Two simple junior high school physics problems, help me solve 2 junior high school physics problems?

1 When the electric fan turns, push the air downward to generate wind, according to the effect of the force is mutual, the air will also react an upward force, acting on the fan, so that the pull force of the boom on the ceiling will be reduced, so F1 is greater than F2

2 Since all objects have inertia, they will remain in their original state of motion, and when the thin line is suddenly broken, the stone will continue to maintain an upward motion due to inertia, but gravity will change this state of motion. Causes the stone to slowly decrease its upward velocity and first slow down and rise, and then start to descend when the upward velocity decreases to 0.

1) After rotation, the fan has a downward force on the wind, and the wind has an upward thrust on the fan, so F1 is greater than F2. (2) The moment the thin line is broken, the stone has speed! The stone is only subjected to gravity (not taking into account air) in a straight downward direction, so it will decelerate evenly until the velocity becomes 0.

Then do a free-fall movement, i.e., a uniform acceleration descent. My shallow opinion, I hope it helps you.

1. When stationary, the fan is in a state of equilibrium, at this time f1=g; According to Newton's third law, when the fan rotates, there is a downward thrust on the air, so the fan will receive an upward thrust, therefore, f2 g f1

2. The rope is suddenly broken, and the stone will continue to move upward due to inertia, why is it to decelerate and rise and then accelerate down, because because the inertia is in the process of rising, the stone is only affected by gravity in the vertical direction, and the direction of gravity is opposite to the direction of movement, so it is decelerated and rising, and after rising to the highest point, the speed of the stone is 0 at this time, and it will fall because it is still subject to gravity, and the direction of gravity is the same as the direction of motion, so it is an accelerated decline.

Hope you're satisfied!

1。The rotation of the electric fan produces a downward thrust on the air, and the action of the force is mutual, so the electric fan is subjected to the upward force of the air, so F1 is greater than F2.

2.Considering the inertia of the object, the moment the rope is broken, the stone has a velocity, it will move upward under the action of gravity, and the direction of velocity is opposite to the direction of acceleration, so it moves in a uniform deceleration until the velocity is reduced to zero, and then the stone is subjected to the action of gravity and makes a uniform acceleration of the free fall motion downward.

Question 1: (1) Because R1R2 is connected in parallel, U1=U2=U=24V, according to I=U R, R2=U I2=24V Ω.

You can find R1 current, add R2 current = total current, total voltage Total current = total resistance, re-derivation formula R2, the process is not written.

2 kick: No, i=u r is Ohm's law, and the definition of the derivation formula r=u i should also be based on Ohm's law: when the voltage is constant, the current is inversely proportional to the resistance; When the resistance is constant, the current is proportional to the voltage.

If you have to say that, it is also when the current is constant, and the resistance is proportional to the voltage; When the voltage is constant, the resistance is inversely proportional to the current.

1).r2=u2 i2=24v beaten. 2).

i1 = u1 r1 = 24v 80 ohms = and = i1 + i2 = . r and = u i and = 24 euros. r2=r1r-(r1-r)=80x48 (80-48)=120 ohms.

2.Wrong. Without combining the laws of physics, deformation in a purely mathematical sense has no physical significance.

Resistance is an inherent property of a conductor, which does not change due to external voltage and current, and cannot be said to be proportional and inverse. Resistance is a constant and does not change with variables. Experiments show that the voltage is n times of the original, the current is also n times of the original, and the ratio is still r.

The circumference of the circle has become n times the original, and the radius is n times the original, has it changed?

Dumbo's good sound is transmitted by the air, and closing the doors and windows isolates most of the c

1.Xiaofei's method is good, because in this way, everyone can take what they need, and it is comfortable.

Wrong, because who is the electronic organ imitating? C: That's right.

1.In order not to affect the hobbies of others and not to torture himself, Xiaofei's method is preferable, although it cannot be isolated, it can effectively reduce the intensity of the sound.

It all makes sense that the first is attenuated at the beginning of the sound source, the second is attenuated during propagation, and the third is attenuated at the reception cEcho is a phenomenon of sound reflection.

1.Used together. Xiaoying's method is to prevent dry sounds, Xiaofei's method is to block the spread of dry sounds, and Xiaohai's method is to prevent dry sounds from entering his ears. So it's better to use them together.

1, Xiaofei is right, because air cannot travel in a vacuum2,c

1.Xiaoying's method is good, directly from the distance to reduce the noise.

1.The amplitude of the first toggle of the saw blade is large, and the second time is small, i.e., the greater the amplitude of the mechanical wave, the greater the intensity of the wave.

2.(1) Vacuum, e.g. on the moon (2) Sound source control, e.g. barrel muffler (3) Acceptance control, e.g. earbuds.

1. Appearance: Echolocation.

2 uh. I don't know. I also have two drops at the beginning of the prime.

1.It shows that the loudness of the sound is related to the amplitude, and the greater the amplitude, the louder the louder!

2.It can be loud enough to cause human hearing!

It can also be a problem with the hearing ability of the human ear, such as deafness.

It can also be the lack of propagation medium, and the vacuum cannot transmit sound!

1) Obtained by =m v: A: B = 1/2: 2/5 = 5:4

2) Obtained by =m v: m is inversely proportional to v when m is certain, and saline water alcohol , so v saline v water v alcohol.

So the highest alcohol.

1. The printing plant generally adopts the electrostatic copying technology of the brothers, in this process, the paper carries a small amount of electric charge, due to the interaction force between the charged bodies (the same charge dust shirt repels each other, and the different charges attract each other), resulting in the quiet cavity paper sticking together.

2. In winter, the temperature of the iron rod is lower than 0, and when the water on the hand touches the iron rod, it quickly solidifies and freezes, so the hand and the iron rod are glued together.

1. Static electricity: Because if it is an electrostatic copier, it uses the principle of static electricity to absorb electric charge, and 2 I have not experienced this.

Because iron is a good conductor of heat, the water on your hands will quickly cool down and solidify when you touch the iron rod with wet hands in winter, causing your hands to become sticky.