physics

100問 • 2年前

問題一覧

you can prove a hypothesis correct with enough supporting evidence?

false

when a ball is hit with a given force, why does contact over a long time impart more speed to the ball than contact over a short time ?

a force sustained for a long time produces more change in momentum than does the same force applied briefly.

why is it a good idea to extend your bare hand forward when you are getting ready to catch a fast moving baseball?

this allows your hand to move backward while catching the ball, greater time of catch means less force on your hand.

why is it poor judgment to have the back of your hand up against the outfield wall when you catch a long fly ball?

with your hand against the wall, there is less time for momentum change, which means more force on your hand.

in boxing why is it advantageous to roll with the punch?

greater time for momentum to change means less force

if a ball has the same speed just before being caught and just after being thrown, in which case does the ball undergo the greatest change in momentum: (1) when it is caught, (2) when it is thrown, or (3) when it is caught and then thrown back.

case (3) because it is case (1) and (2) combined

in which case (1)(2) or (3) in the question above is the greatest impulse required.

case 3 because impulse is greater when the change in momentum is greater.

railroad car a rolls at a certain speed and makes a perfectly elastic collision with car b of the same mass. after the collision car a is observed to be at rest. how does the speed of car b compare with the initial speed of car a.

it is the same speed as momentum is simply transferred from one car to the other.

If the equally massive railroad cars in the previous question coupled together after colliding inelastically how does their speed after the collision compare with the initial speed of car sack a vertical distance of 4m?

speed after coupling is half as much as initially because the mass that moves is doubled.

which requires more work: lifting a 50kg sack a vertical distance of 2m or lifting a 25kg sack a vertical distance of 4m.

it is the same for the product of each is the same (50kg)(2m)=(25kg)(4m)

if a moving car speeds up until it is going three times as fast as it was how much kinetic energy does it have compared with its initial kinetic energy

it has nine times as much KE (as 32=9)

Compared with the work that the brakes must supply to stop a car moving at some original speed, how much more work must the brakes supply to stop that car if it is moving four times as fast as the original speed? How do the respective stopping distances compare?

Its gain in KE will equal its decrease in PE, 10 kJ.

Calculate the change in momentum, when an impulse produced by a 12-N force acts over 2.0 s on an ice puck initially at rest on an air table, the change in momentum

From Ft = ∆mv, ∆mv = Ft = (12 N)(2.0 s) = 24 N × s = 24 kg × m/s.

Calculate the work done when a 20-N force pushes a cart 3.5 m.

W = Fd = (20 N)(3.5 m) = 70 N × m = 70 J

Calculate the gravitational potential energy of a 1000-kg boulder raised 5 m above ground level.

PE = mgh = (1000 kg)(10 N/kg)(5 m) = 50,000 N × m = 50,000 J

Calculate the kinetic energy of a 84-kg scooter moving at 10 m/s.

ke=1/2mv^2= 1/2(84kg)(10m/s)^2=4200kg(m/s)^2=4200j

At a baseball game, a ball of mass m = 0.15 kg, moving at a speed v = 30 m/s, is caught by a fan. a. What is the impulse supplied to bring the ball to a stop? b. If the ball stopped in 0.02 s, calculate the average force on the catcher’s hand.

see problem solution sheet.

From the conservation of momentum, MomentumDaisy = momentum(Jeannie Beanie + Daisy) (20 kg)(6 m/s) = (40 kg + 20 kg)v 120 kg × m/s = (60 kg)v v = (120 kg × m/s)/(60 kg) = 2 m/s

5-kg fish swimming at 1 m/s swallows an absent- minded 1-kg fish swimming toward it at a velocity that brings both fish to a halt. Show that the speed of the smaller fish before lunch was 5 m/s.

see problem solution sheet

Hudson holds a 10-kg barbell 0.3 m above the floor. a. What is the PE of the barbell relative to the floor? b. How much additional work will Hudson do when he lifts the barbell to a height 1.0 m above the floor? c. If Hudson then drops the barbell, what will be its KE of impact with the floor?

a) PE = mgh = (10 kg)(10 N/kg)(0.3 m) = 30 N × m = 30 J (b) Height raised is (1.0 m – 0.3 m) = 0.7 m. W = Fd = mgh = (10 kg)(10 N/kg)(0.7 m) = 70 N·m = 70 J. (c) The KE of the barbell as it hits the floor (where h = 0) equals the PE lost by the barbell in falling, 30 J + 70 J = 100 J. So all that 100 J of PE becomes KE

If you push a crate horizontally with a force of 100 N across a 10-m factory floor, and the friction force between the crate and the floor is a steady 70 N, how much kinetic energy is gained by the crate?

