physics ****ing 100

Free fall is motion in which gravity is the only force acting. Is a skydiver who has reached terminal speed in free fall?Is a satellite above the atmosphere that circles Earth in free fall?

No, because there are still forces acting on the sky diver. Yes the satellite above the atmosphere is in freefall.

What is meant by a system, and how is it related to the conservation of momentum?

A system is any object or collection of objects. Whatever momentum such a system has, in the absence of external forces, that momentum remains unchanged—what the conservation of momentum is about.

When a car is moving in reverse, backing from a driveway, the driver applies the brakes. In what direction is the car's acceleration?

Acceleration (slowing the car) is opposite to velocity (direction car moves).

In terms of impulse and momentum, why do air bags in cars reduce the chances of injury in accidents?

Air bags lengthen the time of impact thereby reducing the force of impact.

In the absence of air resistance, if a ball is thrown vertically upward with a certain initial speed, on returning to its original level it will have the same speed. When air resistance is a factor, will the ball be moving faster, the same, or more slowly than its throwing speed when it gets back to the same level? Why? (Physicists often use a "principle of exaggeration" to help them analyze a problem. Consider the exaggerated case of a feather, not a ball, because the effect of air resistance on the feather is more pronounced and therefore easier to visualize.)

Air resistance decreases the speed of a moving object. Hence the ball has less than its initial speed when it returns to the level from which it was thrown. The effect is easy to see for a feather projected upward by a slingshot. No way will it return to its starting point with its initial speed!

Two golfers each hit a ball at the same speed, but one at 60 ∘ with the horizontal and the other at 30 ∘. Which ball goes farther? Which hits the ground first? (Ignore air resistance.)

Both balls have the same range. The ball with the initial projection angle of 30 degrees, however, is in the air for a shorter time and hits the ground first.

When you are traveling in your car at highway speed, the momentum of a bug is suddenly changed as it splatters onto your windshield. Compared with the change in momentum of the bug, by how much does the momentum of your car change?

By Newton's 3rd law, the force on the bug is equal in magnitude and opposite in direction to the force on the car windshield. The rest is logic: Since the time of impact is the same for both, the amount of impulse is the same for both, which means they both undergo the same change in momentum. The change in momentum of the bug is evident because of its large change in speed. The same change in momentum of the considerably more massive car is not evident, for the change in speed is correspondingly very small. Nevertheless, the magnitude of m∆V for the bug is equal to M∆v for the car! (impulse = m X change in velocuty)

If you stopped an Earth satellite dead in its tracks, it would simply crash into Earth. Why, then, don't the communications satellites that "hover motionless" above the same spot on Earth crash into Earth?

Communication satellites only appear motionless because their orbital period coincides with the daily rotation of the Earth.

5. What Aristotelian idea did Galileo demolish with his experiments with inclined planes?

Galileo demolished the notion that a moving body requires a force to keep it moving. He showed a force is needed to change motion, not to keep a body moving, so long as friction was negligible

4. What Aristotelian idea did Galileo discredit in his fabled Leaning Tower demonstration?

Galileo discredited Aristotle's idea that the rate at which bodies fall is proportional to their weight.

3. A ball rolling along a floor doesn't continue rolling indefinitely. Is it because it is seeking a place of rest or because some force is acting upon it? If the latter, identify the force.

Galileo would say that an unbalanced force must have acted upon the ball to stop it. That force is called friction, and it us in the opposite direction that the ball was rolling.

If a golf ball and a Ping-Pong ball both move with the same kinetic energy, can you say which has the greater speed? Explain in terms of the definition of KE. Similarly, in a gaseous mixture of heavy molecules and light molecules with the same average KE, can you say which have the greater speed?

If KEs are the same but masses differ, then the ball with smaller mass has the greater speed. That is, 1⁄2Mv^2 = 1⁄2mV^2. Likewise with molecules, where lighter ones move faster on the average than more massive ones.

The spool is pulled in three ways, as shown. There is sufficient friction for rotation. In what direction will the spool roll in each case?

In all three cases the spool moves to the right.In the first case there is a torque about the point of contact with the table that rotates the spool clockwise, so the spool rolls to the right. In the second case the pull's line of action extends through (not about) the point of table contact, yielding no lever arm and therefore no torque; but with a force component to the right; hence the spool slides to the right without rolling. In the third case the torque produces clockwise rotation so the spool rolls to the right.

