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physics
mechanics

Questions and Answers of Mechanics

What force causes an automobile to move? A propeller-driven airplane? A rowboat?
Identify the action–reaction pairs in the following situations: a man takes a step; a snowball hits a girl in the back; a baseball player catches a ball; a gust of wind strikes a window.
In a contest of National Football League behemoths, teams from the Rams and the 49ers engage in a tug of war, pulling in opposite directions on a strong rope. The Rams exert a force of 9200 N and
Twenty people participate in a tug-of-war. The two teams of ten people are so evenly matched that neither team wins. After the game they notice that a car is stuck in the mud. They attach the
“When the locomotive in Figure Q5.17 broke through the wall of the train station, the force exerted by the locomotive on the wall was greater than the force the wall could exert on the
An athlete grips a light rope that passes over a low-friction pulley attached to the ceiling of a gym. A sack of sand precisely equal in weight to the athlete is tied to the other end of the rope.
If the action and reaction forces are always equal in magnitude and opposite in direction to each other, then doesn’t the net vector force on any object necessarily add up to zero? Explain your
Can an object exert a force on itself? Argue for your answer.
If you push on a heavy box that is at rest, you must exert some force to start its motion. However, once the box is sliding, you can apply a smaller force to maintain that motion. Why?
The driver of a speeding empty truck slams on the brakes and skids to a stop through a distance d. (a) If the truck carried a load that doubled its mass, what would be the truck’s “skidding
Suppose you are driving a classic car. Why should you avoid slamming on your brakes when you want to stop in the shortest possible distance? (Many cars have antilock brakes that avoid this problem.)
A book is given a brief push to make it slide up a rough incline. It comes to a stop and slides back down to the starting point. Does it take the same time to go up as to come down? What if the
A large crate is placed on the bed of a truck but not tied down. (a) As the truck accelerates forward, the crate remains at rest relative to the truck. What force causes the crate to accelerate
Describe a few examples in which the force of friction exerted on an object is in the direction of motion of the object.
Why does mud fly off a rapidly turning automobile tire?
Imagine that you attach a heavy object to one end of a spring, hold onto the other end of the spring, and then whirl the object in a horizontal circle. Does the spring stretch? If so why? Discuss
Describe a situation in which the driver of a car can have a centripetal acceleration but no tangential acceleration.
Describe the path of a moving body in the event that its acceleration is constant in magnitude at all times and (a) perpendicular to the velocity; (b) parallel to the velocity.
An object executes circular motion with constant speed whenever a net force of constant magnitude acts perpendicular to the velocity. What happens to the speed if the force is not perpendicular to
Explain why the Earth is not spherical in shape and bulges at the equator.
Because the Earth rotates about its axis, it is a non-inertial frame of reference. Assume the Earth is a uniform sphere. Why would the apparent weight of an object be greater at the poles than at the
What causes a rotary lawn sprinkler to turn?
If someone told you that astronauts are weightless in orbit because they are beyond the pull of gravity, would you accept the statement? Explain.
It has been suggested that rotating cylinders about 10 mi in length and 5 mi in diameter be placed in space and used as colonies. The purpose of the rotation is to simulate gravity for the
Consider a rotating space station, spinning with just the right speed such that the centripetal acceleration on the inner surface is g. Thus, astronauts standing on this inner surface would feel
A pail of water can be whirled in a vertical path such that none is spilled. Why does the water stay in the pail, even when the pail is above your head?
How would you explain the force that pushes a rider toward the side of a car as the car rounds a corner?
Why does a pilot tend to black out when pulling out of a steep dive?
The observer in the accelerating elevator of Example 5.8 would claim that the “weight” of the fish is T, the scale reading. This is obviously wrong. Why does this observation differ from that of
If you have ever taken a ride in an express elevator of a high-rise building, you may have experienced a nauseating sensation of heaviness or lightness depending on the direction of the acceleration.
A falling sky diver reaches terminal speed with her parachute closed. After the parachute is opened, what parameters change to decrease this terminal speed?
Consider a small raindrop and a large raindrop falling through the atmosphere. Compare their terminal speeds. What are their accelerations when they reach terminal speed?
On long journeys, jet aircraft usually fly at high altitudes of about 30 000 ft. What is the main advantage of flying at these altitudes from an economic viewpoint?
