01-04-2020-Дедушенко И.С.Обучение чтению и устной речи на английском языке (1171845), страница 6
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The stronger force does not overwhelm theweaker force, or have any effect on the upward motion at all. Theforce components simply add together: Fx,total = F1,x + F2,x whereF1,x = 0, F2,x = 0.Let’s summarize it in this way.A force does not produce any effect on the motion of anobject in a perpendicular direction. The most importantapplication of this principle is that the horizontal motion of aprojectile has zero acceleration, while the vertical motion hasacceleration equal to g. That is, an object’s horizontal and verticalmotions are independent.(1464)Discussion questions to Unit 11. When you stand still, there are two forces acting on you, theforce of gravity (your weight) and the normal force of the floorpushing up on your feet.
Are these forces equal and opposite? DoesNewton’s third law relate them to each other? Explain.2. A tugboat of mass m pulls a ship of mass M, accelerating it.The speeds are low enough that you can ignore fluid frictionacting on their hulls, although there will of course need to befluid friction acting on the tug’s propellers.(a) Analyze the forces in which the tugboat participates.
Don’tworry about vertical forces.(b) Do the same for the ship.36(c) Assume now that water friction on the two vessels’ hulls isnegligible. If the force acting on the tug’s propeller is F, what isthe tension, T, in the cable connecting the two ships?3. Ginny has a plan. She is going to ride her sled while herdog pulls her. However, Ginny hasn’t taken physics, so there maybe a problem: she may slide right off the sled when the dog startspulling.(a) Analyze all the forces in which Ginny participates.(b) Analyze all the forces in which the sled participates.(c) The sled has mass m, and Ginny has mass M.
Thecoefficient of static friction between the sled and the snow is µ1,and µ2 is the corresponding quantity for static friction betweenthe sled and her snow pants. Ginny must have a certain minimummass so that she will not slip off the sled. Find this in terms of theother three variables.(d) Under what conditions will there be no solution for M?4. When you fire a gun, the exploding gases push outward inall directions, causing the bullet to accelerate down the barrel.What third law pairs are involved? [Hint: Remember that thegases themselves are an object.]5. Tom grabs Sarah by the hand and tries to pull her. She triesto remain standing without moving.
A student analyzes thesituation as follows. “If Tom’s force on Sarah is greater than herforce on him, he can get her to move. Otherwise, she’ll be able tostay where she is.” What’s wrong with this analysis?6. Kinetic friction is usually more or less independent ofvelocity. However, inexperienced drivers tend to produce a jerkat the last moment of deceleration when they stop at a stop light.What does this tell you about the kinetic friction between thebrake shoes and the brake drums?7. You hit a tennis ball against a wall. Explain any and allincorrect ideas in the following description of the physicsinvolved: “According to Newton’s third law, there has to be aforce opposite to your force on the ball. The opposite force is theball’s mass, which resists acceleration, and also air resistance.”378.
A student states that when he tries to push his refrigerator,the reason it won’t move is because Newton’s third law saysthere’s an equal and opposite frictional force pushing back. Afterall, the static friction force is equal and opposite to the appliedforce. How would you convince him he is wrong?9. The earth is attracted to an object with a force equal andopposite to the force of the earth on the object. If this is true, whyis it that when you drop an object, the earth does not have anacceleration equal and opposite to that of the object?38SUPPLEMENTARY READING TO UNIT 2Read the texts and discuss the problems given below.Text 1A. Numerical Scale of EnergyEnergy comes in a variety of forms, and physicists didn’tdiscover all of them right away.
They had to start somewhere, sothey picked one form of energy to use as a standard for creating anumerical energy scale.One practical approach is to define an energy unit based onheating water. The SI unit of energy is the joule, J, (rhymes with“cool”), named after the British physicist James Joule.
One Joule isthe amount of energy required in order to heat 0.24 g of water by1 C. Note that heat, which is a form of energy, is completelydifferent from temperature, which is not. Twice as much heatenergy is required to prepare two cups of coffee as it takes to makeone, but two cups of coffee mixed together don’t have double thetemperature. In other words, the temperature of an object tells ushow hot it is, but the heat energy contained in an object also takesinto account the object’s mass and what it is made of.Once a numerical scale of energy has been established forsome form of energy such as heat, it can easily be extended toother types of energy.
Here are some examples of other types ofenergy that can be measured using the same units of joules:chemical energy, nuclear energy. New types of energy are keptgetting added to the list.To establish the existence of a new form of energy, a physicisthas to (1) show that it could be converted to and from other forms39of energy; and (2) show that it related to some definitemeasurable property of the object, for example its temperature,motion, position relative to another object, or being in a solid orliquid state.For example, energy is released when a piece of iron issoaked in water, so apparently there is some form of energyalready stored in the iron.
The release of this energy can also berelated to a definite measurable property of the chunk of metal: itturns reddish-orange. There has been a chemical change in itsphysical state, which we call rusting.It might seem that if the principle of conservation of energyever appeared to be violated, we could fix it up simply byinventing some new type of energy to compensate for thediscrepancy.
In the 20th there were experiments that suggestedenergy was not conserved in radioactive processes. Precisemeasurements of the energy released in the radioactive decay of agiven type of atom showed inconsistent results. One atom mightdecay and release, say, 1.1 × 10–10 J of energy, which hadpresumably been stored in some mysterious form in the nucleus.But in a later measurement, an atom of exactly the same typemight release 1.2 × 10–10 J. Atoms of the same type are supposedto be identical, so both atoms were thought to have started outwith the same energy.
If the amount released was random, thenapparently the total amount of energy was not the same after thedecay as before, i.e., energy was not conserved. Only later was itfound that a previously unknown particle, which is very hard todetect, was being spewed out in the decay.The particle, now called a neutrino, was carrying off someenergy, and if this previously unsuspected form of energy wasadded in, energy was found to be conserved after all. Thediscovery of the energy discrepancies is seen with hindsight asbeing a step in the establishment of a new form of energy, and thediscovery of the neutrino was that step.(2726)40Text 2.
Energy and Relative MotionAlthough Einstein’s theory of relativity was mentioned above,it’s more relevant right now to consider how conservation ofenergy relates to the simpler Galilean idea that motion is relative.Galileo’s Aristotelian enemies would probably have objected toconservation of energy. After all, the Galilean idea that an objectin motion will continue in motion indefinitely in the absence of aforce is not so different from the idea that an object’s kineticenergy stays the same unless there is a mechanism like frictionalheating for converting that energy into some other form.More subtly, however, it’s not immediately obvious thatwhat has been learnt so far about energy is strictlymathematically consistent with the principle that motion isrelative. Suppose we verify that a certain process, say thecollision of two pool balls, conserves energy as measured in acertain frame of reference: the sum of the balls’ kinetic energiesbefore the collision is equal to their sum after the collision.
(Inreality it is necessary to add in other forms of energy, like heatand sound, which are liberated by the collision.) But what if wewere to measure everything in a frame of reference that was in adifferent state of motion? A particular pool ball might have lesskinetic energy in this new frame; for example, if the new frameof reference was moving right along with it, its kinetic energy inthat frame would be zero.
On the other hand, some other ballsmight have a greater kinetic energy in the new frame. It’s notimmediately obvious that the total energy before the collisionwill still equal the total energy after the collision. After all, theequation for kinetic energy is fairly complicated, since itinvolves the square of the velocity, so it would be surprising ifeverything still worked out in the new frame of reference. Itdoes still work out.(1570)41Text 3.
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