∆KE = work done = Fd = (100 N – 70 N)(10 m) = (30 N)(10 m) = 300 N×m = 300 J

How many watts of power do you expend when you exert a force of 6 N that moves a book 2 m in a time interval of 3 s?

see problem 64 ch.3 solutions

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

Airbags lengthen the time of impact thereby reducing the force of impact.

Why are today’s autos designed to crumple upon impact?

Crumpling allows more time for reducing the momentum of the car, resulting in a smaller force of impact on the occupants

If you throw a raw egg against a wall, you’ll break it; but if you throw it with the same speed into a sagging sheet, the egg won’t break. Explain, using concepts from this chapter.

Although the impulses may be the same for the two cases, the times of impact are not. When the egg strikes the wall, impact time is short and impact force is correspondingly large. The egg breaks. But when the egg strikes the sagging sheet, impact time is long and the impact force is correspondingly small.

A pair of skaters who are initially at rest push against each other so that they move in opposite directions. What is the total momentum of the two skaters as they move apart? Is there a different answer if their masses are not the same?

The total (net) momentum is zero. That’s true whatever their masses, for each will have the same amount of momentum but in the opposite direction from the other.

If you throw a ball horizontally while standing on a skateboard, you roll backward with a momentum that matches that of the ball. Will you roll backward if you hold on to the ball while going through the motions of throwing it? Explain in terms of momentum conservation

If no momentum is imparted to the ball, no oppositely directed momentum will be imparted to the thrower. Going through the motions of throwing has no net effect. If at the beginning of the throw you begin recoiling backward, at the end of the throw when you stop the motion of your arm and hold onto the ball, you stop moving too. Your position may change a little, but you end up at rest. No momentum given to the ball means no recoil momentum gained by you.

If your friend pushes a stroller four times as far as you do while exerting only half the force, which one of you does more work? How much more?

Your friend does twice as much work (4d ´ half the force > d ´ the same force)

Two people of the same weight 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 a cannon with a long barrel is fired, the force of expanding gases acts on the cannonball for a long distance. What effect does this have on the velocity of the emerging cannonball? (Do you see why long-range cannons have such long barrels?)

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

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 possess?

The KE of a pendulum bob is maximum where it moves fastest, at the lowest point; PE is maximum at the highest 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. (Via energy conservation: Total energy = KE + PE.)

On a playground slide, a child has potential energy that decreases by 1000 J while her kinetic energy increases by 900 J. What other form of energy is involved, and how much?

The 100 J of potential energy that doesn’t go into increasing her kinetic energy goes into thermal energy—heating her bottom and the slide

When the mass of a moving object is doubled, with no change in speed, by what factor is its momentum changed? By what factor isits kinetic energy changed?

When the mass is doubled with no change in speed, both momentum and KE are doubled.

When the velocity of an object is doubled, by what factor is its momentum changed? By what factor is its kinetic energy changed?

When the velocity is doubled, the momentum is doubled and the KE is increased by a factor of 4. Momentum is proportional to speed, and KE to speed squared

How does the force of gravity between two bodies change when the distance between them is tripled?

The force of gravity is one-ninth as much.

Where do you weigh more, at sea level or atop one of the peaks of the Rocky Mountains? Defend your answer

You’re closer to Earth’s center at sea level, so you weigh more there than on any mountain peak.

Would the springs inside a bathroom scale be more compressed or less compressed if you weighed yourself in an elevator that accelerated upward? Accelerated downward

Springs would be more compressed when accelerating upward; less compressed when accelerating downward.

Would the springs inside a bathroom scale be more com- pressed or less compressed if you weighed yourself in an elevator that moved upward at constant velocity? In an elevator that moved downward at constant velocity?

There are no changes in compression when moving at constant velocity

A stone is thrown upward at an angle. Neglecting air resistance, what happens to the horizontal componentof its velocity along itstrajectory?

With no air resistance the horizontal component of velocity remains constant.

A stone is thrown upward at an angle. Neglecting air resistance, what happens to the vertical component of its velocity along its trajectory?

