Pleading for a few 8th grade physics questions... Keep it simple... 20

1. The speed of sound propagation in iron is 5200m s, and the propagation speed in air is 340m s, so the time difference is 10000 340-10000 5200=

2. When a person stands in the middle, the sound of the two stone walls should theoretically be heard at the same time, so four echoes are counted, and the sound travels the distance between the two stone walls four times, so it is s=340 multiplied and divided by 4=

3.To catch up with the problem, the distance the student has traveled before is s1 = 2 8 = 16 km, and before the motorcycle catches up, he has walked s2 = 8 1 60 = 2 15 km, and the speed of the motorcycle is v=(16+2 15) 1 60=968km h ) to solve the equation.

1.I checked the speed at which sound travels in the air at 340 per second and in steel at 5000 per second, so do the math.

t=10000÷(5000-340)≈

3.The distance walked by students = 8 (2 + 1 60) = 242 15km242 15 1 16km min

I don't know if the answer is right or not, I hope it can help you.

First of all, the valency of this chemical formula is 0Let the valency of x be x, then n+(-2(2n-1))+x=0

n-4n+2+x=0

x=3n-2

Because n,x are integers greater than 0, assuming n=1.

1.Suppose n=1, x=1

2.Assuming n=2, x=4

3.Assuming n=3, x=7

Then compare the options in turn, and only AB satisfies them.

The multiple-choice questions should not be too complicated, and the process should be fast and accurate. This problem can be substituted, so that n=2, then the chemical formula is H2XO3, which can be carbonic acid (H2CO3) or sulfurous acid (H2SO3), both acids are produced by oxides and water, and the corresponding oxides are CO2 and SO2 respectively, then the answer is B

Hope it helps. (I was tricked into thinking it was physics.) Chemistry is bad).

First of all, the valency of this chemical formula is 0Let the valency of x be x, then n+2(-(2n-1))+x=0, calculate x, and try it in the answer.

1. V=ST, the sound propagation is the same as the flight time of the aircraft, then V is proportional to S, that is, V sound V machine = S machine S sound = tan 30° (times).

2. t=s v=100 340=, the real score is.

3. Equipment: iron frame, scale, reading glasses.

Steps: a. Fix the reading glasses on the iron frame and let the reading glasses face the sun, b. Move the reading glasses to form a minimum and brightest spot on the ground.

c. Use a scale to measure the distance from the spot to the reading lens, that is, the focal length f.

Formula: Power of reading glasses = 100 f

4. M water = m mercury, water sh water = mercury sh mercury, mercury) = xh mercury.

H mercury = 5, let the time of the messenger chase the team t1, then v2t1=v1t1+l, t1=l (v2-v1);

Let the time taken for the communicator to return from the head of the team to the end of the team is t2, then v1t2+v2t2=l, t2=l (v2+v1);

The total distance traveled by the team is s=v1(t1+t2)=2v2l (v22-v12).

Let the time of the aircraft be from the top of Xiao Ming's head to the direction of the ground about 60° above him, then: tan60° = (v speed of sound t) (v aircraft t), i.e., v speed of sound = tan 60° v aircraft.

1150 kg. If you want to have a constant speed, the pulling force is equal to the friction force, and the friction coefficient can be found by pulling a 100-kilogram sled by 40 oxen, and then divide 500 by the friction coefficient to obtain a total weight of 1250 kilograms of the sled and the cargo, and subtract 100 kilograms of the sled to obtain a cargo mass of 1150 kilograms.

Solution: m1gu=f1 100*10*u=40 u=

m1+m2)gu=f2 (100+m2)*10* m2=1150 kg.

The mass of the cargo on the sled is 1150 kg.

Let the whole process s=2x

The whole journey takes t=x v1+x v2=x(v1+v2) (v1*v2) (v1*v2) and the average speed of the whole process v=s t=2x [x(v1+v2) (v1*v2)]=2v1*v2 (v1+v2).

The whole journey time is set to be 2t

The whole journey is long'=v3*t+v4*t=(v3+v4)tThe average speed of the whole process is v'=s'/(2t)=(v3+v4)t/(2t)=(v3+v4)/2

(1) Close the switch first, observe the movement of the wire, then switch the positive and negative binding posts, close the switch, and then observe the movement of the wire.

2) Close the switch, observe the movement of the wire, then turn the north and south poles of the magnetic field, close the switch, and then observe the movement of the wire.

3) The direction of the current and the north and south poles of the magnetic field.

4) Control variable method.

(1) In order to improve the relationship between the direction of force and the direction of the current of the energized wire, his practice is: - Replace the binding post and observe whether the direction of movement of the metal rod changes ---

2) In order to improve the relationship between the direction of force on the energized wire and the direction of the magnetic field, his approach is: -- Place the magnet upside down, and compare and observe whether the direction of movement of the metal rod changes ---

3) Experiments show that the direction of force on the energized wire in the magnetic field is related to the direction of the --- current --- 、-- the direction of the magnetic field ---.

4) The ** method used in this experiment is ---empirical method (you can also fill in: observation method) - not calculus reasoning method).

After 5 seconds, the driver hears an echo, indicating that the time it takes for the sound to reach the mountain is seconds, and the speed of the sound in the air is 340 per second, so we derive that the distance from the boat to the mountain is 850 meters multiplied by 340.

Suppose the boat is at a distance of x m from the mountain when the horn is sounded

The sound went first x m

After hitting the mountain, return and meet the boat that has gone for 5 seconds, so the total distance traveled by the sound and the boat in 5 seconds is exactly 2x

5×340+5x10=2x

x=（5×340+5×10）÷2

x = 875 meters.

5 340 = 2x-5 10 (the distance traveled by the sound wave from disembarkation to the return of the ship = 2 the distance traveled by the ship in 5 seconds when the horn is pure - the distance traveled by the ship in 5 seconds).

x=（5×340+5×10）÷2

x=875