Analyze the motion of a rock falling through water in terms of its speed and acceleration as it falls. Assume that the resistive force acting on the rock increases as the speed increases.
“If the current position and velocity of every particle in the Universe were known, together with the laws describing the forces that particles exert on one another, then the whole future of the
When a particle rotates in a circle, a force acts on it directed toward the center of rotation. Why is it that this force does no work on the particle?
Discuss whether any work is being done by each of the following agents and, if so, whether the work is positive or negative: (a) A chicken scratching the ground, (b) A person studying, (c) A crane
When a punter kicks a football, is he doing any work on the ball while his toe is in contact with it? Is he doing any work on the ball after it loses contact with his toe? Are any forces doing work
Cite two examples in which a force is exerted on an object without doing any work on the object.
As a simple pendulum swings back and forth, the forces acting on the suspended object are the gravitational force, the tension in the supporting cord, and air resistance.(a) Which of these forces, if
If the dot product of two vectors is positive, does this imply that the vectors must have positive rectangular components?
For what values of θ is the scalar product? (a) Positive and (b) Negative?
As the load on a vertically hanging spiral spring is increased, one would not expect the Fs -versus-x graph line to remain straight, as shown in Figure 7.10d. Explain qualitatively what you would
A certain uniform spring has spring constant k. Now the spring is cut in half. What is the relationship between k and the spring constant k- of each resulting smaller spring? Explain your reasoning.
Can kinetic energy be negative? Explain.
Discuss the work done by a pitcher throwing a baseball. What is the approximate distance through which the force acts as the ball is thrown?
One bullet has twice the mass of a second bullet. If both are fired so that they have the same speed, which has more kinetic energy? What is the ratio of the kinetic energies of the two bullets?
Two sharpshooters fire 0.30-caliber rifles using identical shells. A force exerted by expanding gases in the barrels accelerates the bullets.
The barrel of rifle A is 2.00 cm longer than the barrel of rifle B. Which rifle will have the higher muzzle speed?(a) If the speed of a particle is doubled, what happens to its kinetic energy? (b)
A car salesman claims that a souped-up 300-hp engine is a necessary option in a compact car, in place of the conventional 130-hp engine. Suppose you intend to drive the car within speed limits ((65
Can the average power over a time interval ever be equal to the instantaneous power at an instant within the interval? Explain.
In Example 7.15, does the required power increase or decrease as the force of friction is reduced?
The kinetic energy of an object depends on the frame of reference in which its motion is measured. Give an example to illustrate this point.
Words given precise definitions in physics are sometimes used in popular literature in interesting ways. For example, a rock falling from the top of a cliff is said to be “gathering force as it
In most circumstances, the normal force acting on an object and the force of static friction do zero work on the object. However, the reason that the work is zero is different for the two cases.
“A level air track can do no work.” Argue for or against this statement.
Who first stated the work–kinetic energy theorem? Who showed that it is useful for solving many practical problems? Do some research to answer these questions?
If the height of a playground slide is kept constant, will the length of the slide or the presence of bumps make any difference in the final speed of children playing on it? Assume the slide is slick
Explain why the total energy of a system can be either positive or negative, whereas the kinetic energy is always positive.
One person drops a ball from the top of a building while another person at the bottom observes its motion. Will these two people agree on the value of the gravitational potential energy of the
Discuss the changes in mechanical energy of an object–Earth system in (a) Lifting the object, (b) Holding the object at a fixed position, and (c) Lowering the object slowly. Include the muscles in
In Chapter 7, the work–kinetic energy theorem, W = ∆K, was introduced. This equation states that work done on a system appears as a change in kinetic energy. This is a special- case equation,
If three conservative forces and one non-conservative force act within a system, how many potential-energy terms appear in the equation that describes the system?
If only one external force acts on a particle, does it necessarily change the particle’s (a) Kinetic energy? (b) Velocity?
A driver brings an automobile to a stop. If the brakes lock so that the car skids, where is the original kinetic energy of the car, and in what form is it after the car stops?
Answer the same question for the case in which the brakes do not lock, but the wheels continue to turn. You ride a bicycle. In what sense is your bicycle solar powered?
