Junior high school physics questions, urgent!!! Tell me why .

Reason B is chosen for the first question, because when the fan is rotating, the air velocity above the blade surface will be greater than the air velocity below due to the shape of the fan blades. According to the fact that the higher the air velocity, the smaller the atmospheric pressure, the atmospheric pressure above the leaf surface will be less than the atmospheric pressure below, resulting in an upward buoyancy f. Overall, the fan is stationary in one place.

So by the equilibrium force, that is to say, the upward force and the downward force are equal. f+f float = g. Therefore, if the tensile force f is less than g, b should be chosen.

The second question is reason C. When a hydrogen balloon is released, it rises because hydrogen is less dense than air and it is subjected to much more buoyancy than its gravity. As the altitude increases, the outside air pressure decreases, the volume of the balloon increases, and although the buoyancy increases, the air resistance will also increase.

When the addition of air resistance and gravity equals buoyancy, the balloon rises at a uniform speed. But when it reaches a certain point, the drag will go up again, so the balloon will fall at a uniform speed. It's just that the balloon doesn't have a lot of range to move up and down, so it can be seen as rising to a certain distance.

So choose C.

1）a．2）a．

When the hydrogen balloon rises to a certain height, it floats and should be stationary there.

But the balloon originally had an upward velocity. Therefore, even when it reaches the position of equilibrium force, it cannot stop immediately, but continues to move upwards due to inertia. Until the upward velocity is 0.

However, the force here is that the downward force is greater than the upward force. The object will start to do a downward acceleration motion.

For the same reason, when the equilibrium force is reached, it cannot be stopped and ...... due to inertia.

So the balloon will keep rising and falling, rising and falling again and again at the point of equilibrium.

Because at this time it is in a state of two-force equilibrium.

i.e. f=g, so the pulling force is equal to the gravity of the fan.

Because the air pressure in the balloon is equal to the atmospheric pressure on the ground, and with the increase of altitude, the outside air pressure is gradually decreasing, while the air pressure in the balloon remains unchanged, so the air pressure in the balloon is greater than the outside air pressure, causing the balloon to burst, so it can only rise to a certain height.

The first question is c, because the ceiling fan is stationary, which means that it is affected by the balance force. So g=f.

Question 2 c: When the balloon only rises to a certain height, it will not burst because the density of the internal air is greater than the density of the external air, resulting in the air pressure inside the balloon being greater than the external air pressure.

Hopefully. Thank you...

Select B for the first question and C for the second question

Question 1: When the ceiling fan is running, the air flows downward, so at the same time, the air gives the ceiling fan an upward reaction force, so the force on the ceiling fan is less than the gravitational force.

The second question, before the release of hydrogen, the balloon accelerates upward with constant acceleration, after the release of gas, the buoyancy is less than the gravity, and the acceleration is downward, so the deceleration movement is done to the highest point, and then the downward acceleration movement. until it hits the ground.

Question 1 B: We all know that the wind blows downward when the ceiling fan rotates, according to "the effect of the force is mutual", it can be seen that the wind has a downward force, so the ceiling fan moves slightly upward (the amplitude is very subtle) because of the reaction force of the wind, so F is smaller than G.

Question 2A: The buoyancy problem is quite complicated (it is quite difficult to explain), ask the teacher, it must be more accurate.

If the ground is not smooth, there will be friction on the object, if the thrust f<= that is, thrust = static friction, then its motion state remains unchanged. Only when the thrust is greater than the maximum static friction is it manifested as dynamic friction, and then the thrust is dynamic friction and the state of motion changes. So it's not necessarily going to change.

Item b A pair of equilibrium forces is defined as two forces of the same magnitude and opposite directions acting on the same straight line and on the same object. Item B does not emphasize the same object, so it is not necessarily a balancing force.

C The three elements of force are the powerful action point, direction, and magnitude. It is certain that the object is subjected to twice the force in one direction, and then the object will not be stationary, so it is not a balanced force.

