Physics for junior high school, pulley problems, physics pulley practice questions for junior high s

Look at the number of strands of the moving pulley rope.

Apparently this one has 3 shares.

So the pulling force is (500+10) 6=85N.

6 6 = 1 meter.

Fainting, g matter = 500n f = (g matter + g motion) 3 = 510 3 h = 2m

If the direction of the pull is the same as the direction in which the object rises, the number of ropes is counted.

If the direction of the pull is inconsistent with the direction of the object's rise, then subtract the total number of strands of the rope by 1, don't look at the following.

It's a very simple question, and I loved it when I was in junior high school.

Look at the ** number is mainly to see that there are a few ropes on the moving pulley to pull it, like this question, it looks like three strands.

Analysis: The number of ropes that bear the moving pulley by the formula f = (weight of the moving pulley + weight of the object). As can be seen from the diagram, there are three ropes that bear the movable pulley, so f=(500+10) 3=170N.

The distance the weight moves = the distance traveled by the free end of the rope The number of ropes that bear the movable pulley. So 6 3 = 2m.

1.170n s=nh Analysis 1 (n is the number of strands h is the distance at the free end) Analysis 2 6=3 times h h=2 from s=nh to determine the number of strands, see how many ropes are connected to the movable pulley, there may be one on the axis of the movable pulley, and there must be two on the edge of each movable pulley. There is a maximum of 3 movable pulleys and a maximum of 5 movable pulleys for two movable pulleys.

f＝510/3 n

When calculating the pulling force, you can count the number of strands of rope that are "connected" between movement and stillness, noting that you can't count the strands at the free end, because this strand is not connected. This question is 3

Rose 6 3m

When calculating distances, the numbers are the same.

Answer: Solution: (1) From the question, regardless of the friction and rope weight between the pulley and the shaft, f = (g wheel + g object).

That is: 500N = (G wheel + 900N), the weight of the movable pulley:

g wheel = 100n;

2) The distance that the pull force moves:

s=2h=6m, tensile work:

W total = FS = 500N 6m = 3000J, tensile power power:

p= = =200w；

3) Useful work done by using pulley blocks:

W has = GH = 900N 3m = 2700J, the mechanical efficiency of the pulley block:

Answer: (1) The weight of the movable pulley is 100N;

2) The power of the pulling force is 200W;

2) The mechanical efficiency of the pulley block is 90%.

This question is about force analysis, and it is necessary to learn to apply force analysis and understand the general process of force analysis.

1. Analyze the motion state of the object: uniform linear line, rest (the force is balanced in these two states), and the motion state outside these two motion states (such as: linear motion, uniform motion, acceleration motion, variable speed motion, deceleration motion, etc., the force in these states is not necessarily balanced).

When the forces are balanced, it is enough to list the equation, force = force or force + force = force + force.

2. Conduct force analysis.

1》Gravity, in general, there is, unless the title indicates that it is not subject to gravity.

2》Analyze the force applied to the object.

In addition to the earth and magnets, to exert force on other objects, it must be contacted, so first analyze which objects are in contact with the stressed object; Secondly, the shape of these objects in contact with the stressed objects is analyzed.

Common force objects: support surface, rope (the rope is loose and has no force on the object, and it is possible to exert force if it is tight), spring (the original length, there is no force on the object, and the force will be applied to the object only if it is elongated or compressed), wooden sticks, etc.

3》List the relation.

3. According to the relational formula, combined with the problem, the relational formula to answer. (This step is more lively, and it requires that you must be familiar with the formula.) ）

Let's analyze this topic.

1. Status. You didn't make it clear, it should be a constant rise, or a standstill. So the force is balanced.

2. Force analysis.

Who is analyzed? Person.

1> man is subjected to the gravity of the earth on him straight downwards

2. > has a face that touches a person, and has a vertical upward force to the person; There is a rope that comes into contact with the person, and there is a vertical upward pull to the person.

3. Column relation.

g = f branch + f pull.

4. Combine with question answers.

Find f pull, because f pull and f branch are both produced under the action of a rope, and the force at both ends of a rope is equal, so f branch = f pull. i.e. g = f pull + f pull = 2f pull f pull = 1 2g

Add this group, and if you have any questions, you can discuss 61295786 here

The object and the person are regarded as a whole, and there are 2 ropes connected to it, which bear the gravitational force of the object and the person together, so the pulling force of the person is 1 2f.

Perform a force analysis on yourself, the downward gravity, and the upward are two ropes, one of which bears g 2

The person pulls the rope, and the rope pulls the person, so the object is subjected to two equal upward pulling forces.

The person is subjected to an upward pull f, and the person and the thing that supports it are borne by two strands of rope, so that the top of each strand is f=1 2g

It is enough to perform a force analysis on the person. In the vertical direction, the person is subjected only to the downward gravitational force g, the upward force to the support force of the person by the object on which the person is standing, f and the friction force of the opponent of the rope fAccording to the characteristics of the fixed pulley, f=f.

Since the whole device is in a state of force equilibrium, there is f f=g, f=fThe solution is f=g 2

Because the last free end of the rope is on the person's own body, there are two ends of the rope. So it's 1 2f

The force on the same rope is equal everywhere, and the force on different ropes is not necessarily equal.

In the first diagram, there is only one rope, which means that the tensile force of each rope is f. If the weight is in the air, then there is a G slip + G = 2F balance, if the figure is water, but also take into account the buoyancy of the heavy object, it should be G slip + G = 2F + F float to balance.

In the second figure, there are two ropes, the one on the right is fb, the one on the left is 2fb+g wheel, and the two ropes hang a weight together, so there is 3fb+g slip = g object.

Total gravity g = 200 + 600 + 40 = 840n

The movable pulley, the man, and the frame are considered as a whole, connected to three ropes, and each rope has equal pulling force. So gravity is borne by a total of 3 strands of rope. f=g 3=280nUnderstood?

Draw a line from the middle, and the fixed pulley is equivalent to three strands of rope to bear, then the calculation is:

200n+600n+40n)/3=280n

It must be three strands, with the middle of the two pulleys as the boundary, there are three sections on the upper pulley, so there are three forces in total. That's another way of saying it in your resource book. Force analysis, (200 + 40 + 600) 3, can be. Because the fixed pulley itself is balanced by force, it is not counted.

Because the rope also gives people a pulling force, which relieves the pressure on the floor.

Overcoming movable pulleys and objects to redo work accounts for 90

The extra work generated by overcoming friction and rope weight accounts for 1 9 total work of overcoming the work of moving pulley and object weight = work of overcoming movable pulley and object weight * 10 9, column formula: 50 g [(g+2)*10 9] 80, and the < of Ox = g< = 16 Ox.

Let the tensile force be f, and the gravity of the hanging object is g, =g 3f, and the total external power is 2n+3f 10, that is, 2n+3f 10+g=3f, and g is used to represent f, and =g 3f can be brought in according to 50 mechanical efficiency, and 80 columns of inequalities.

In fact, there is an easy way to do this, which is "odd and even". That is to say, if n is an odd number, start with the moving pulley, and if it is an even number, start with the fixed pulley. And the n 5 you gave is an odd number, directly from the moving pulley to start drawing, there are 5 lines in the picture to hang the moving pulley, so n 5, I hope it can help you, thank you!

Start connecting with the hook of the moving pulley. Connect 5 movable pulleys.

Because odd and even, and because 5 is an odd number, start drawing from the moving pulley.