High School Physics Problems Mechanics with regard to energy .

Answer: a, b, c

Analysis: The resultant force of the block is f-f, and the opposing position is shifted to (s+l).

The work of the tensile force f w=f(s+l), and the work of the friction force f w2=-f(s+l), so option c is correct;

Applying the kinetic energy theorem to the block, (f-f)(s+l)=ek-0, so option a is correct;

The resultant external force on the trolley is frictional force f, and the opposing position is shifted to s.

Apply the kinetic energy theorem to the trolley, fs=e'k-0, so option b is correct;

Friction does work on the system -fs, and f does work on the system f(s+l), so the mechanical energy of the system increases f(s+l)-fs, option d is wrong;

According to the law of conservation of energy, the amount of heat generated by the system is equal to the work done by the system to overcome the frictional force fs, so option e is wrong.

a. For the analysis of the force of the block, the horizontal constant force f direction is to the right, the friction force f direction is to the left, the kinetic energy of the block is the work done by the constant force f - the work done by the friction force f, and the movement distance of the block is the length of the trolley + the trolley travel distance. mv = f fit * s f fit = f f s = l + s correct.

b. For the trolley, because the horizontal plane is smooth, the horizontal direction of the trolley is only affected by the friction force f of the block on it, and the distance of the trolley movement is S mv = fs is correct.

c. The work done by the block to overcome the friction force is the work done by the friction force on the small block w friction = f*(l+s) is correct.

d. The mechanical energy increased by the block and the trolley = kinetic potential energy + gravitational potential energy The gravitational potential energy has not changed, and the kinetic potential energy should be the kinetic energy increased by the trolley + the kinetic energy increased by the block is the sum of the two answers ab, so it is not true.

e. The heat generation of the system should be the relative displacement of the block and the trolley * the friction force should be FL, so it is wrong.

The correct answer is ABC

The maximum static friction is equal to the sliding friction, the neutral component of the static friction force along the inclined surface downwards and mgsin30=

Look at the picture and speak.

The figure is an analysis of the force when the object just remains at rest, because the object is stationary, so there is a tendency to slide downward, then there must be an upward maximum static friction f', according to the figure, f'＝mgsin30°。When the block starts to slide, the sliding friction is also equal to the maximum static friction, so f f'＝mgsin30°。There are two states of the block, rest and motion, so there are two force analyses, both of which must be drawn!

If it helps you, don't forget to take it.

First of all, when there is no pulling, the object is just stationary on the inclined plane, which means that the friction force is equal to the component of gravity along the inclined plane, and the friction force at this time is also the maximum static friction force (because it is just right), so the sliding friction force is, and the slow motion of the object is a state of equilibrium, and it is not difficult to balance f is mg according to the force balance. There are often some key words in high school physics, which should be carefully examined, and these are often manifested as a critical state, which is a breakthrough in doing problems.

At the beginning, it didn't mean that it happened to be stationary, which meant that the maximum static friction was reached at this time.

(1) When the ball is just thrown, the velocity is in the horizontal direction, the resistance f=kv0 is in the horizontal direction, and the gravitational force mg is vertically downward, which is combined by Newton's second law a=f m=sqrt(g 2+(kv0 m) 2) (sqrt: open squared).

2) Apply the kinetic energy theorem to the ball from throwing to landing: mgh-w resistance = mv 2-m(v0) 2 (1).

The ball has a uniform velocity before hitting the ground, so the resistance and gravity are balanced, mg=kv (2) synthesis(1)(2) obtains, the work done by the ball to overcome the air resistance during the process from throwing to landing.

W resistance = mgh + m(v0) 2 2-m 3*g 2 k 2 2(3) The trajectory of the ball is a curve, the velocity is in the tangential direction of the curve, and the resistance f direction is always in the opposite direction of the velocity. This problem only needs to analyze the vertical motion and stationary release when the horizontal throw is thrown. When the horizontal throw is thrown, the speed of the vertical motion increases from zero, and the resistance in this direction also increases

Gravity is the cause of acceleration, and vertical drag is the cause of downward motion, exactly as the ball is released from rest. As a result, the time it takes for both balls to land is the same and the velocity at which they hit the ground. The principle applied in the analysis of this question is the independence of sub-motion.

(1) g 2+(kv0 m) 2] (is the meaning of the root number).

2)w=mgh -

3) With the same velocity, it takes a long time for a statically released block to fall.

Explanation: (1) The block is subject to gravity and resistance, both perpendicular to each other, and according to the Pythagorean algorithm, the vector sum of its acceleration can be calculated.

