Junior high school physics questions, please explain in detail Thank you

1. Select B, indicating that the sound needs to vibrate, and other options: A means that the sound propagation needs a medium, C: The sound emitted by different vibration frequencies is different, and D means that the sound can be transmitted in different media.

2. Choose B, the answer does not need to be explained.

3. Choose C, alcohol can definitely boil, but water will not, because water boiling needs 100 degrees, and the heat source for heating in the test tube is also 100 degrees, the water in the test tube itself will evaporate and cool, the closer to 100 degrees, the faster the evaporation and cooling, so the water in the test tube can only be close to 100 degrees, and can not reach 100 degrees.

4. The first horizontal bar: the sun, lightning, the night pearl, the second horizontal bar: (8 * 60 + 20) * 300,000 * 1000 = ?

5. The first horizontal bar: north and south, and the second horizontal bar: the light rays travel in a straight line.

6, first: sound propagation needs a medium, there is no air in space, and there is no medium that can transmit sound, it is impossible to hear the first sound, and second: even if there is a medium, it is impossible to hear the first sound and then see the flash, because the propagation speed of light is much higher than the propagation speed of sound.

7. There is antifreeze in the car water tank, and the mixed liquid is reduced because of evaporation, and the boiling point of the antifreeze is higher than the boiling point of water, so when the water boils, the antifreeze does not boil, and from the evaporated gas, the content of water vapor is much higher than the content of the antifreeze, so the content of the antifreeze in the water tank is getting larger and larger, resulting in an increase in the boiling point of the mixed liquid.

4.The pearl of the sun and the night 5 billionEast-West Light travels in a straight line 6 1There is no medium in the universe, no sound can be heard 2The light should be seen first 7There is less oil, more water, and the specific heat of the water is greater.

Both the state of uniform linear motion and the state of rest are states of equilibrium! The resultant force is zero! Theorem!

Drop the air resistance up, rise up and down! Make your own force analysis diagram of the two states before and after!

The resultant force is when two or more forces acting on an object have the same effect as one force, then this force is the resultant force of those forces.

F is the air resistance, G is the total weight of the balloon, and G2 is the remaining total weight.

Re-analysis: Because the balloon descends at a uniform speed, the balloon is affected by the equilibrium force, then the total weight of the balloon is equal to the sum of the buoyancy and drag force experienced by the balloon, then the drag = 1500n-1400n = 100n

Let the balloon rise at a constant speed and the maximum remaining total weight be g2, and the buoyancy and resistance during the ascent are unchanged.

At this point, buoyancy is up, drag is down, and gravity is down.

It is easy to know that buoyancy is equal to the remaining gross weight plus drag.

then 1400N=G2+100N

Therefore, if the remaining total weight is up to 1300N, the goods that should be thrown are 200N

The combined force is the sum of all external forces. Judgment is based on the relationship between force and motion, as well as simple addition and subtraction. The topic objects do.

Uniform linear motion indicates that the force is balanced, so the net force is 0

The resultant force of the topic includes gravity, buoyancy and force.

air resistance, and gravity includes the gravity of the balloon itself and the gravity of the cargo. The law of the synthesis of the same straight line: the direction is the same, the resultant force is the sum of the magnitude, and the direction is the same as the two forces.

The direction is opposite, the resultant force is the difference of magnitude, and the direction is the same as the strong force.

So the analysis that can lead to the answer.

A force that produces the same effect as several forces acting together is called the resultant force of several forces.

The direction of the resultant force should refer to the direction after the synthesis of several forces, if it is two forces, the direction of the diagonal is the direction of the resultant force.

If the two forces are balanced

forces), the resultant force is zero.

The resultant force has the same effect as several of its components.

f generally refers to various resistances, the force that hinders the movement of objects, called "resistance", also known as post-drag force. A train running on a straight stretch of railroad is subjected to the traction of the locomotive and at the same time by the resistance of air and rails to it. Traction and drag force are in opposite directions, traction increases the speed of the train, while drag decreases the speed of the train.

If traction and drag are balanced against each other, their effects on the train cancel each other out and the train remains in a constant linear motion or stationary state [1]. When an object moves in a liquid, the moving object is subjected to a force acting on the fluid, which reduces its velocity, and this force is also a drag force. For example, when rowing a boat, between the oars and the water, the force of the water that prevents the oars from moving backwards is the resistance.

Another example is when an object moves in air and is dragged by friction with the air.

G refers to the total gravity of the balloon and the cargo, and G2 refers to the gravitational force of the thrown cargo. Gravity is the force that the object on the Earth shows to be subjected to the Earth's vertical downward direction.

These contents will be focused on in the first year of high school, and it is helpful for high school to understand first in junior high school, you can borrow some books from the first year of high school).

The center of gravity of the bar is at point c in the middle of the bar, which is obtained by the equilibrium condition.

G rod * OC g * OA

i.e. g bar *[(ab 2) oa] g*oag bar *[(5*oa 2) oa] g*oa gets g bar 2*g 3 2*120 3 80 Newtons.

Obtained from equilibrium conditions.

G rod * OC g * OA

G bar * [(ab 2) oa] g * oag bar * [(5 * oa 2) oa] g * oag bar 2 * g 3 2 * 120 3 80 Newton 4 5x * 1 2 * 4 5y) = 1 5x * 1 2 * 1 5y) + 120 * 1 5y).

Solution: x=80 n

Let the weight of the lever itself be g, and according to the condition of the lever equilibrium, it is obtained: 120N x OA+1 5G x1 2 OA=4 5G x2 OA, and G = 80N

You should give the diagram, or you have to describe it in detail, otherwise you won't be able to give you an answer.