Supplementary Question Item AB Inertia is an intrinsic property of an object, not a force, and cannot be used"Subjected to inertia"To describe, can only say"Subjected to the action of *** force"

The object of the two forces in item c is different and does not conform to the equilibrium force"The same object"Definition.

Item D Li Ning is still subject to gravity, but at this time he (she) is weightless, that is, although he or she is subjected to gravity, he or she cannot feel it.

ba, if the thrust is less than the maximum static friction or stationary, the state of motion or stationary c, the three elements in are the same, which is the same force.

d, an object that does a uniform linear motion may be subjected to multiple pairs of equilibrium forces, such as on a horizontal ground, push an object with f to make it have a uniform velocity, then. Thrust and friction, pressure and support are balanced.

Question 1 is selected b

A No, when a stationary object is thrust, if the friction between the object and the ground cannot be overcome, the state of motion will not change. You have to exceed the maximum friction to get moving!

c is not the same for the three elements of the three elements of the two forces, which means that they are the same force. The direction is the same. Whereas, the equilibrium force is in the opposite direction.

D is not right, it will still move in a straight line.

For the second question, choose A, and you just need to specify option B.

Inertia is not a force", it cannot be said that "acted upon by inertia"!

And I'll give you a summary. The misconceptions about inertia are: inertia is generated. subject to inertia. Subjected to inertiaInertial force. Correct to say: due to inertia. It has inertia. (The essence of the teacher's summary).

Disclaimer: The title says "then the object may no longer move in a straight line" instead of saying "then the object may no longer do a uniform "linear motion"! So some of the netizens below are wrong!

DA not necessarily.

c are the same two forces that cannot be a balanced force.

b does not say that it acts on the same object.

d For example, the ball rolls frictionlessly on the table, and is balanced by gravity and support, and will be thrown flat after the support force is removed.

Supplementary Question A

b is not accurate. c. The stressed object is not uniform.

d Pure.

Question 1 D

A Not necessarily, it may be pushed but not moved.

b does not say the same point of action, nor does it say the same object.

cThe equilibrium force is in opposite directions.

So, choose D for the second question A

b It should be said "because of inertia" or "due to inertia", but it cannot be said "because it is acted upon by inertia".

c does not act at the same point of action.

He has always had inertia.

Question 1 is selected b

Analysis: A: If the thrust is less than the maximum static friction force, the object is still stationary, that is, the thrust is too small!

b: If two forces act on the same object, it is a pair of balanced forces! Therefore, correct c: if the three elements are the same, then the direction is the same, and it cannot be a balanced force!

D: Force is what changes the state of motion of an object! If a force is removed and the net force is not zero, then the motion state of the object must change!

The change of the state of motion includes the change of the direction of motion and the magnitude of the velocity! If the direction of the remaining force is in the same straight line and the direction of the object's motion is the same, then the object moves in a straight line with acceleration, to be precise, it moves in a straight line with uniform acceleration, if the direction is opposite, it moves in a straight line with uniform deceleration, and if the direction of the force is not in the same straight line, then the object moves in a curve!

There is no correct answer to the second question.

A: It is also subject to gravity and support.

B: Inertia is a property, not a force, and it cannot be said that it is affected by inertia, but it should be said that it is due to inertia!

C: It should be that the pull of the horizontal bar on him and the gravity on him are a pair of balancing forces.

d: Everything on the earth's surface is subject to gravity.

B is right. Because the fault is that it is acted on by the force, but as long as it is not greater than the static friction force, it will not move. c The mistake is that the three elements include the direction of the force, while the equilibrium force is in the opposite direction. The mistake is that you don't necessarily receive only one set of equilibrium forces. Thank you...

Inertia has been seen as a force in the BAB.

cTwo stressed objects.

dWhenever an object is subjected to gravity.

Ba: If the force f friction force f may be stationary.

C: Didn't say "same object".

d: Definitely no longer do uniform linear motion, uniform linear motion objects are only subject to the equilibrium force, lose one of the forces will not be affected by the equilibrium force, no longer do uniform linear motion.