Neglecting air resistance, the vertical component of velocity decreases as the stone rises, and increases as the stone falls, the same as with any freely falling object

Using the formula for gravity, calculate the force of gravity on a 1-kg mass at Earth's surface. You need to know that the mass of Earth is 6 X 1024 kg, and its radius is 6.4 X 106 m.

see ch4 solutions #36

Calculate the force of gravity on the same 1-kg mass if it were 6.4 X 106 m above Earth's surface (that is, if it were two Earth radii from Earth's center)

see ch4 #37

Suppose you stood atop a ladder that was so tall that you were three times as far from Earth's center as you presently are. What would your weight be compared to the present value?

see ch4 #42

What happens to the gravitational force between two planets if the masses of both planets are doubled but the distance between them stays the same.

see ch4 #42

An airplane is flying horizontally with speed 1000 km/h (280 m/s) when an engine falls off. Neglecting air resistance, assume that it takes 30 s for the engine to hit the ground. a. Calculate the altitude of the airplane is 4.4 km. (Use g = 9.8 m/s2) b. Calculate the horizontal distance that the airplane engine travels during its fall is 8.4km. c. If the airplane somehow continues to fly as though nothing had happened, where is the engine relative to the airplane at the moment the engine hits the ground?

see ch4 #49

A rock thrown horizontally from a bridge hits the water below. The rock travels a smooth parabolic path in time t. a. Show that the height of the bridge is ½ gt2. b. What is the height of the bridge if the time the rock is airborne is 2 s? c. To solve this problem, what information is assumed here that wasn't in Chapter 2?

Because of the independence of horizontal and vertical components of velocity, the falling time depends only on height y. Since there is no initial velocity in the vertical direction, the vertical distance is simply y = 1/2 gt2 (as learned in Chapter 2). (b) y = 1/2 gt2 = 1/2(9.8 m/s2)(2 s)2 = 19.6 m (or 20 m for g = 10 m/s2) (c) The horizontal and vertical components of velocity are independent of each other, which means falling distance is not affected by the horizontal component of velocity. Hence the bridge height is simply the height required for a freely falling rock to fall a vertical distance y. That’s y = 1/2 gt2 (as we learned in Chapter 2

What would be the path of the Moon if somehow all gravitational acting forces on it sank to zero

The path would be a straight line in accord with the law of inertia.

Consider a space pod somewhere between Earth and the Moon, at just the right distance so that gravitational_ attraction to Earth and gravitational attraction to the Moon are equal. Is this location nearer Earth or nearer the Moon?

Nearer the Moon.

If Earth somehow expanded to a larger radius, with no change in mass, how would your weight be affected? How would it be affected if Earth instead shrunk? (Hint: Let the equation for gravitational force guide yourthinking.)

Your weight would decrease if the Earth expanded with no change in its mass and would increase if the Earth contracted with no change in its mass. Your mass and the Earth’s mass don’t change, but the distance between you and the Earth’s center does change. Force is proportional to the inverse square of this distance.

Phil works on the 15th floor of an office building, and his wife Jean works on the 30th floor, which is twice as high as Phil's workplace. Is the force of gravity half as much in Jean's workspace as in Phil's? Explain why or why not.

Gravitational force is measured with respect to distance from Earth’s center. Since the workplaces are practically the same distance from Earth’s center any difference due to gravity is negligible

Earth is not exactly a sphere but, rather, bulges outward at the equator. How does this bulge affect the relation- ship between a person's weight in Singapore and his or her weight in Hong Kong?

Due to the bulge a person in Singapore is farther from the Earth’s center and therefore weighs less there. The spin of the Earth contributes to even less weight in equatorial regions, although in practice these effects are minuscule

If you were in a freely falling elevator and you dropped a pencil, it would hover in front of you. Is there a force of gravity acting on the pencil? Defend your answer

The pencil has the same motion that you have. The force of gravity on the pencil causes it to accelerate downward alongside of you. Although the pencil hovers relative to you, the pencil and you are falling relative to Earth

3. Chuck releases a ball near the top of a track and measures the ball's speed as it rolls horizontally off the end of the table. Students make measurements to predict where a can must be placed to catch the ball. How will the ball's speed affect the time it takes to reach the can once the ball leaves the end of the table? (Does a faster ball take a longer time to hit the floor?) Defend your answer.

Since the ball moves horizontally off the edge of the table, speed does not affect the time to reach the can. Time of fall is independent of horizontal velocity

In the absence of air resistance, why does the horizontal component of a projectile's motion not change, while the vertical component does?

