Problems in junior high school physics and mechanics, junior high school physics problems

8. A person standing on the ground uses a pulley group composed of two fixed pulleys and two movable pulleys, and forcefully lifts the weight of 2 103 Ox on the ground to a constant speed of 4 meters high, regardless of friction, and the person does 1 104 joules. Seeking:

1) How much pull force does a person actually use?

2) If this pulley group is used to raise another weight to the same height at a constant speed, and the person does work 104 joules, find the mechanical efficiency at this time?

9. There is a volume of 3 meters, weighing 6860 ox of heavy objects accidentally fell into the water, now use the illustrated pulley group to salvage, and lift it up 2 meters in the water at a uniform speed. Seeking:

1) What is the buoyancy of the heavy object and the pulling force of the pulley hook a?

2) If the weight of each pulley is 120 N, not counting the rope weight and friction, f does more work.

10. On the dock shore, someone pulls the boat with a rope through a fixed pulley at a constant speed to make the ship dock, regardless of friction, when the rope is angled to the horizontal plane, what is the speed of the ship? The speed of the ship has become larger, or has it become smaller, 165 high.

11 Give you two pulleys, string, 11 plastic bags, and explain how to divide a pile of small parts into three equal parts. (Pulleys, strings, plastic bags are not counted, friction is not counted) old information.

Lever Exercise 1 A hard rod, under the action of force, if this hard rod can be called a lever; The five elements of leverage are:

2 The equilibrium condition for leverage is: Its mathematical expression is:

3. Pry a stone with a mass of 1t with a lever, if the power arm is 200cm, and the resistance arm is 20cm, then at least the power can be used to pry the stone.

4. The use of lever should be labor-saving, and the lever of the power arm and resistance arm should be used; If a strenuous lever is used, the power arm should be made resistance arm, with which the balance can be leveraged; No amount of leverage can be both

5 In the following examples, leverage is used to work ( ).

a Paddle with paddle in the lake water.

b Measure the gravity of an object with a spring dynamometer.

c Pump up your bike with a pump.

d A parachutist uses a parachute to drop from the sky.

6 The following are the following labor-saving levers ( ).

a Tweezers with an alcohol cotton ball.

b Boom on crane.

c A claw hammer with nails drawn.

d Shovels to feed coal into the boiler.

7 Two children sit on a seesaw and balance in a horizontal position, at this time ( ).

a The gravity of two children is equal.

b The distance between the two children and the fulcrum must be equal.

c The quality of the two children must be equal.

d The gravitational force of the two children is equal to the product of their respective distances to the fulcrum.

Lever Answer 1 Turn around a fixed fulcrum Power Resistance Power Arm Resistance Arm.

2 Power Power Arm Resistance Arm F1 L1 F2 L2

3．1000n

4 Greater than less than provincial distance Equal arm labor-saving distance.

5．a 6．c 7．d 8．b

Buy this practice book yourself, and train you to compete in a new way, which is good.

There are no difficult problems in junior high school physics.

1. The pressure formula knows p=f s (p is the pressure, f is the pressure, and s is the action area).

Since it is known that the maximum pressure that the ground on which the track is laid is to the power of 5 PA, and the pressure is.

The total weight of the ancient building together with the foundation, so the area of pressure of the track on the ground is at least.

s=fp=7th power n5th power p=20 (m-squared).

2. The formula by work w=f s (f is the force, s is the displacement in the direction of the force, and w is the work done by the force f).

The title already told Jack thrust f= 8th Newton and Displacements s=50 m

Therefore, the work done by the jack on the ancient building.

w = 8 to the power n 50m = 10 to the power j

3. The formula of power p=w t (p is the power, the silver dust t is the action time, and w is the work done in the time t).

As we know from the above, the work required to move the ancient building 50 meters w=10th power j

And the power of the motor to do work on the pulley block is 80kw, and the efficiency is 80%, then the real power used is.

p=80kw × 80% =64000w

So, time t=w p= to the 10th power j 64000w = to the 5th power s

Then, the speed of movement of the ancient building v=50m to the power of 5 s = 2 to the power of -4 m s of 10

The third question, my answer may be problematic, because the speed is too slow, I think it is unlikely, please forgive me if I am wrong!!

Suppose the sphere volumes are v1 and v2 respectively by the question in the vacuum.

f= k p1v1*p2v2

The key is the force of the liquid on the globules. Consider that at this time, the liquid has two pores, which can be completely replenished and completely homogeneous f total = 0, that is, f total = f porous case + f liquid pellet = 0

That is, the other liquids produce a gravitational balance that is equivalent to the repulsive force and the liquid ball

Thus we get f porous liquid = - f liquid pellet = - k p2v1* p2v2 = - p2*p2* f (p1*p1).