Supplementary question: AB: It can only be said that the student has inertia, and there is no such thing as "being affected by inertia". It's not right c:

Question 1 choose b, explain: a not necessarily if the force is less than the frictional force the object is not moving, c the balance force three should be 1 the force is opposite in direction and the same magnitude acts on the same object. d may also move in a straight line, but it won't be at a constant speed!

Question 2 Option B Explanation A is not on the same object when it is subjected to gravity, air resistance, and C forces at the same time, D then he can't fly!! Hehe, simple answer, I hope it helps you!!

b Well, that's the concept of equilibrium.

The first one is b

The second is c

Solution: (1) The volume of a full tank of water:

v = 200l = 200 10-3m3, the mass of a full tank of water:

m= water v=, the temperature of a full tank of water rises from 15 to 65, and the heat absorbed is q suction = c water m t =, when 10 water heaters are filled with water, the heat absorbed by the water:

q=；(2) According to the question, the solar energy obtained by 10 water heaters is equivalent to the heat released by the complete combustion of natural gas with mass m, that is: q discharge = mq = q = q=, m = q release q=

Answer: (1) When 10 water heaters are filled with water, the temperature rises from 15 to 65 to absorb heat;

2) If these calories are provided by completely burned natural gas, 5kg of natural gas is required

(1) m = v = 1000kg m cubic x 200 1000m cubic = 200kg

Q suction = cm (t-to) = 4200x10x200 (65-15) = 420000000j

2) Because Q inhalation = Qm gas.

Therefore, m gas = q inhalation q420000000 84000000 = 5kg

The blacksmith was seen forging the iron for the last time while the blow was heard, followed by two blows.

It can be seen that the last blow was transmitted to this person 2 seconds later.

Therefore: s=vt=340*2=680m.

The answer came out at a glance, because I heard it twice, and the last time it was sent out was the last time I saw it.

Once per second, twice is 2 seconds, so 2*340=680

The sound of the last wrought iron, (and then two more blows were heard) was the second of those two blows, one every second. So 2 seconds elapsed from the time the sound was made to the time it was heard, and the distance was: 340m s times 2m = 680m

That's what I did in the first question.

Once per second, twice is 2 seconds, so 2*340=680

1 has current, no voltage, and the rest has no current and voltage.

2. There is no current, there is voltage, and the rest has current and voltage.

3 There is current, no voltage, and the rest has current and voltage.

4. No current, no voltage, and the rest have no current voltage.

1. There is current, there is voltage (power supply voltage), no current voltage 2, no current, no voltage, there is current voltage.

3. There is current, no voltage, there is current and voltage.

4. None at all.

1 The resistor of the short circuit has current passing through, there is no voltage at both ends, and the other resistor has no current2 The open circuit resistor has no current passing through, there is no voltage at both ends, and the other resistor is normal3 The resistor of the short circuit has current, no voltage, and the other is normal4 The resistor of the short circuit has no current, there is no voltage at both ends, and the other resistor has no current and voltage.

1 Start 15 minutes.

s=260m,t=15min

Speed v=s t=260m 15min=

2. 25 minutes in the middle.

s=440m,t=25min

Speed v=s t=440m 25min=

3. The whole process. Finally, the average velocity in 20 minutes is, t=20min, v=s=vt=15*20=300m

v=s/t v1=260m/900s=v2=s/t=440/1500s=

s3=v3t3= 900s=225m

v=s/t=(260m+440m+225m)/(900s+1500s+9000m)=

A: The average speed of his first 15 minutes, the average speed of the middle 25 minutes, and the average speed of the whole course is.

28 degrees. A 100-40 = 60

B 100-50=50

If the ratio of heat emitted by two copper blocks is 6:5, then the ratio of heating the liquid is also 6:5, B warms the liquid by 10 degrees, then A warms the liquid by 12 degrees.

So before the liquid was 40-12 = 28 degrees.

Boiling in boiling water for a considerable period of time means that the temperature of copper reaches 100 degrees Celsius, let the specific heat of copper be C1, the specific heat of liquid C2, and the initial temperature t

c1 × 100-40）= c2 × 40-t）c1 × 100-50）= c2 × 50-40）t=28