Senior 1 Physics, Urgent !! Who will do 2 or 4 questions! Be concise and explain

BCB is right: the explanation is as follows: 2. To maintain a uniform circular motion, the linear velocity is inversely proportional to the radius, so the larger the radius, the smaller the linear velocity.

That is, a and c are wrong, according to f=ma, the higher the satellite leaves the ground, the smaller f is, and its mass is constant, so a is reduced. i.e. d is wrong. According to GMM R2=mv2r, ek=12mv2

ek=gmm2r and the gravitational potential energy of the satellite ep=-gmm r (with infinity as the zero point of potential energy, this formula can be obtained).

So the mechanical energy e=ep+ek=-gmm2r, so the greater r, the greater e, b is correct.

3. The formula for kinetic energy is: e=1 2m*v*v. So e1:

e2=v*v :(3v)*(3v), i.e., e1:e2=1:

9, so the increment is 8 times the ek answer choose c (note that it is an increment, this is the test point of this question.) ）。4. Since the spring is compressed, the elastic potential energy increases for sure, so B is chosen.

The reason why the remaining three things are wrong: the gravitational potential energy is directly proportional to the height, and when the height decreases, the gravitational potential energy can only decrease. A false.

Kinetic energy is directly proportional to velocity, velocity reduction is zero, and kinetic energy is also zero. D false. To sum up the above situation, that is, the gravitational potential energy decreases, the kinetic energy is zero, and the mechanical energy is increased.

Therefore c is wrong.

2. The gravitational force is a centripetal force, which can be known from the centripetal force formula.

gmm/r^2=mv^2/r

gm r under the root number is derived with v = root, g is the fixed value, m is the mass of the earth, so the larger the radius r is, the smaller v is, so a and c are wrong;

gmm/r^2=ma

A=GM R 2, so the larger R is, the smaller A is, so D is wrong.

gmm/r^2=mv^2/r，ek=1/2mv^2

ek=gmm2r and the gravitational potential energy of the satellite ep=-gmm r (with infinity as the zero point of potential energy, this formula can be obtained).

So the mechanical energy e=ep+ek=-gmm2r, so the greater r, the greater e, b is correct.

2m(3v)^2-1/2mv^2=4mv^2=8·(1/2mv^2)=8ek

c correct. 4、a.When the object falls, the gravitational potential energy must decrease, a false.

b.When the object falls on the spring, the deformation of the spring gradually increases, and the elastic potential energy must increase, b is correct.

c.From a, we can know that the gravitational potential energy of the object decreases, and the kinetic energy decreases due to the decrease in velocity, and the mechanical energy is equal to the gravitational potential energy plus kinetic energy, so the mechanical energy decreases, and c is wrong.

d.The velocity of the object keeps decreasing, and the kinetic energy ek=1 2mv2, so the kinetic energy keeps decreasing, d is wrong.

The answer is B

F direction = mv 2 r centripetal force.

f=gmm r 2 gravitational force.

mv^2/r=gmm/r^2

v^2*r=gm

The larger the r, the smaller the v.

The answer should be B, and the potential energy is greater at this time.

The kinetic energy is proportional to the square of the velocity, v becomes 3v, e becomes 9e, and its increment is 8e, and the answer is c

This process of falling objects reduces the gravitational potential energy. The spring is compressed and the elasticity is able to increase. The potential energy of the spring into which the mechanical energy of the object is converted into the mechanical energy decreases. Finally to the stationary object kinetic energy becomes 0

So the answer is B

2.If you choose b, gmm r 2=mv 2 r v 2=gm r r r, the smaller v, a, c is wrong, acceleration a=gm r 2, the same becomes smaller.

The higher it is, of course, the greater its mechanical energy.

3.Select C, ek1=1 2mv 2 ek2=1 2m(3v) 2=9 2mv 2 ek2-ek1=4mv 2=8ek

4.Option B, the object is descending, the gravitational potential energy is decreasing, A is wrong, part of the mechanical energy of the object is converted into the elastic potential energy of the spring, decreasing, C is wrong, the object has been decelerating, the kinetic energy is decreasing, D is wrong.

Classmates, study hard, the first and second years of high school are very important, I dare to say that it is more important than the third year of high school, the topic is better to do it yourself, besides, if you don't give me the topic, how do I know how to do it?

