Physics in the second year of junior high school. Ask for help Ah Rush 50

3000+120*2) 100=meter seconds.

A train with a length of 120m crosses a bridge at a constant speed, and the speed of the train crossing the bridge is 100s when the train passes completely, and the length of the bridge is found.

A train with a length of 120 m crosses a 3000 m long bridge at a constant speed, and the speed of the train crossing the bridge is the time it takes for the train to completely cross the bridge.

A train with a length of 120 m crosses a bridge at a constant speed, and the value of the speed is , plus the length of the train is 11 150 for the length of the bridge, and the distance and time are found.

Answer: If the bridge is x meters long and the time is t, then (120+x) t=t 120= x (11 150).

s total = 3000 + 120 = 3120

s total = 3120, t = 100

Solution: As shown in the figure, a train with a length of 120m crosses a 3000m long bridge at a constant speed, and the time it takes for the train to completely pass through the bridge is 100s.

Known: S1 = 120m, S2 = 3000M, T3 = 100s to find the solution of T4 S3 = S1 + S2 = 120M + 3000M = 3120MV4 = V3 = S3 T3 = 3120M 100s = S4 = 3000m - 120m = 2880M

t4=s4/v4=2880m/

As shown in the figure, a train with a length of 120m passes through a 3000m long bridge at a constant speed, and the time taken by the train to completely pass the bridge is 100s.

The speed is meters per second, the distance is 3240 meters, and what you said about asking for time is what Shenma means.

A 120m train passes through a bridge at a constant speed, and the time it takes to pass completely is 100s, and the speed is, find the distance - 100s*

A 120m long train passes through a 3000m bridge at a constant speed, and the average speed of the complete passage is, find the time - t total = (3000m + 120m).

1 A train with a length of 120m crosses a 3000m long bridge at a constant speed, and the speed of the train crossing the bridge is 312m s, find the bridge crossing time?

2 A train with a length of 120m crosses a bridge at a constant speed, the speed of the train crossing the bridge is 312m s, and the time taken for the train to completely pass the bridge is 100s, find the length of the bridge?

A train with a length of 120 meters crosses a bridge at a speed of 100 seconds per meter, and the length of the bridge is 3000 meters.

A 120-meter-long train crosses a 3,000-meter bridge at a speed of meters per second, and the time to pass is 100 seconds.

1.How far did the train travel in the first ten seconds? 2.What is the speed of a train?

[For your reference].

1. C (three of the three phenomena in A, B, and D are formed by the propagation of light in a straight line, only D is not, and D is caused by the continuous refraction of sound in the clouds).

2. D (light can propagate in a vacuum, and the speed is the fastest, and the propagation of light does not require any medium, so the three answers a, b, and c are incorrect).

3. [You can draw a picture to do this question by yourself, but because there is no picture, I tried to draw it myself to get this answer, I don't know if it is correct] b

4. C (The answer to C reflects the refraction of light when it passes through different media).

5. D (Answer D and answer A have the same point, so the angle of incidence in answer D should be 0°).

6. Omitted. 7. A (B and D answers are direct light exposure, C answer is the refraction of light).

8, B9, B (Diffuse reflection in answer A, although diffuse reflection in answer C reflects light irregularly, it also follows the law of reflection of light, and answer D I don't know how to explain it, it's not good to be conscious).

10. Omitted. 11. (1) Reflection, (2) Convergence, (3) Void.

12. (1) above, (2) don't know (3) don't know (bad consciousness).

13. (1) Mirror, (2) Virtual, (3) Solid, (4) Reflection of light, (5) Light propagation in a straight line.

14, (1) 6, (2) 12, (3) unchanged, (4) unchanged.

19, omitted. [The answers are for your reference, I am not sure about the answers to some questions, and there are others that I don't know, and I am not aware of them].

[You can draw a picture to do this question yourself, but because there is no picture, I tried to draw it myself to get this answer, I don't know if it's correct] b

4. C (The answer to C reflects the refraction of light when it passes through different media).

5. D (answer D and answer A have the same, so the angle of incidence in answer D should be 0°).

6. Omitted. 7. A (B and D answers are direct light exposure, C answer is the refraction of light).

8, B9, B (Diffuse reflection in answer A, although diffuse reflection in answer C reflects light irregularly, it also follows the law of reflection of light, and answer D I don't know how to explain it, it's not good to be conscious).

