Junior High School Physics Exercise Problem 1 Urgent Detailed Problem Solving Process and Analysis!!

8.A and B two cars are 200m apart from each other before and after the first day of the same speed, it is known that car A is in front, car B is behind, and the speed of car A is 72km h, the speed of car B is 36km h, if car A is taken as a reference, the direction of movement of car B is backward, the speed of car B is 36 km h, if car B is taken as a reference, the direction of movement of car A is forward, and the speed is 36 km h.

17.A large wooden box floats on the surface of the river and drifts downstream with the river, and there is a small boat jumping upstream and downstream of the wooden box, and the distance between the two boats and the wooden box is equal. If the speed of the two boats in still water is equal, then when both boats row towards the wooden box at the same time (b).

a.The upstream boat fished the wooden box B. firstThe downstream boat fished the wooden box first.

c.Both ships arrived at the same time at the wooden box DIt is impossible to tell which boat arrived first in the wooden box.

18.Television stations transmit television signals to high-altitude communication satellites, which in turn transmit signals to other areas on the ground, thus realizing the relay of satellite television. During the commissioning process, the TV station itself also needs to receive the signal from the satellite and compare it with the signal from the cable in the studio.

At this time, it will be found that the changes in the person's movements on the screen from the satellite always occur later than the corresponding changes on the screen from the cable.

1) From the point of view of a person on the ground, the state of motion of such a communication satellite is (a).

a.Eastward movement bWestward movement cStatic DMovement in a north-south direction.

25.The car was driving on a straight road at a speed of 36 km/h when a passenger dropped an object from the window and fell to the ground. At this point, how many meters did the car advance?

What is the horizontal distance between the object and the car window? If the car is traveling on the road at a speed of 72 m h and the passenger drops the same object from the window, how many seconds does the object fall to the ground?

In seconds, x=36*, h=10*

Asking too much, I'm going to sleep and continue tomorrow.

8.72km h-36km h=36km h The relative speed of the two cars is 36km h, so both blanks are filled with 36km h

17.Choose C, because the wooden box is stationary relative to the river water, and the speed of the two boats relative to the river water is also the same.

I regret it and didn't answer... Play went.

8. The first question is -36, and the second question is 36

(1) Circuit diagram B is more in line with the actual situation (when the hair dryer blows cold air, the fan needs to work alone, and in diagram A, whether the switch is placed between 1, 2 or 2, 3, the heating wire is turned on, which does not meet the requirements).

2) When blowing hot air, the fan and heating wire of the hair dryer are working, and when blowing cold air, the fan is working alone, so the power of the heating wire is the power when blowing hot air minus the power when blowing cold air is the power of the heating wire (460w-20w) = 440w, the voltage of household electricity is generally 220v, then the resistance of the heating wire is the square of the voltage divided by the power is the resistance value (220 v 440w = 110), the last few words of the question you asked are not typed, If you ask the heat generated by the current passing through the heating wire in one minute, it is the power of the heating wire multiplied by the time (minutes should be converted to seconds in SI units, that is, 1min = 60s), 440w 60s = 26400j

Answer:1It should be Figure B, Note that when the switch is connected to 2, Figure A only heats up and is not connected to the motor. When connecting 2 and 3, both pictures A and B indicate that hot air is blowing.

2.When blowing hot air, the motor and the heating wire work at the same time.

There is P total = P motor + P heating wire.

So P heating wire = 460-20 = 440W

The home circuit voltage is 220V

So r=u 2 p=110 ohm i=u r=2a, so q=i 2rt=4*110*60=26400j

(1) Figure B is more in line with the actual situation. Because as shown in Figure A, the heating wire heats up when the switch is connected between them, but the motor.

It does not turn, and no wind blows out; When connected between the heating wire and the motor, both work, and the hot air is blown out.

2) As shown in Figure B, the power when blowing cold air is 20W, the power of the motor when blowing hot air is 460W, and the total power of the motor and the heating wire, therefore, the power of the heating wire is P=460-20=440 W, the resistance of the heating wire R=U squared, P=220 squared, 440=110 euros, the heat generated by the current passing through the heating wire within 1min Q=PT=440*1*60=26400 J

Figure B is more in line with the requirements of point blowing, which is that it must be blown first and then heated, or it must be blown and heated at the same time, but there must not be a situation where it is only heated and not blown, so Figure A is unreasonable.

The power of the spot heat wire is equal to the total power minus the power of the fan (the power when blowing cold air). 460w-20w=440w

The resistance r is equal to the square of the resistance divided by the power, which is equal to 110 euros, and the heat of one minute is equal to its power, multiplied by the time, q=440wx60s=26400j

A freight train with a length of l = 1500 m travels at a constant speed on a straight track with a drag force of f = 4 105 n. When you want to pass a cross-river bridge with a length of l=600m, in order to remind the pedestrians on both sides of the railroad tracks on the bridge to pay attention, the driver at the front of the locomotive blows the whistle when the locomotive reaches the bridge, and the escort at the rear of the car hears the whistle after t1=4s, and the sound speed in the air is known to be v1=340m s

1) The speed of the train v2;

2) The time when the train passes through the bridge t2.