1.Solution: The falling process of the iron chain is a free fall motion, so the time required for the upper end of the iron chain to fall to the suspension point t1 = root number (2h g) = take g = 10m s 2).

Therefore, the time elapsed by the lower end of the chain just to reach the place under the suspension point t2=;

The distance the chain falls within this second is the length of the chain.

l=2.Solution: After the second object falls time t, the rope is straightened, and the distance between the two objects is exactly 95m

Therefore gt 2-g(t-1) 2=95;

From this equation we can solve t=; (g takes 10 m s2).

Emphasize that this kind of question is not very difficult, but it is more time-consuming, I am bored to come and see, too lazy to do it, here only provide ideas for solving the problem.

The common point of these two questions is probably the lack of knowledge points of the questioner, and my personal feeling should be that the coordinate system is not very clear when solving the problem.

First of all, see this kind of problem After reading the problem, first establish the disintegration coordinate system, which is the basis for solving the problem related to the force, especially the body type that combines force and speed, and the solution idea of a good problem built by the coordinate system is clear at a glance.

The most conventional method is to draw the coordinate system horizontally and vertically, and an axis coincides with the direction of the initial velocity of the object, which can be said to be a universal solution to any type of problem, as long as the horizontal and vertical are calculated separately, and the final vectors can be added.

Another way of thinking is to take the direction of the resultant force as the y-axis, which is a bit winding, but the fixed force can be approximated as parabolic motion, which is simpler than the seemingly complex problem.

If the question type is complex, like the second kind, or more complex, it is necessary to divide the whole process according to the force situation, one step within a scene, for example, the analysis of the second question can be divided into three scene analysis: before the electric field, in the power plant, and after the electric field, the speed does not change during the conversion, but if the problem is complex enough, considering the momentum, then it is necessary to separate the before and after the sudden change in velocity, and it should be noted that the first way of thinking should consider whether the process meets the conditions of vector addition, and there is no foreign quantity or mutation or anything. Otherwise, it's definitely not right, and if there is, you have to divide the process into details.

This question can be used to make a rough V-T diagram to simplify the thinking process.

Solution: The velocity direction of the diamond cutter is decomposed into the direction of the knife and the direction perpendicular to the direction of progress. Then by forming a right-angled triangle, it can be obtained:

cos@=2/10=1/5

arccos(1/5)

Therefore, the angle between the direction of the knife and the direction of movement of the glass plate is arccos(1 5), and the velocity of the vertical glass plate forward direction can be obtained through the right triangle is:

v= (10 2-2 2)=4 6 m s, so the cutting time is:

t=s/v=9/(4√6)

3√6)/8 s

s

If the ring moves from a place with a sparse magnetic induction intensity in the horizontal direction to a dense place, the induced current of the ring will generate a magnetic field in the same direction as the original magnetic field by Lenz's law; If it is densely diluted, a magnetic field in the same direction is generated. AB can then be judged by the right-hand rule. The ring moves in a circular motion, and since the strength of the magnetic field in the vertical direction does not change, the movement in the vertical direction does not produce the Lorentz force, only the movement in the horizontal direction, so the Lorentz force is always perpendicular to the horizontal direction.

And the speed does a circular motion, choose D

Because the magnetic induction intensity is always vertical, the ampere force direction is always horizontal, according to Lobida's law.

Descending at a constant speed indicates that the component of gravity is balanced with the frictional force. That is, the friction force is f1=mg*sino; When sliding upwards at a constant speed, the block is also balanced. Then at this time there is:

2*mg*sinos=f*cos (friction + component of the gravity of the block = f*cos) gives f=2mgsino cos so when =0°, f is the smallest, which is 2mgsinoThe frictional force is mg*sinoThe o in the above formula is the wooden wedge inclination is completed, and the work is over!

In the end, you got the velocity right, and the direct momentum is conserved.

The kinetic energy lost by the wooden block is actually e1=1 2m(vt -v0), and the kinetic energy increased by the wooden board e2=1 2mv0

The internal energy is e= e1- e2

Let the average induction time potential be e, then according to Faraday's law of electromagnetic induction, there is e=n δt=nbs2w =40 volts and the instantaneous time potential of 2 is the maximum, and the height is em

em=nblv=nbsw=20v