2) The energy of the object is analyzed, and the gravitational potential energy of the object is converted into kinetic energy and impedance heat energy. If the object finally moves at a uniform velocity, it means that the drag force is equal to gravity at this time, and the value of the final stable velocity can be solved, then there is the following equation: total energy = work to overcome the resistance + kinetic energy + potential energy (at this time the potential energy is equal to 0), and the result can be calculated.

3) The velocity is the same as it is easy to understand because the equation of state for ultimate stability is the same.

Time: When the flat throwing motion is in motion, the drag acceleration is the velocity reversal, and the vertical component of the resistance produces the drag acceleration in the vertical direction. When falling at rest, vertical drag acceleration = drag acceleration. So the fall is slow and takes a long time.

At the same time, the large average drag acceleration when falling at rest explains the fact that the block enters equilibrium earlier, so there is no need to worry about the block not satisfying the equilibrium equation.

Feel free to ask.

My idea is to decompose the resistance into vertical and horizontal directions, and the motion into horizontal and vertical acceleration, and then subject to two resistances.

1) There is no drag acceleration if there is no velocity in the vertical direction when it is just thrown, so the total acceleration is the combination of the gravitational acceleration in the vertical direction and the acceleration provided by the resistance in the horizontal direction.

2) The horizontal resistance reduces v0 to 0, and the vertical direction is constant, so the work done by the resistance is 1 2mv0 2+mgh-1 2m(mg k) 2

3) The displacement is the same, the former is a uniform motion in the vertical direction, and the latter is a uniform motion with 0 acceleration, so the time is the same.

,a=f/m

2.Mechanical energy before landing: w=mgh+mv*v 2When landing, kv1=mg, w1=mv1*v1 2, resistance workmanship w2=w-w1

Your first sentence is correct, the mistake lies in the second sentence, because you are a little confused about the concept of functional principle. The work done by gravity and the change in gravitational potential energy in mechanical energy are considered repeatedly.

To get the work done by hand, there are two algorithms:

The first kinetic energy theorem: 1 2mv -0=w-mgh, from which w=mgh + 1 2mv is obtained

The second functional principle: w = mechanical energy change = (mgh + 1 2mv ) - 0 + 0) The answer is exactly the same.

It doesn't seem to be explained.

Do you think that you have been holding it at a certain height, and your hands are sore? This is the negative work of gravity.

You get the kinetic energy of the object, and you still don't understand the kinetic energy theorem and the conservation of energy.

Your hand is the negative work of overcoming gravity, which is equivalent to the internal force you exert on the object. The energies in the system can be converted into each other.

The work done by the hand is the total increase in energy, including kinetic energy and potential energy, and the work done by gravity is subtracted from the increase in kinetic energy.

The change in mechanical energy is equal to the work done by the resultant external force other than gravity. Seeing the object and the earth as a whole is only a special case that will not be covered in a general topic....

If you use the kinetic energy theorem to do it, you can understand that the sum of the work done by the initial kinetic energy and the final kinetic energy of each force (positive function, negative function) is 0 1 2mv 2= mgh

The first question mark: not necessarily, if negative work is done, the energy of the object decreases.

The second question mark: it is not necessarily internal energy, it can also be mechanical energy, potential energy, etc....

The third question mark: the piston inflatable ball, the person does positive work on the piston, which is converted into the mechanical energy of the piston and a part of the heat energy generated by friction (the pump will be very hot after pumping is friction heat), and the mechanical energy of the piston is converted into the internal energy of the gas.

The fourth question mark: the teacher is right.

The fifth question mark: the external body does negative work on the object, then the energy of the object decreases, because the negative work done by the outside world on the object is equivalent to the positive work done by the object on the outside world.

The sixth question mark: friction does negative work, the internal energy of the object decreases, friction produces heat volatilization, so that the energy of the object is reduced, such as the pump, the friction between the piston and the cylinder wall does negative work, is the reduction of piston mechanical energy; Friction force does positive work, the internal energy of the object increases, at this time friction is beneficial, such as the rear wheel of the car, the friction between the ground and the wheel forward, do positive work to increase the mechanical energy of the car forward.

The seventh question mark: the work done by the outside world on the object depends on whether the outside world does positive or negative work on the object, and the energy of the work done by the outside world is transferred to the object, which is the increase in the energy of the object. Negative work, on the other hand, causes the energy of the object to be transmitted to the outside world.

The eighth question mark: 1) Do positive work corresponding to energy output, for example, the engine piston does positive work on the crank connecting rod, so that the mechanical energy of the rod increases and outputs energy outward to let the car move forward, and the mechanical energy of the car increases. 2) Doing negative work corresponds to energy input, or the example of an engine, the fuel does positive work on the piston, that is, the piston does negative work on the fuel, and the internal energy of the fuel is input into the mechanical energy of the piston.

