000 ft) up a ski slope and has a total of 200 chairs evenly spaced every 12 m (about 40 ft) apart. On a busy weekend, the ski lift is at full capacity
A 70-kg skydiver reaches a terminal velocity of 50 m/s. At what rate is the drag force dissipating his kinetic energy? How does this rate compare with his rate of change in potential energy?
The world record for the highest-altitude skydive was made by Joseph Kittinger in 1960. Kittinger jumped from a high- altitude balloon (Fig. P6.97) at a height of 102,800 ft (31,330 m) above sea
The NEAR spacecraft flew by the asteroid Eros in 1997 (Fig. P6.98). The fly by allowed the craft to measure the asteroid???s density at 2.7 g/cm3. Assume most asteroids have similar composition and
Aerobic workout. After a 5-min workout on a Climb Max stair machine, the readout panel (Fig. P6.99) reports that the 75-kg user burned 19.7 Calories (see Problem 85) and climbed a total of 180
The expression for the force due to air drag described in Chapter 3 (Eq. 3.20) ignores the aerodynamic shape of an object. Some objects, such as a race car, are shaped so as to minimize the drag
For a car moving with speed v, the force of air drag is proportional to v2. If the power output of the car’s engine is doubled, by what factor does the speed of the car increase?
The Roche limit. Some ???bodies??? in the solar system are a collection of rocks held together solely or mainly by their gravitational attraction. When such an object comes close to a planet,

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