Assist in solving junior high school physics problems, and solve junior high school physics exercise

1.Why is the dynamometer not 0 when it is subjected to the balance force, but who told you that the dynamometer measures the force it receives, and it measures the tensile force on the hook.

If the elevator is straight up and down, Xiao Ming is only affected by two forces, gravity and support, and the balance depends on whether he rises at a uniform speed. b. If Xiao Ming is sitting on the staircase-type elevator, it will be subject to an additional static friction.

3.The student is the object of my research, I see the student as a whole, I see what force he is subjected to, and I don't care about the force he acts on something else, he is only affected by the gravity of the earth and the friction of the flagpole on him, and the uniform speed rises to push the two forces into a balance, equal in magnitude and in opposite directions.

4.There is no answer, the object of study is the fan, when it is stationary, it is subjected to a pulling force equal to the gravitational force (this is no problem) and only these two forces, but when the fan is rotated, in addition to the gravity and tension force, due to the downward blowing of the wind, the wind will give the fan an upward force fNow it is a state of equilibrium of three forces, i.e., f plus the pulling force equals the gravitational force.

5.Bottom line: The state of motion of an object reflects its state of motion, and the state of motion of an object determines its state of motion.

Question 1, Conclusion 5 is easy to understand, because the dynamometer is at rest, so the two forces are balanced. Conclusion 1, if we hang it on the wall, also measure 10n objects. The downward pull of the dynamometer hand is 10N (the indication is 10N), and the matter remains stationary, so the upward pull is ten Ox.

In the same way, the right pull force is 10N, the force gauge shows one side, and the left pull force is 10N.

Gravity Support Force No, because he stands still because it is constant velocity, so gravity is equal to friction, so the friction force is 500n direction downwards 5Yes, because of the balance of forces (which is the only explanation) Man stands on the ground (as long as it is some motionless object).

Explanation 1It's vague and can't say it by feeling and your own years of experience 2This is common sense and does not need to be explained in the textbook Just memorize it 3Explained 4Two-Force Balance The gravitational force is equal to the pulling force.

The first question is obviously to keep the two objects and the spring in balance. I choose C gravity directly, support power!

Friction = gravity = mg) 500n The direction is opposite to the direction of the force, and the friction force goes up along the bamboo pole.

t=g t'(Pull & Gravity) The book is placed on the table (Gravity & Support) ......The size is equal and the direction is opposite, acting on the same straight line!!

It is better at high temperatures, because the speed of sound in the air is affected by the temperature, the higher the temperature, the greater the speed of sound, so that the aircraft is less likely to approach the surrounding speed of sound when flying at a limited speed, thus reducing the air resistance.

The key to this question is a combination of "the relationship between the speed of sound and temperature in the air (textbook knowledge)" and "when the flight speed of an aircraft is close to the speed of sound around it, it will experience considerable resistance" (the information provided in the question).

Our teacher just talked about this two days ago.

Answer: In a high-temperature environment.

Because the higher the temperature, the faster the speed of sound.

Hypothesis: It's high temperature, and the speed of sound is 360ms. Then when the plane flies to 340m s, the plane achieves supersonic flight (the teacher said that supersonic flight is to reach 340m s, or more).

Because of the large difference with the speed of sound, the aircraft will not be subject to wear and tear. (The aircraft is dragged when approaching the speed of sound, which causes wear).

Let's assume again: if it's low temperature now, the speed of sound is 310ms. Then, when the plane flies to 310m s, there will be resistance, and it will no longer be able to continue flying. (It is impossible for an airplane to fly together to reach 340 m s, so it has to accelerate slowly.) ）

Therefore, in a high-temperature environment, the experiment is more likely to be successful. (It's easier to reach 340m s.) ）

High temperatures are easier, and the higher the temperature, the less dense the air is, and the less it is relatively thin, the less resistance it will be.

Because the flight speed of the aircraft is close to the surrounding sound speed, it will be subject to considerable resistance, in order to reduce the resistance, high temperature should be selected, but it should also be considered in combination with the actual situation and a variety of factors.

Microtherm. The speed of sound propagation is low at low temperatures, and for aircraft with limited flight speed, if you want to exceed the speed of sound, it is naturally easier to surpass it when the speed of sound is low.

It is easier to succeed at high temperatures; Because the speed of sound usually refers to the speed of sound at room temperature and pressure of 340 meters per second, supersonic flight as long as it exceeds this number, then it is necessary to start from reducing air resistance, and at high temperatures, the air is thin, low density, and low resistance, which is convenient for the aircraft to gradually accelerate, so it is easier to succeed at high temperatures.

It should be high temperature and limited flight speed requires as little resistance as possible High temperature, low air pressure, thin, and low resistance.

High temperature Because when it is hot, the air is thinner and has less resistance.