10. Omitted. 11. (1) Reflection, (2) Convergence, (3) Void.

12. Above (1), the gods (2) don't know (3) don't know (bad consciousness).

13. (1) Mirror, (2) Virtual, (3) Solid, (4) Reflection of light, (5) Light propagation in a straight line.

14. (1) 6, (2) 12, (3) unchanged, (4) unchanged.

How convenient it is to copy homework at school, you have to post it on the Internet...

Oh my God, you're a problem, you're letting someone else do your homework for you, don't do it again, you've ruined it.

。Because the copper rod is a conductor, and the human body is also a conductor, the electricity will be transmitted to the human body, and the copper rod will not be electrified.

2.The amount of electric charge is a physical quantity used to measure the amount of electric charge, which is represented by the letter q. The unit is c (coulombs).

e is the special amount of charge, which is the amount of electricity carried by an electron, e =

1, d is not charged, because the copper rod is a conductor, 2, the amount of charge that the object carries, is identified by e.

V1 = 54 * 1000 3600 = 15m s, v2 = 10m SL1 = 200cm = 2m , L2 = 300cm = 3m [The length of the train is too short, leave it alone for the time being, first calculate according to the number you give].

Wrong train trip = the sum of the lengths of the two trains.

The relative velocity of the opposite direction is the sum of the speeds of the two trains

The relative speed of travel in the same direction is the speed difference between the two trains

(1) It should be car A meets car B! And your unit is messy (I calculate the length by meters) first convert the speed of the two cars to meters, the speed of car A is 15m s, and the speed of car B is 10m s, then the time required in the opposite direction is (200 + 300) (15 + 10) = 20s (2) The time required in the same direction is (200 + 300) (15-10) = 100s

1.The filament evaporates.

When incandescent lamps work, the temperature of the entire filament cannot be completely uniform everywhere. When the temperature in a place is too high, the filament (called a hot spot) in that place evaporates faster due to the higher temperature. After a certain period of time, the hot spot cross-section becomes smaller and the resistance increases, and the temperature is higher and the evaporation is faster.

If this cycle continues, it won't be long before the filament at the hot spot will burn out and become a breaking point, resulting in the end of the bulb's life.

2.High current shock when the light is turned on.

The basis for this is that the inrush current of turning on the lamp is more than ten times that of the normal working current, and such a large current filament is difficult to bear. Many light bulbs break at the moment the light is turned on.

3.Physical** leads to filament breakage.

When the filament is subjected to a large physical shock, it can cause the filament to break.

Conjecture 1: The filament sublimates at high temperatures, which thinner and breaks.

Conjecture 2: The filament has a large resistance, and according to Joule's law, it can be seen that the heat generated is more, and it burns out.

Conjecture three; The filament may have melted at high temperatures and thus broken.

1. Air enters the bulb: At the moment when the light is turned on, the oxygen in the air will oxidize the filament.

2. The voltage applied to the two sections of the bulb is too high: it exceeds the rated power of the bulb.

3. The current through the bulb is too large: the rated power of the bulb is exceeded.

1.Objects Objects mutually.

2.Movement state Shape changes.

3.Size, Direction, and Effect Point are three elements.

4.Newton Ox n Newton.

5.Dynamometer Spring dynamometer.

6.The force on the object due to the attraction of the earth The weight of the object The earth is vertically downward The center of gravity g=mg

7.(1) An object that undergoes elastic deformation will exert a force on the object that comes into contact with it, and (2) it will act in the opposite direction of deformation.

3) Direct contact and elastic deformation.

8. g m

n kg can be varied without change.

Dynamometer balances.

g=mg9.There is relative sliding, relative sliding, pressure, roughness of the contact surface, and relative direction of motion.

10.(1) Increase the pressure (2) Increase the roughness of the contact surface 11(1) When the roughness of the friction surface is the same, reduce the pressure (2) When the pressure is unchanged, reduce the roughness of the friction surface.

3) Change sliding friction to rolling friction.

4) Dry friction is wet friction.

It's all the basics, read the book, and you can solve it yourself.