There are no forces horizontally (neglecting air resistance), so there is no horizontal acceleration, and hence the horizontal component of velocity doesn’t change. Gravitation acts vertically, which is why the vertical component of velocity changes

A heavy crate accidentally falls from a high-flying air- plane just as it flies directly above Mike's shiny red Corvette defensively parked in a car lot. Relative to the Corvette, where does the crate crash?

The crate will not hit the Corvette, but will crash a distance beyond it determined by the height and speed of the plane.

When you jump upward, your hang time is the time your feet are off the ground. Does hang time depend on the vertical component of your velocity when you jump, on your the horizontal component of your velocity, or on both? Defend your answer.

Hang time depends only on the vertical component of your lift-off velocity. If you can increase this vertical component from a running position rather than from a dead stop, perhaps by bounding harder against the ground, then hang time is also increased. In any case, hang time depends only on the vertical component of your lift-off velocity

The hang time of a basketball player who jumps a vertical distance of 2 feet (0.6 m) is about 0.6 s. What is the hang time if the player reaches the same height while jumping 4 ft (1.2 m) horizontally?

The hang time will be the same, in accord with the answer to the preceding exercise. Hang time is related to the vertical height attained in a jump, not on horizontal distance moved across a level floor.

A friend claims that bullets fired by some high-powered rifles travel for many meters in a straight-line path before they start to fall. Another friend disputes this claim and states that all bullets from any rifle drop beneath a straight-line path a vertical distance given by ½ g? as soon as they leave the barrel and that the curved path is apparent for low velocities and less apparent for high velocities. Now it's your turn: Do all bullets drop the same vertical distance in equal times? Explain.

For very slow-moving bullets, the dropping distance is comparable to the horizontal range, and the resulting parabola is easily noticed (the curved path of a bullet tossed sideways by hand, for example). For high speed bullets, the same drop occurs in the same time, but the horizontal distance traveled is so large that the trajectory is “stretched out” and hardly seems to curve at all. But it does curve. All bullets will drop equal distances in equal times, whatever their speed. (It is interesting to note that air resistance plays only a small role, since the air resistance acting downward is practically the same for a slow-moving or fast- moving bullet.)

A park ranger shoots a monkey hanging from a branch of a tree with a tranquilizing dart. The ranger aims directly at the monkey, not realizing that the dart will follow a parabolic path and thus will fall below the monkey. The monkey, however, sees the dart leave the gun and lets go of the branch to avoid being hit. Will the monkey be hit anyway? Does the velocity of the dart affect your answer, assuming that it is great enough to travel the horizontal distance to the tree before hitting the ground? Defend your answer.

The monkey is hit as the dart and monkey meet in midair. For a fast-moving dart, their meeting place is closer to the monkey’s starting point than for a slower-moving dart. The dart and monkey fall equal vertical distances—the monkey below the tree, and the dart below the line of sight—because they both fall with equal accelerations for equal times

Is inertia a property of matter or a force of some kind?

Inertia is a property of matter.

When a bird lets go of a branch and drops to the ground below, why doesn’t the moving Earth sweep away from the dropping bird?

Both the bird and Earth have the same speed horizontally before, during, and after the bird drops.

If the mass of a sliding block is tripled at the same time that the net force on it is tripled, how does the resulting acceleration compare to the original acceleration?

Acceleration is the same as before.

What is the net force that acts on a 10-N freely falling object?What is the net force that acts on a 10-N freely falling object?

The net force is 10 N.

Why doesn’t a heavy object accelerate more than a light object when both are freely falling?

A heavy object has proportionally more mass. The greater weight is offset by the greater mass, so the ratio of force/mass remains the same.

What is the net force that acts on a 10-N falling object when it encounters 4 N of air resistance? 10 N of air resistance?

2. Net force is 10 N – 4 N = 6 N. When air resistance reaches 10 N, the net force is 10 N – 10 N = 0.

What is the acceleration of a falling object that has reached its terminal velocity?

Acceleration is zero

Why does a heavy parachutist fall faster than a lighter parachutist who wears the same size parachute?

A heavier parachutist must fall faster for air resistance to balance weight.

A boxer can hit a heavy bag with great force. Why can’t he hit a sheet of tissue paper in midair with the same amount of force?