Let the weight of AB be Xn and the length of AB be Y, then the weight of OB is 4 5Xn and the length is 4 5Y, and the weight of OA is 1 5Xn and the length is 1 5Y;

According to the principle of equilibrium: 4 5x * 1 2 * 4 5y) = 1 5x * 1 2 * 1 5y) + 120 * 1 5y).

Solution: x=80 n

Note: The moments of each segment are in parentheses.

Principle: l1*f1=l2*f2

Therefore: left = right (the length of ab is denoted as l, the weight of ab is denoted as ab), that is: 2 5l * 4 5ab = 1 10l * 1 5ab + g * 1 5l get: ab = 2 3g i.e. ab = 80n.

It's been a few years since I graduated from college, and I forgot about it, which should be right

Let the weight of the rod be g, and consider the two sections of ao and bo separately, goa=1 5g, gob=4 5g, because of the balance, so.

4 5g*2=1 5g*1 2+120n, the solution is g=80n

g*ao=rod.

Find out the center of gravity of the stick, it's as simple as that.

If the two poles adsorbed with iron filings are close together, A and B become a large magnet, and the N pole of A and the S pole of B become the middle part of the large magnet, and the strongest magnetism of the magnet is at the poles at both ends, and the magnetism in the middle is the weakest, so it is less than before.

As shown in Figure B, a soft iron rod with a length slightly greater than the distance between the two magnets is gently rested on the two magnets, and the soft iron rod will also be magnetized, so that the three form a large magnet, so it is the middle part of the large magnet, the magnetism is the weakest, and the iron filings are reduced.

If the two poles adsorbed with iron filings are placed close together, the iron filings adsorbed at the connection will be - (decrease) optionally "increase", "unchanged" or "decrease"); Because the pole part on the magnet is the most magnetic. When the poles are close together, the butt part is no longer a magnetic pole. So the magnetism will weaken.

If a soft iron rod with a length slightly greater than the distance between the two magnets is gently rested on the two magnets, the adsorbed iron filings will (increase) optional: "increase", "unchanged" or "decrease"). Because the soft iron rod will be magnetized after being placed on the two magnets, the two ends of the soft iron rod will become magnetic poles, and because the length of the iron rod is slightly larger than the distance between the two magnets, the magnetic field of this part will be superimposed, and the magnetism will also be enhanced.

Hope it helps.

After getting closer, you can see the two magnets as a new magnet, so the middle part is the weakest magnetic, so that the attracted iron filings are reduced, and after adding a soft iron rod, the magnetic pole magnetism is enhanced and attracts more.

The first, unchanged, does not change the magnetic field at the point of contact.

The second, increasing, the strength of the magnetic field becomes larger.

This is a heat transfer phenomenon, the soymilk is in full contact with the environment, and the larger the temperature difference, the faster the energy is transferred. So hot soy milk cools slower in the summer and cools faster in the winter.

This kind of question belongs to the ** question, and the theory should be applied to practice according to the information given by the question, combined with the knowledge learned and the actual life.

Yes! Because he's asking: Hot soy milk cools slower in the summer and cools faster in the winter.

The biggest difference between winter and summer is the difference in temperature (there is also a temperature difference), so you should grasp the temperature to ask; The speed of cooling of soy milk is judged by time, which should grasp the length of cooling time rather than the speed of cooling to ask questions, so that the answer is c

The ambient temperature outside is low, and the hot soybean milk has a heat exchange with the environment, and the heat is transferred to the environment, so as to achieve the same temperature as the environment.

A household energy meter is marked with "220V; 10 (20)a". When some electrical appliances are working in the home, the total power of the electrical appliances used in these works is 800 W if the power is consumed within 15 minutes; The home can add up to 3,600 W of appliances to work at the same time.

1) Power consumption within 15min, power consumption in 1 hour:

2) p=ui=220*20=4400, the family can increase up to 4400-800=3600w

Total power p total

The total power of the electrical appliances allowed to be connected to the electric energy meter is 220V·10A=2200W=

It is also possible to increase the power to p

Solution: As can be seen from the figure, if there are 3 strands of rope bearing frictional resistance, then f=f, the frictional force of the ground facing the object m f=3f=3 50n=150n;

From the question, the object m velocity v object =, the trolley speed v car =, assuming that the object is stationary, the speed of the small car relative to the object in Xiaohe: v = v car v object =, after 2s, the object is relative to the human movement hall The distance is raised: s1 = vt =, and the free end of the rope moves to the land moving distance:

s2=3s1=3×

So the answer is: 150;

1, a water buoyancy on the wooden ball = the gravity of the wooden ball, the buoyancy of the iron ball is less than the gravity of the iron ball, 2, d, the buoyancy should be between to, if the object floats on the water, the buoyancy is, if it sinks, the buoyancy is greater than or equal to less.

3,c The object floats in the liquid, and the surface, no matter how much is underneath the liquid, the buoyancy is equal to the gravitational force of the object.

4, a, not very sure.

1 A wooden block floats after resting, so f wood = g wood block = g iron.

Because the iron block sinks, the F iron g iron.

So f Fe2 d is not sure if the water in the beaker is full or not, f floats

3 c is floating, floating according to g=f, and the mass of the block remains the same.

4 c Pumping machine utilizes the principle of atmospheric pressure.

1a Because the floating force of the wooden ball indicates that the buoyancy force is greater than the gravitational force, and the sinking of the iron ball indicates that the buoyancy force is less than the gravitational force. And they have equal gravity, so wood is greater than iron.

Select B, as long as the container does not touch the bottom of the sink, H will not change. As long as water is filled into the container, the container will definitely become heavier, and the container will become heavier, and it will definitely sink, so h decreases.