6. When any object is in mechanical equilibrium, what can be correctly said about all the forces that act on it? Must the net force necessarily be zero?

In mechanical equilibrium, the vector sum of all forces, the net forces, necessarily equals zero: ΣF=0

When you pedal a bicycle, maximum torque is produced when the pedal sprocket arms are in the horizontal position, and no torque is produced when they are in the vertical position. Explain.

In the horizontal position the lever arm equals the length of the sprocket arm, but in the vertical position, the lever arm is zero because the line of action of forces passes right through the axis of rotation. (With cycling cleats, a cyclist pedals in a circle, which means they push their feet over the top of the spoke and pull around the bottom and even pull up on the recovery. This allows torque to be applied over a greater portion of the revolution.)

Which contains more apples, a 1-pound bag of apples on Earth or a 1-pound bag of apples on the Moon? Which contains more apples, a 1-kilogram bag of apples on Earth or a 1-kilogram bag of apples on the Moon?

Items like apples weigh less on the Moon, so there are more apples in a 1-pound bag of apples there. Mass is another matter, for the same quantity of apples are in 1-kg bag on the Earth as on the Moon. *( weight is a relevant term, mass is not)*

Can a satellite maintain an orbit in the plane of the Arctic Circle? Why or why not?

No, for an orbit in the plane of the Arctic Circle does not intersect the Earth's center. All Earth satellites orbit in a plane that intersects the center of the Earth. A satellite may pass over the Arctic Circle, but cannot remain above it indefinitely, as a satellite can over the equator.

When you rub your hands together, can you push harder on one hand than the other?

No, for each hand pushes equally on the other in accord with Newton's third law—you cannot push harder on one hand than the other.

You push a heavy car by hand. The car, in turn, pushes back with an opposite but equal force on you. Doesn't this mean that the forces cancel one another, making acceleration impossible? Why or why not?

No. You can't cancel a force exerted on the car with a force exerted on you. In order for forces to cancel, the forces have to be equal and opposite and act on the same object.

2. Consider the normal force on a book at rest on a tabletop. If the table is tilted so that the surface forms an inclined plane, will the magnitude of the normal force change? If so, how?

Normal force is greatest when the table surface is horizontal, and progressively decreases as the angle of tilt increases. As the angle of tilt approaches 90°, the normal force approaches zero. When the table surface is vertical, it no longer presses on the book, which then freely falls.

Three identical blocks are pulled, as shown, on a horizontal frictionless surface. If tension in the rope held by the hand is 30 N, what is the tension in the other ropes?

Note that 30 N pulls 3 blocks. To pull 2 blocks then requires a 20-N pull, which is the tension in the rope between the second and third block. Tension in the rope that pulls only the third block is therefore 10 N. (Note that the net force on the first block, 30 N - 20 N = 10 N, is the force needed to accelerate that block, having one-third of the total mass.) it splits the force equal if mass is the same and frictionless.

Why aren't high ocean tides exactly 12 hours apart?

Ocean tides are not exactly 12 hours apart because while the Earth spins, the Moon moves in its orbit and appears at its same position overhead every 25 hours, instead of every 24 hours. So the two-high-tide cycle occurs at about 25-hour intervals, making high tides about 12.5 hours apart.

Friction is needed for a car rounding a curve. But, if the road is banked, friction may not be required at all. What, then, supplies the needed centripetal force? (Hint: Consider vector components of the normal force on the car.)

On a banked road the normal force, at right angles to the road surface, has a horizontal component that provides the centripetal force. Even on a perfectly slippery surface, this component of the normal force can provide sufficient centripetal force to keep the car on the track.

Why does the force of gravity do work on a car that rolls down a hill but no work when it rolls along a level part of the road

On the hill there is a component of gravitational force in the direction of the car's motion. This component of force does work on the car. But on the level, there is no component of gravitational force along the direction of the car's motion, so the force of gravity does no work in this case.

An ice sailcraft is stalled on a frozen lake on a windless day. The skipper sets up a fan as shown. Discuss the advisability of simply removing the sail and compare it to the situation with the sail where all the wind bounces backward from the sail and the situation where the air is bought to a halt by the sail without bouncing. Consider if the craft will be set in motion without the sail? If so, in what direction?

Removing the sail and turning the fan around is the best means of propelling the boat! Then maximum impulse is exerted on the craft. If the fan is not turned around, the boat is propelled backward, to the left. (Such propeller-driven boats are used where the water is very shallow, as in the Florida Everglades.)