Although the boxer-bag interaction can be great, the tissue-boxer interaction can only be small due to the small mass of the tissue

Consider hitting a baseball with a bat. If we call the force of the bat against the ball the action force, what is the reaction force?

The reaction force is ‘the ball against the bat.’

How does a helicopter get its lifting force?

A helicopter pushes air down, and as a reaction, air pushes the helicopter up

Previously acceleration was defined as 𝑎 = ∆% ∆& . What is the acceleration of a cart on an inclined plane that gains 6.0 m/s each 1.2 s?

ch2#35

Newton’s second law gives another way to calculate acceleration; 𝑎 = '()* + . Calculate the acceleration of the cart in the previous problem if a net force of 15 N is exerted on the 3.0-kg cart.

ch2 #36

A simple rearrangement of Newton’s second law gives 𝐹-.& = 𝑚𝑎. What is the net force exerted on a 12-kg package if its acceleration is 7.0 m/s2 (Remember a 𝑁 = 23 + 45 ).

Fnet = ma = (12 kg)(7.0 m/s2) = 84 kg·m/s2 = 84 N

a 120-kg astronaut recedes from her spacecraft by activating a small propulsion unit attached to her back. The force generated by a spurt from this device is 30 N. What is her acceleration?

ch2 #41

Sophia pushes with a 40-N horizontal force on a 4.0-kg box resting on a lab bench. The box slides against a horizontal friction force of 24 N. What is the acceleration of the box?

ch2#43

A business jet of mass 30,000 kg takes off when the thrust for each of two engines is 30,000 N. What is the jet’s acceleration?

ch2#44

Leroy, who has a mass of 100 kg, is skateboarding at 9.0 m/s when he smacks into a brick wall and comes to a dead stop in 0.2 s. What is the deceleration (negative acceleration)? What is the force of impact?

ch2#47

For 3.0 s, Allison exerts a net force of 10.0 N on a 6.7-kg shopping cart that was initially at rest. Find the acceleration of the cart, and the distance it moves

ch2 #48

. If you stand next to a wall on a frictionless skateboard and push on the wall with a force of 40 N, how hard does the wall push on you? If your mass is 80 kg, what is your acceleration away from the wall?

ch2 #51

Note that the weight of the 80-kg firefighter is mg = 80 kg ´ 10 m/s2 = 800 N. So Fnet = (800 N – f) = ma = (80 kg)(4 m/s2) = 320 N. So f = 800 N – 320 N = 480 N

Note that the weight of the 80-kg firefighter is mg = 80 kg ´ 10 m/s2 = 800 N. So Fnet = (800 N – f) = ma = (80 kg)(4 m/s2) = 320 N. So f = 800 N – 320 N = 480 N. Note that the weight of the 80-kg firefighter is mg = 80 kg ´ 10 m/s2 = 800 N. So Fnet = (800 N – f) = ma = (80 kg)(4 m/s2) = 320 N. So f = 800 N – 320 N = 480 N.

With no force of gravity on the Moon it would travel in a straight-line path.

You exert a force to overcome the force of friction. This makes the net force zero, which is why the wagon moves without acceleration. If you pull harder, then net force will be greater than zero and acceleration will occur.

Your empty hand is not hurt when it bangs lightly against a wall. Why does your hand hurt when it bangs lightly against the wall while carrying a heavy load? Which of Newton’s laws is most applicable?

Mainly the first law, for the bag in motion tends to continue in motion, which results in a squashed hand.

What is the net force acting on a 1-kg ball in free fall? What is the net force if it encounters 2 N of air resistance?

The net force is mg, 10 N (or more precisely, 9.8 N). With air resistance the net force is 10 N – 2 N = 8 N (or more precisely 9.8 N – 2 N = 7.8 N).

For each of the following interactions, identify the action and reaction forces. a. A hammer strikes a nail. b. Earth’s gravity pulls down on a book. c. A helicopter blade pushes air down.

a) Action; hammer strikes nail. Reaction; nail strikes hammer. (b) Action; Earth pulls down on a book. Reaction; book pulls up on Earth. (c) Action; helicopter blade pushes air downward. Reaction; air pushes helicopter blade upward. (In these examples, action and reaction may be reversed—which is labeled which is unimportant.)

Why does a cat that falls from the top of a 50-story building hit a safety net below no faster than it would if it fell from the 20th story?