When the speed of the aircraft is close to the speed of sound around it, there will be considerable resistance, in the middle of the last century, people tried to fly at supersonic speeds, at that time when the speed of the aircraft was limited, was it more likely to succeed in hot or low weather? Why?

1(1) Because p=ui, i=p u=1(2) Because q=uit, so =q lamp q power=u lamp it u power supply it=u lamp u power supply=

2. First of all, we should look at the figure, this is an L1 and L2 in parallel, L1 emits light normally, then U1=U2=, L1=; The ammeter measures the trunk current, i=, then i2=i-i1=; Voltmeter measures L2 voltage (not used).

r2=u2/i2=

There is no diagram and no solution, at least the rated power and rated voltage of the lamp L1 should be said.

How are L1 and L2 connected?

i1=p1 u1=, the total voltage of the parallel circuit = the voltage at both ends of each branch; Trunk current = current of the sum of the currents of each branch l2: i2=i-i1=

r2=u i2=oh).

Normal luminescence, so the voltage and power are rated. Then i=p u= w(l)=p(l)t= w(total)=u(total)it=3v*0.2a*10s=6j power conversion rate=w(l) w(total)=5j 6j= ask for figure b, and then I'll help you answer.

Or you tell me how to connect the two lights.

1. Left 2, OA and OC

3、f1×l1＝f2×l2

4. It will not affect the balance of the lever, because the line of action of this force is through the fulcrum, and its force arm is 0.

Directly come up with the whole question, and you will be tired when you look at it.

Hehe, I'm sorry, I don't like such people the most.

From Newton's second law: f-mg=ma

Solved tensile force: f=mg+ma mg

Therefore, when the object rises, the force gauge will be greater than the gravitational force.

When moving at rest or at a constant speed, there are two forces in equilibrium: f=mg

So it can only be read if the object is at rest or moving in a straight line at a uniform speed.

This involves acceleration, and the so-called acceleration is the amount of velocity change in an object in a certain amount of time, and the amount of velocity change is divided by the amount of time change, and this is the calculation formula, and the definition is the resultant external force of the object, and the mass of the object, when the velocity or stationary is 0, so the reading is accurate. Why does the indicator become larger when an object accelerates upward? Because the acceleration is upward and greater than gravity, the spring dynamometer shows a resultant force rather than gravity, so it will be greater than the actual gravity (this phenomenon is a phenomenon of superweightlessness, which you will learn in high school physics).

f=mg+ma a is the acceleration, and only when a is 0 is the indication is mg

40 km/h means that you can travel 40 km/h for an hour at this speed.

And this lady's speed is 60 kilometers per hour, which means that if she drives at this speed for one hour, the distance will reach 60 kilometers, exceeding the speed limit of 40 kilometers per hour, which is the so-called.

Speeding. "That's it.

40 km/h and 60 km/h are calculated speeds, not 60 km/h for 7 minutes! 40 km/h.

And. 60 km.

Two quantities are different concepts.

D volume vm p

According to the title, the mass of ball A is 3 times that of ball B.

Whoever has the highest density will have a smaller volume.

But this is a solid ball made of two different materials.

So the density is not clear. It is impossible to judge.

The circuit of the choice d this question is a series first and then parallel mixed circuit, in order to make the resistance to the maximum, the resistance in the two branches should be relatively balanced, a total of six resistors R is first 3 R in series short circuit in each branch One R, that is, 2R and 3R parallel resistance is the largest.

rm=2r·3r (2r+3r)=6r 5 If you want to minimize the resistance, you need to make the resistance in the two branches unbalanced, first 1 R on one branch and 5 R on the other branch, and then short circuit 4 R on the 5 R branch, that is, 1 R parallel resistance of each branch is the smallest.

rs=r/2

So choose D

Students in physics and mathematics focus on two words abstract.

Just take the phenomenon that you see, get rid of some of the complex appearances, and abstract the simple parts to solve.

This problem can be abstracted as follows.

Any two short posts are equivalent to the two ends of a resistor rx.

At the moment, it seems that the resistance value of this resistor rx can be made up of 5r and 1r, 4r and 2r, 3r and 3r (because we don't include zero resistance), and then you can calculate it according to the parallel formula.

In addition, I attach you a little knowledge about paralleling, the total resistance of parallel connection is less than any resistor being paralleled, I hope it will help you summarize your knowledge.

First of all, let's clarify the meaning of this question.

It's a wire in the middle of a hexagon and you put it in the circuit.

You draw a hexagon.

Label a, b, c, d, e, f, g

You connect the AC.

Then put it in the circuit.

The positive electrode is connected to B, and the negative electrode is connected to C

This is the minimal.

The biggest one is the positive connection B negative connection E

That's the answer.