1) According to the topic, the relationship between speed, road type and time is:

l=(v1+v2)t1

Substituting the data yields: v2=35m s

2) According to the topic, the relationship between speed, distance and time has:

l +l=v2t2

Substituting the data yields: t2=60s

If the speed of the lead train is v2, then the speed at which the sound of the whistle is transmitted to the escort is (v1+v2). Therefore t=l (v1+v2), i.e., 1500 Qimin(340+v2)=4, and the solution v2=35m s

The time when the train crosses the bridge'=(l1+l2)/v2=2100/35=60s

1. The density of gold should be gold = . Aluminum = .

Let the mass of aluminum be m aluminum, then the mass of gold is (157g-m aluminum), the volume of aluminium v aluminium = m aluminium = m aluminum, the volume of gold v gold = m gold gold = (157g-m aluminum);

and v total = v aluminium + v gold = 52cm, so m aluminium + 157g-m aluminum) = 52cm solution m aluminium = , then m gold = .

2. Let the volume of sand in the Yellow River water of 10dm be v sand, then the volume of water is (sand), the mass of sand m sand = sand v sand = sand, the mass of water m water = water v water = sand), and m total = m sand + m water =, so sand + (sand) =.

The solution gives v sand = , then m sand = sand v sand = =.

The sediment content of the water of the Yellow River is (1m).

1.The mass of aluminum can be set to be xg and the mass of gold to ygYes.

x+y=157,x, solving the system of equations gives x=,y= that is, the mass of aluminum is, and the mass of gold is, but I remember that it should be the density of mercury.

2.The volume of sand in the Yellow River water at 10dm3 is VDM3, then, v=, its mass is, and 1 cubic meter is 1000dm3, then it contains 30 kg of sand, that is, the sediment content of the Yellow River water is 30kg, children, study hard, these are the basic knowledge.

Aluminum mass m1 gold mass m2 m1 + m2 = 157 -3 to the power of 10 kg....

Aluminum volume v1 gold volume v2 v1 + v2 = 52 -6 power m3 of 10....

and gold and aluminum can be found.

The above 2 solutions are good, you can refer to them.

1.Let the mass of aluminum be xg and the mass of gold be yg

x+y=157,x/

x=, y=

That is, the mass of aluminum is and the mass of gold is.

2.The volume of sand in the Yellow River water located at 10dm3 is vdm3, v=, and its mass is, and another 1 cubic meter is 1000dm3, then it contains 30 kg of sand, that is, the sediment content of the Yellow River water is 30kg

1.Ad. If the quality.

Bai is the same, then the water of 0 is frozen into 0 and has to absorb heat. It shows that the internal energy of DAO water is more than once.

2.Ad. By replying to the transmission power p=ui, the voltage is increased to 10 times the original, and the current drops to the original 1 10.

If the line resistance r does not change, then the power p = i r of the line heating loss. So the loss of power is only the original loss of power is indeed 100

3.Aa. Movable pulley, labor-saving but not effort-saving.

4.Answer: The rope pull force is one-third of the gravitational force of the object, and only one-quarter of the force of gravity in B.

1.The internal energy of the object bai and the temperature of the object, the body du

product, the number of molecules contained in the DAO (mass) DAO, the higher the temperature.

The greater the internal energy, the larger the volume, the greater the internal energy, the greater the mass, the greater the internal energy. Because of ice and water of the same mass, v ice:v water = 10:

9, so when their masses are equal, v ice is greater than v water, and because the mass of ice is greater than the mass of water, the internal energy of ice must be greater than that of water.

2.Because the power is certain, from p=ui, it can be seen that when u is increased to 10 times, i is reduced to 1 10 of the source current. The original electric energy loss is i*i*r, and the electric energy loss after the voltage change is (1 10i)*(1 10i)*r=1 100*i*r, so the current electric energy loss is 1 100 times the original loss.

Think from the point of view of the power plant).

3.Because of the smooth bevel, the friction and pulley gravity are not taken into account so the two cases =1So w=w'=gh

And because the length is 3h, the height is h, and the smooth bevel is h, so f=1 3g. Movable pulley, f=1 2g, so f f'

F = 1 3g in B, F = 1 4g in B.

1.The internal energy of the object and the temperature of the body, volume du

The higher the temperature, the greater the internal energy, the larger the volume, the greater the internal energy, the larger the mass quantity, the greater the internal energy. Because of ice and water of the same mass, v ice:v water = 10:

9, so when their masses are equal, v ice is greater than v water, and because the mass of ice is greater than the mass of water, the internal energy of ice must be greater than that of water.

Answer: (1) 15min=900s

According to v=s t, the average velocity v=9000m, 900s=10m, s(2)g=mg=9000kg*

p=f/s=g/s=88200n/

3) Power: P=W, T=FS, T=FVT, T=FV=4500*12=54KW

Hope it helps.

When the plate wipe moves downward, the friction force is upward, f=g+f=;

When the plate rub moves upward, the friction force is downward, fb=g+f=

f Mo = b = F Mo +

Draw a force analysis by yourself and you will understand.

1.Buoyancy Gravity 4*10 40n

So the volume of drained water is 40 10 4l

So the lifebuoy volume 4 (1 4)=16

So the lifebuoy density is 4 (16)=, i.e. 250 kg cubic meters.

2.The maximum buoyancy can be generated 16*10 160n, so the mass of the little one is 160-40 120n, that is, 12 kg 3The volume of the person is 50, and the volume of the total discharge of boiling water is 49 + 4 53, so the maximum buoyancy is 53 * 10 530, and the total gravity is 50 * 10 + 40 540 is greater than the buoyancy, so it is not safe.