The front wheels of a racing vehicle are located far out in front to help keep the vehicle from nosing upward when it accelerates. What physics concepts play a role here?

Rotational inertia and torque are most predominantly illustrated here, and the conservation of angular momentum also plays a role. The long distance to the front wheels increases the rotational inertia of the vehicle relative to the back wheels and also increases the lever arm of the front wheels without appreciably adding to the vehicle's weight. As the back wheels are driven clockwise, the chassis tends to rotate counterclockwise (conservation of angular momentum) and thereby lift the front wheels off the ground. The greater rotational inertia and the increased clockwise torque of the more distant front wheels counter this effect.

At what point in its motion is the KE of a pendulum bob at a maximum? At what point is its PE at a maximum? When its KE is at half its maximum value, how much PE does it have relative to its PE at the center of the swing?

The KE of a pendulum bob is maximum where it moves fastest, at the lowest point; PE is maximum at the uppermost points. When the pendulum bob swings by the point that marks half its maximum height, it has half its maximum KE, and its PE is halfway between its minimum and maximum values. If we define PE = 0 at the bottom of the swing, the place where KE is half its maximum value is also the place where PE is half its maximum value, and KE = PE at this point. (By energy conservation: Total energy = KE + PE.)

Your friend says that the law of momentum conservation is violated when a ball rolls down a hill and going momentum. What do you say?

The Law of Momentum Conservation states that if no external forces act on a system, then the system's momentum will not change. If you treat the ball as your system, it is clear that the gravitational force is acting on the ball, provides an impulse, and therefore changes the ball's momentum. But this is not a violation of the Conservation of Momentum because there is an external force acting.

Since the Moon is gravitationally attracted to Earth, why doesn't it simply crash into Earth?

The Moon's tangential velocity is what keeps the Moon coasting around the Earth rather than crashing into it. If its tangential velocity were reduced to zero, then it would fall straight into the Earth!

What is the acceleration of a rock at the top of its trajectory when it has been thrown straight upward? (Is your answer consistent with Newton's second law?)

The acceleration at the top or anywhere else in free fall is g, 10 m/s2, downward. The velocity of the rock is momentarily zero, but the rate of change of velocity is still present. Or better, by Newton's 2nd law, the force of gravity acts at the top as elsewhere; divide this net force by the mass and you have the acceleration of free fall. That is, a=F(net)/m=mg/m=g.

An apple falls because of the gravitational attraction to Earth. How does the gravitational attraction of Earth to the apple compare? (Does force change when you interchange m1 and m2 in the equation for gravity - m2 m1 instead of m1 m2 ?)

The forces between the apple and Earth are the same in magnitude. Force is the same either way, but the corresponding accelerations of each are different.

Consider the two forces acting on the person who stands still-namely, the downward pull of gravity and the upward support of the floor. Are these forces equal and opposite? Do they form an action-reaction pair? Why or why not?

The forces must be equal and opposite because they are the only forces acting on the person, who obviously is not accelerating. Note that the pair of forces do not comprise an action-reaction pair, however, for they act on the same body. The downward force, the man's weight, Earth pulls down on man; has the reaction man pulls up on Earth, not the floor pushing up on him. And the upward force of the floor on the man has the reaction of man against the floor, not the interaction between the man and Earth. (If you find this confusing, you may take solace in the fact that Newton himself had trouble applying his 3rd law to certain situations.

Most people today know that the ocean tides are caused principally by the gravitational influence of the Moon, and most people therefore think that the gravitational pull of the Moon on Earth is greater than the gravitational pull of the Sun on Earth. What do you think?

The gravitational pull of the Sun on the Earth is greater than the gravitational pull of the Moon. The tides, however, are caused by the differences in gravitational forces by the Moon on opposite sides of the Earth. The difference in gravitational forces by the Moon on opposite sides of the Earth is greater than the corresponding difference in forces by the stronger pulling but much more distant Sun.

Explain why a long pole is more beneficial to a tightrope walker if the pole droops.

The long drooping pole lowers the CG of the balanced system—the tightrope walker and the pole. The rotational inertia of the pole contributes to the stability of the system also.

If a Mack truck and a MiniCooper have a head-on collision, which vehicle will experience the greater force of impact? The greater impulse? The greater change in momentum? The greater deceleration?

The magnitude of force, impulse, and change in momentum will be the same for each. The MINI Cooper undergoes the greater acceleration because its mass is less.

