Physics test questions are so difficult to kneel and beg for answers Come 20

1. Incorrect, when the object is at rest, the size of the resistance increases with the increase of the tensile force f, but when the object is in motion, the resistance is a certain value, and the sliding friction is only related to the mass and the coefficient of friction.

2. Design a group of experiments: (1) Use a spring to weigh the object horizontally pulled on a rough plane, and observe the change of the spring scale pointer: when the tensile force of the spring scale increases, the object has no movement, indicating that the object is in a state of force equilibrium, and the resistance on the level is equal to the tensile force.

So the resistance is changing; When the spring is used to pull the object horizontally to do a uniform motion, the force on the object is still balanced, and the resistance on the level is equal to the tensile force.

3. When the object moves before, the resistance is equal to the tensile force, and the tensile force increases. But the maximum friction of different objects is different, the maximum friction of rough surface objects is large, and the maximum friction of smooth surface objects is small.

Partially correct, the conjecture is right to have resistance, and the later conjecture is conditional.

It should be said that this is the case before the movement of the object.

2.According to Newton's first law, when the object is subjected to zero and external force or is not subjected to force, it always keeps the state of motion unchanged, then the object remains stationary under the action of tensile force, then the net force in the horizontal direction is zero, and it is easy to analyze that the horizontal direction is only subject to tension and resistance, so the tensile force is equal to the resistance, that is, the increase of the tensile force, the increase of the resistance.

3.The tensile force increases until it reaches its maximum when the object begins to move, consistent with Xiaoqiang's conjecture.

The rougher the plane, the greater the pulling force.

Well, the name is single-minded, hee-hee-hee-hee-hee-

Uncertainty is transmitted, and it hurts.

You can only choose C, and middle school physics is generally idealized analysis. Liquid pressure p= gh, during the rain, the water surface outside the car is constantly **, note that it is a heavy rain, the speed of the liquid level outside the car is higher than the speed of the liquid level in the car, due to the difference between internal and external pressure, water ingress, so a right; Of course, the rain process h is getting bigger and bigger, and the pressure naturally becomes larger, b is right; Option d you have solved it yourself; The key c, the process of rain retreat is relatively slow, and this process can be treated as dynamic equilibrium, that is, the height of the total water surface is decreasing, but the inner and outer liquid levels are connected and remain flat in this process of slow change. The difficulty of middle school physics lies in understanding the contradiction between idealized theories and real phenomena.

Hello, you are very attentive and love to use your brain, not bad!

There is nothing wrong with choosing D for your analysis of A and C, especially C, your teacher is not comprehensive enough, however, B is also right, from the point of view of the topic, the stressed object is actually a car, which should be fine.

For D, it should be said that the pressure on the car is towards the interior of the car, not in all directions, so D is wrong.

I also think that B is incorrect, C is just like you analyzed, A is also a strong external pressure, and water will enter the carriage. The stressed object of option b should obviously be the car, but what is the pressure of the car, if it is under the car, of course, it is getting bigger and bigger, because the depth of the water is larger, according to the liquid pressure formula, h increases, the pressure increases, but if it is the roof, there is no water on the roof, and the pressure does not increase or decrease.

The bulb is connected in series with the fixed value resistance r, the sliding rheostat is adjusted to make the voltage representation number at both ends of the bulb in parallel when the number is U, then the voltage at both ends of the fixed value resistance is measured with a voltmeter, the current is calculated according to Ohm's law, that is, the rated current of the small bulb is measured, the rated power of the small bulb is measured, the experimental circuit is shown in the figure to calculate the current in the circuit i=u r, and then the rated power of the bulb is calculated p=ui=u u r

1 All 1, when swinging, the mechanical energy is conserved:

v=period: t=2 root number l g root number.

2. Conservation of mechanical energy:

After 1s, the stone falls h=

ek v=3, friction is also 5n, f=ma

a=f/m＝5/2=

vt2-vo2=2as

s=5m

1、e=1/2(mv^2)

Speed: v= (2e m).

Period: t = 2 l g

2. E initial = 1 2*m*v1 2, m=, v1=17 m s;

Vertical direction: h=1 2 g t 2;

E end = e beginning + mGH;

v2= (2e end m);

vvertical=gt;

v2= (v1 2+v vertical 2).

v2= m/s

3. F Mo = F Push = 5N;

a=f m m;

distance s=v 2 2a;

You're not going to be testing, are you?!

1 All (1) The tensile force is equal to the spring scale on the left side;

2) equals; 3) sliding friction is related to the pressure on the object; The higher the pressure, the greater the sliding friction.

4) Sliding friction is related to the roughness of the contact surface; The smoother the contact surface, the less sliding friction.

1) The less resistance;

2) will maintain a uniform linear motion.

(1) Iron ring;

2) Reduce the time required to heat to boil;

3) If the air pressure is lower than 1 atm, the boiling point of water measured in the other groups will not all be 100;

4) Compare the temperature of the same glass of liquid with the thermometer of your own group and the thermometer of other groups at the same time