The terminal speed attained by the falling cat is the same whether it falls from 50 stories or 20 stories. Once terminal speed is reached, falling extra distance does not affect the speed. (The low terminal velocities of small creatures enable them to fall without harm from heights that would kill larger creatures.)

2. Free fall is motion in which gravity is the only force acting. a. Explain why a skydiver who has reached terminal speed is not in free fall. b. Explain why a satellite circling Earth above the atmosphere is in free fall.

a) A skydiver encountering NO air resistance is in free fall. But a diver falling in air at terminal velocity encounters air resistance and is not in free fall. (b) The only force acting on a satellite is that due to gravity, so a satellite is in free fall (more about this in Chapter 4).

You tell your friend that the acceleration of a skydiver decreases as falling progresses. What is your response when your friend asks if this means the skydiver is slowing down?

say to your friend that the skydiver is still speeding up but at an ever-slower rate of increase, which means acceleration is decreasing. Eventually the acceleration will become zero, in which case the diver has reached terminal velocity

When you stand on a floor, does the floor exert an upward force against your feet? How much force does it exert? Why does this force not move you upward?

Yes, an upward support force acts on you while standing on a floor, which is equal and opposite to the force of gravity on you—your weight. You are not moved upward by this force because it is only one of two vertical forces acting on you, making the net force zero.

Why can you exert greater force on the pedals of a bicycle if you pull up on the handlebars?

When you pull up on the handlebars, the handlebars in turn pull down on you. This downward force is transmitted to the pedals.

Suppose two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is released. How fast does the heavier cart roll compared with the lighter cart?

As in the preceding exercise, the force on each cart will be the same. But since the masses are different, the accelerations will differ. The twice-as-massive cart will undergo only half the acceleration of the less massive cart and will gain only half the speed

If a Mack truck and a motorcycle have a head-on collision, upon which vehicle is the impact force greater? Which vehicle undergoes the greater change in its motion? Defend your answers.

In accord with Newton’s 3rd law, the force on each will be of the same magnitude. But the effect of the force (acceleration) will be different for each because of the different mass. The more massive truck undergoes less change in motion than the motorcycle.

The auto in the sketch moves forward as the brakes are applied. A bystander says that during the braking interval, the auto's velocity and acceleration are in opposite directions. Do you agree or disagree? Explain

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

If it were not for air resistance, would it be dangerous to go outdoors on rainy days? Why?

With air resistance water drops fall at a tolerable terminal speed. Without air resistance water drops would be in free fall, and depending on the height of the clouds, speeds of impact would be hazardous

When you toss a coin upward, what happens to its velocity while it is ascending? What happens to its acceleration? (Neglect air resistance.) What is its acceleration at the top of its path?

When the coin goes up the velocity decreases by 10 m/s each second. Its acceleration, however, remains constant (which is why the constant change of 10 m/s each second). At the top its acceleration is 10 m/s2 .

Does a stick of dynamite contains force? Similarly, does a fist contain force? A hammer? Why? Why not?

Neither a stick of dynamite, a fist, a hammer, nor anything else “contains” force. We will see later that a stick of dynamite contains energy, which like a fist or hammer is capable of producing forces when an interaction of some kind occurs

Why do you lurch forward in a bus that suddenly slows down? Why do you lurch backward when it picks up speed? What law applies here?

When the bus slows, you tend to keep moving at the previous speed and lurch forward. When the bus picks up speed, you tend to keep moving at the previous (lower) speed and you lurch backward. The law of inertia applies in both cases

If you drop an object, its acceleration toward the ground is 10 m/ s2 . If you throw it down instead, would its acceleration after leaving your hand be greater than 10 m/ s2? Ignore air resistance. Why or why not

After leaving your hand its speed would be greater than if dropped, but its acceleration would be the same 10 m/s2 downward because only Earth’s pull acts on it.

You throw a ball straight upward. Compared with its initial speed when thrown, how fast does it return to its starting point? Answer this for the case of no air resistance, and for the case where air resistance does affect motion

With no air resistance the ball returns as fast as it was thrown. When air resistance affects motion, the ball returns to its starting level with less speed than its initial speed,With no air resistance the ball returns as fast as it was thrown. When air resistance affects motion, the ball returns to its starting level with less speed than its initial speed,

100

If a 2-kg mass has 40 J of potential energy relative to the ground, how high above the ground is(it-'?~

see week3 homework

exam3

ユーザ名非公開 · 27問 · 1年前

exam3

27問 • 1年前

ユーザ名非公開