Gravitational force acts on all bodies in proportion to their masses. Why, then, doesn't a heavy body fall faster than a light body?

The reason that a heavy body doesn't fall faster than a light body is because the greater gravitational force on the heavier body (its weight), acts on a correspondingly greater mass (inertia). The ratio of gravitational force to mass is the same for every body—hence all bodies in free fall accelerate equally. And it's true not just near the Earth, but anywhere.

At which of the indicated positions does the satellite in elliptical orbit experience the greatest gravitational force? Have the greatest speed? The greatest velocity? The greatest momentum? The greatest kinetic energy? The greatest gravitational potential energy? The greatest total energy? The greatest angular momentum? The greatest acceleration?

The satellite experiences the greatest gravitational force at A, where it is closest to the Earth, the perigee; and the greatest speed and the greatest velocity at A, and by the same token the greatest momentum and greatest kinetic energy at A, and the greatest gravitational potential energy at the farthest point C. It would have the same total energy (KE + PE) at all parts of its orbit, likewise the same angular momentum because it's conserved. It would have the greatest acceleration at A, where F/m is greatest.

Two 100-N weights are attached to a spring scale as shown. Does the scale read 0, 100, or 200 N, or does it give some other reading? (Hint: Would it read any differently if one of the ropes were tied to the wall instead of to the hanging 100-N weight?)

The scale will read 100 N, the same it would read if one of the ends were tied to a wall instead of tied to the 100-N hanging weight. Although the net force on the system is zero, the tension in the rope within the system is 100 N, as shown on the scale reading.

The strong man will push the two initially stationary freight cars of equal mass apart before he himself drops straight to the ground. Is it possible for him to give either of the cars a greater speed than the other? Why or why not?

The strong man can exert only equal forces on both cars, just as your push against a wall equals the push of the wall on you. Likewise for two walls, or two freight cars. Since their masses are equal, they will undergo equal accelerations and move equally.

What aspect of physics was not known by the writer of this newspaper editorial that ridiculed early experiments by Robert H. Goddard on rocket propulsion above Earth's atmosphere? "Professor Goddard . . . does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react . . . he seems to lack the knowledge ladled out daily in high schools."

The writer apparently didn't know that the reaction to exhaust gases does not depend on a medium for the gases. A gun, for example, will kick if fired in a vacuum. In fact, in a vacuum there is no air drag and a bullet or rocket operates even better.

Explain why a centripetal force does not do work on a circularly moving object.

There is no component of force parallel to the direction of motion, which work requires.

When a rifle with a longer barrel is fired, the force of expanding gases acts on the bullet for a longer distance. What effect does this have on the velocity of the emerging bullet? (Do you see why long-range cannons have such long barrels?)

When a rifle with a long barrel is fired, more work is done as the bullet is pushed through the longer distance. A greater KE is the result of the greater work, so of course, the bullet emerges with a greater velocity. (Note that the force acting on the bullet is not constant, but decreases with increasing distance inside the barrel.)

Two people who weigh the same climb a flight of stairs. The first person climbs the stairs in 30 s, and the second person climbs them in 40 s. Which person does more work? Which uses more power?

Work done by each is the same, for they reach the same height. The one who climbs in 30 s uses more power because work is done in a shorter time.

When the space shuttle coasts in a circular orbit at constant speed about Earth, is it accelerating? If so, in what direction? If not, why not?

Yes, the shuttle is accelerating, as evidenced by its continual change of direction. It accelerates due to the gravitational force between it and the Earth. The acceleration is toward the Earth's center.

Suppose that you and two classmates are discussing the design of a roller coaster. One classmate says that each summit must be lower than the previous one. Your other classmate says this is nonsense, for as long as the first one is the highest, it doesn't matter what height the others are. What do you say?

You agree with your second classmate. The coaster could just as well encounter a low summit before or after a higher one, so long as the higher one is enough lower than the initial summit to compensate for energy dissipation by friction.

Your friend says that the primary reason astronauts in orbit feel weightless is that they are beyond the main pull of Earth's gravity. Why do you agree or disagree?

You disagree, for the force of gravity on orbiting astronauts is almost as strong as at Earth's surface. They feel weightless because of the absence of a support force.

1. Your friend says that inertia is a force that keeps things in their place, either at rest or in motion. Do you agree? Why or why not?

You should disagree with your friend. In the absence of external forces, a body at rest tends to remain at rest; if moving, it tends to remain moving. Inertia is a property of matter to behave this way, not some kind of force.