In an earthquake, a large amount of energy is “released” and spreads outward, potentially causing severe damage. In what form does this energy exist before the earthquake, and by what energy
A bowling ball is suspended from the ceiling of a lecture hall by a strong cord. The ball is drawn away from its equilibrium position and released from rest at the tip of the demonstrator’s nose as
Roads going up mountains are formed into switchbacks, with the road weaving back and forth along the face of the slope such that there is only a gentle rise on any portion of the roadway. Does this
As a sled moves across a flat snow-covered field at constant velocity, is any work done? How does air resistance enter into the picture?
You are working in a library, re-shelving books. You lift a book from the floor to the top shelf. The kinetic energy of the book on the floor was zero, and the kinetic energy of the book on the top
A ball is thrown straight up into the air. At what position is its kinetic energy a maximum? At what position is the gravitational potential energy of the ball–Earth system a maximum?
A pile driver is a device used to drive objects into the Earth by repeatedly dropping a heavy weight on them. By how much does the energy of the pile driver–Earth system increase when the weight it
Our body muscles exert forces when we lift, push, run, jump, and so forth. Are these forces conservative?
A block is connected to a spring that is suspended from the ceiling. If the block is set in motion and air resistance is neglected, describe the energy transformations that occur within the system
Describe the energy transformations that occur during(a) The pole vault (b) The shot put (c) The high jump. What is the source of energy in each case?
Discuss the energy transformations that occur during the operation of an automobile.
What would the curve of U versus x look like if a particle were in a region of neutral equilibrium?
A ball rolls on a horizontal surface. Is the ball in stable, unstable, or neutral equilibrium?
Consider a ball fixed to one end of a rigid rod whose other end pivots on a horizontal axis so that the rod can rotate in a vertical plane. What are the positions of stable and unstable equilibrium?
Does a large force always produce a larger impulse on an object than a smaller force does? Explain.
If the speed of a particle is doubled, by what factor is its momentum changed? By what factor is its kinetic energy changed?
If two particles have equal kinetic energies are their moment necessarily equal? Explain.
While in motion, a pitched baseball carries kinetic energy and momentum. (a) Can we say that it carries a force that it can exert on any object it strikes? (b) Can the baseball deliver more kinetic
An isolated system is initially at rest. Is it possible for parts of the system to be in motion at some later time? If so, explain how this might occur.
If two objects collide and one is initially at rest, is it possible for both to be at rest after the collision? Is it possible for one to be at rest after the collision? Explain.
Explain how linear momentum is conserved when a ball bounces from a floor.
A bomb, initially at rest, explodes into several pieces. (a) Is linear momentum of the system conserved? (b) Is kinetic energy of the system conserved? Explain.
A ball of clay is thrown against a brick wall. The clay stops and sticks to the wall. Is the principle of conservation of momentum violated in this example?
You are standing perfectly still, and then you take a step forward. Before the step your momentum was zero, but afterward you have some momentum. Is the principle of conservation of momentum violated
When a ball rolls down an incline, its linear momentum increases is the principle of conservation of momentum violated in this process?
Consider a perfectly inelastic collision between a car and a large truck. Which vehicle experiences a larger change in kinetic energy as a result of the collision?
A sharpshooter fires a rifle while standing with the butt of the gun against his shoulder. If the forward momentum of a bullet is the same as the backward momentum of the gun, why isn’t it as
A pole-vaulter falls from a height of 6.0 m onto a foam rubber pad. Can you calculate his speed just before he reaches the pad? Can you calculate the force exerted on him by the pad? Explain.
Firefighters must apply large forces to hold a fire hose steady (Fig. Q9.15). what factors related to the projection of the water determine the magnitude of the force needed to keep the end of the
A large bed sheet is held vertically by two students. A third student, who happens to be the star pitcher on the baseball team, throws a raw egg at the sheet. Explain why the egg does not break when
A skater is standing still on a frictionless ice rink. Her friend throws a Frisbee straight at her. In which of the following cases is the largest momentum transferred to the skater? (a) The skater
In an elastic collision between two particles, does the kinetic energy of each particle change as a result of the collision?
Three balls are thrown into the air simultaneously. What is the acceleration of their center of mass while they are in motion?
A person balances a meter stick in a horizontal position on the extended index fingers of her right and left hands. She slowly brings the two fingers together. The stick remains balanced and the two

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