The ninth question mark: It's very tangled if you can't figure it out, what can you do if it's swollen? Analyze it slowly, for example, if a meteorite falls from space to the earth, where does the kinetic energy of the meteorite come from?

The meteorite has no engine, so it is obviously the potential energy that is converted into kinetic energy! And then what? What else does it matter?

Does the meteorite fall have to rub against the air, the friction does negative work, the friction generates heat, and the meteorite is very hot and woody? Where does the heat come from? Apparently it was converted from kinetic energy!

How does a meteorite move if it has no kinetic energy? How to rub against air? And then what?

Meteorites hit the earth, the sky was full of dust and rocks, and the dinosaurs were extinct. How does the dust get up? Obviously, the energy of the impact is converted into the kinetic energy and potential energy of the dust!

That is, the mechanical energy of dust. How did the big hole smashed by the earth come about? The original matter in the pit has been moved, and the energy is converted from the kinetic energy of the meteorite!

Finally, the potential energy of the meteorite is converted into heat and dissipated in the atmosphere, which is converted into the mechanical energy of the dusty stone .........Mechanical energy = kinetic energy + potential energy).

Finally: Think about it slowly! Conservation of energy means that energy changes to change and satisfies conservation.

First, familiarize yourself with the concept, second, memorize the formula, imagine that the amount of energy is fixed and is just constantly transferring, exchanging, just like bartering, third, find a very difficult and very complex problem that can contain the knowledge points of momentum energy, from beginning to end by yourself to do each step by yourself, don't be afraid of wasting time, after doing it right, sort it out a few times, think about how to do other questions, and then find a little more difficult problem to make it yourself, after doing so, it's OK, When you do it yourself, you must not ask others about it, and if you don't understand it, you must read theorems and example problems, and you must rely on yourself. Finally, summarize the key words by yourself, find some theorems that will be used when the words appear, memorize them skillfully, and the questions will be much faster, and you must use your brain in class, listen carefully, and sleep must be sufficient!

Momentum is the formula, the same as kinetic energy, the key is the breakthrough, the formula mv0 = mv1 + mv2 and then find the relationship according to other conditions, most of the momentum and are kinetic energy combined. Judging when energy is conserved, it is transformed when there is no heat energy loss, and it is also associated with kinetic energy.

Of course, a good way to remember the formula is to finish a difficult question (preferably get it wrong) and compare the answers, and then slowly think about it until you think it clearly, and think clearly about every process of energy transformation, and this question is almost fine.

Momentum is conserved, energy is conserved. Stress condition. Analyze them one by one in the order in which the blocks move.

The momentum of the collision is conserved, the force on it is analyzed, and the acceleration is considered to see what kind of motion it does. Energy is conserved throughout the process. I don't care about how many questions I do, I have to think about and summarize each question.

In order to learn physics well, in addition to doing more problems, that is, reading more example problems, listening to lectures in books, and finally doing more problems.

Upstairs is so handsome.,It's a high school student.,You think it's for undergraduates.。。。 It seems that you are not asking for the calculation, you are counting how long it takes to get to the top, the up and down time is not equal, and you can't multiply it by 2

It's very simple and can be solved with elementary math.

1。For the whole process, the momentum theorem is used, i weight + i gas = mv2-(-mv1) is in the downward direction.

That is, MGT + I gas = M (V2 + V1).

Since the drag force is proportional to the velocity, let f=kv, the whole process can be divided into several small segments, and the time of each small segment is infinitely short, it can be considered that the velocity is unchanged in the infinitesimal time, and the object has a constant velocity, then f(n)=kv(n) in the n segment, so the total air impulse i gas in the whole process = kv(n)t(n)=k h(n), v(n)t(n) is the displacement h(n) in the nth sub-segment, and the total displacement of the whole process is zero, that is, h(n)=0, so the total impulse of the air resistance in the whole process is zero, that is I gas = k v (n) t (n) = k h (n) = 0

So there's mgt=m(v2+v1), and you get t=(v1+v2) g, ha, that's just as interesting as a free-fall, right?

2。This is calculated using the conservation of angular momentum. i.e. mv·r=i +m r·r, the moment of inertia of the cylinder around the central axis is i=mr 2 2, which can be calculated by taking it in =2mv (mr+2mr).

The energy loss can be calculated directly, that is, the initial kinetic energy minus the kinetic energy of the last cylinder and bullet rotation, e=mv 2 2-[m( r) 2 2+i 2 2], and the above data can be calculated to obtain the δe value.