From this, the magnitude of the force exerted on one of the balls = f + f porous liquid = [1-(p2*p2) (p1*p1)] f

Dude, this is a junior high school question......Khan ......I'm good at physics, and I can't even understand the questions. Grab a sofa, and I'll learn it when I have the answer.

According to the conclusion that I have come to for many years, generally this kind of problem is equal, and if you are wrong, you will smoke me.

1.After the soldering iron is energized, is the internal energy that the temperature increases by work or heat transfer?

2.Can it increase the amount of heat absorbed?

3.After boiling two copper blocks of equal mass A and B in boiling water for a considerable time, first take out the copper block A and put it in some kind of liquid, and wait for the temperature of the liquid to rise to 40 after heat equilibrium; After taking out A, put the copper cube into this liquid and wait for the temperature of the liquid to rise to 50 (excluding heat loss) after heat equilibrium, what is the temperature of this liquid?

1. The current first does work through the heating element to increase its internal energy, and then the heating element transmits heat to the shell to increase the internal energy of the shell.

2. Not necessarily, work can be done on objects.

After the copper block is taken out from the boiling water, the temperature of the liquid and the temperature of A after immersing in the liquid equilibrium are 40, and the heat released by A is equal to the heat absorbed by the liquid, and the equation is (the copper block has the same mass, set to M copper).

c copper * m copper * (100-40) = c liquid * m liquid * (40-t) -

After putting B into the liquid equilibrium, the temperature of the liquid and B are both 50, and the heat released by B is equal to the heat absorbed by the liquid, and the equation is.

C copper * m copper * (100-50) = C liquid * M liquid * (50-40) ——

From , c copper * m copper = liquid * m liquid, substitute , eliminate c liquid * m liquid to obtain t = 28

Three identical thermometers, the first in a 25 degree Celsius room, the second in alcohol in a 25 degree Celsius room, and the third under a fan in a 25 degrees Celsius room. Q: Which thermometer has a lower indication?

Answer: The second thermometer is lower because the alcohol evaporates and absorbs its own heat.

Check the past year exam questions yourself.

When a ray of light is shot from air into the water, the angle of incidence is 40 degrees, on the surface of the water, one part of the light is reflected and the other part is refracted against, then what is the angle between the reflected light and the refracted light?

A: Between 100 degrees and 140 degrees.

1) Take C as the research object.

Kinetic energy theorem: mg2l = (1 2) mv0 2, solution v0 = 2 gl2) takes the system composed of b and c as the research object.

At the moment when the rope is straightened, the momentum of the system is conserved: mv0=(m+3m)v, and v=v0 4=(1 2) gl is obtained

3) After B glides on A and reaches the maximum displacement, the velocity of the system composed of A and BC is always equal, so the acceleration of the system composed of A and BC is equal, that is, (mg- 3mg) 4m = 3mg 6m, and the solution is =.

If B glides on A and L does not reach the maximum displacement, then B will slide off A.

The acceleration of a is a1 = 3 mg 6 m = 1 m s 2, the acceleration of the system composed of b and c is a2 = (kmg- 3 mg) (km + 3m), the time from the rope tension to the slip is t, the displacement of a is x1, and the displacement of bc is x2.

a1t=v+a2t

x1=(1/2)a1t^2

x2=vt+(1/2)a2t^2

x2-x1=l

The four-formula coupling gives k = 27 7.

The third question seems to be a bit problematic, so I'll change it. Don't be in a hurry.

Is this called a puzzle?

Don't you know the 3 major theorems of mechanics?

Acting force = Reaction force.

So the force experienced by both sides of the tug-of-war through the rope is equal.

There are wins and losses because of the different friction on their feet.

You go to a tug-of-war with Hercules and get him to wear a pair of roller skates on his feet, and you can still win him.

The tug-of-war is mainly to compare the frictional force experienced by each team, the pull of A to B and the pull of B to A are interactive forces, so the force of A and B is the same.

The action of force is reciprocal How much force A exerts on B, how much reaction force B exerts on A.

A will win because the friction between A and the ground is greater than B.

The friction is not only because the shoe is rough, but also because of his gravity and the direction of his force, and if the force is not in the right direction, even if the foot rubs the same friction, the center of gravity of the body will lean forward, and it will be pulled over.

It's all about arm strength.

First of all, A and B are equal by the pull of the rope, but the friction is not equal, whoever is subject to friction is large, who wins, so in the tug-of-war competition, it is generally necessary to choose a person with a larger weight to participate, in order to increase the pressure, but also to wear non-slip shoes, that is, to increase the roughness of the contact surface. You can try, if two people weigh about the same, sprinkle some sand under one person's feet, let two people tug-of-war, it will definitely be a loss if you step on sand under your feet.

The action and reaction forces are well explained.

A exerts force on B, and B is also subjected to the reaction force of A.