Why is the dynamometer pulled at a constant speed when measuring the mechanical efficiency of a pull

The effect of an object's gravity being changed by a block of pulleys"I think the meaning of this sentence refers to the amount of useful work done when using a pulley set. Useful work is the work that is useful to people, that is, the work done to achieve the goal directly with the hands without using machinery. When using a pulley block to lift a heavy object, the useful work is equal to gh, and it should be said that in the case of lifting the same object to the same height, the magnitude of the useful work is unchanged.

Therefore, this statement is false.

The main reason for pulling the dynamometer at a uniform speed when measuring the mechanical efficiency of the pulley block is: only when pulling at a uniform speed, the size of the tensile force is constant, and the indication of the spring dynamometer is unchanged, so that the magnitude of the tensile force can be quickly and accurately measured.

Supplements: 1) The purpose for which people set the concept of mechanical efficiency:

A physical quantity that describes the performance of machinery, that is, the percentage of the work that is useful (required to achieve the purpose) when used to the total work done by the machine. The greater the efficiency of the machine, the less extra work is required to be used by the machine. Anyone who wants to use machinery with high mechanical efficiency when using it.

This is also a reference quantity for people to buy machinery.

2) In the measurement, if the acceleration movement is done, the total work required by the tensile force will inevitably increase, and in the case of lifting the same height, the useful work will remain unchanged, and the mechanical efficiency will become smaller, so the mechanical efficiency measured by uniform speed pulling is the largest.

When the hook rises at a constant speed, the spring dynamometer is equal to the tension of the rope (due to friction when pulling, the tension at rest reading is less than the actual tension).

If you're in high school, you know.

f=ma There is an acceleration when you are not at a constant velocity.

At this time, it is necessary to look at it in the non-inertial frame, there will be an "inertial force", which is "equivalent to" doing more work, thus affecting the effect.

In the third year of junior high school, I also learned Newton's first theorem, only when the resultant external force is 0, the original state is maintained, which you can understand as when measuring the state, the resultant external force should be 0.

I don't know if this is good or not.

Pulling at a constant speed is for the convenience of reading, and if it is not at a constant speed, it is not easy to read.

Tension and (the gravity of the object and the gravity of the moving pulley and the gravity of the rope) are also tension and gravity. The object is not rubbed when it is at rest in a duel. For example, people stand on the ground without friction. When there is a trend of movement or is in motion, it is subject to friction resistance.

When pulling at a constant speed, the velocity remains the same, and the increased or decreased kinetic energy does not need to be included in the calculation of mechanical energy, and the initial velocity and final velocity of the rising object must be measured when accelerating or decelerating, which is very troublesome, so the constant velocity; One of the two forces in the two-force balance is the pulling force, and the other is the resultant force of the gravitational force of the lifted object and the frictional force of the pulley block.

To measure the mechanical efficiency of the pulley, why pull the rope at a constant speed? - Keep the system in equilibrium so that the readings are accurate.

What happens if the velocity is not uniform? - The dynamometer is constantly changing, and the specific value cannot be read.

But increase the speed at which the rope is pulled, and the mechanical power remains the same. - Error, the mechanical work remains the same, the mechanical efficiency remains the same, and the mechanical power increases.

Why should the spring dynamometer be pulled upwards at a constant speed to measure the mechanical efficiency of the pulley block? What does acceleration or deceleration cause?

Answer: The spring dynamometer for measuring the mechanical efficiency of the pulley group should be pulled upwards at a uniform speed.

1) It's to make the problem more simplified and simpler. It is convenient to provide a theoretical basis for the transformation of pulley blocks.

2) However, if you really want to accelerate or decelerate, the pulling force will change due to different accelerations, and there are many kinds of values for total work.

There is only one useful function, and the mechanical efficiency will vary, so it cannot be determined, which complicates the research problem.

4) Physics takes uniform motion as the ideal model, and with uniform velocity as the benchmark, the problem is simplified and simple.

1.It is necessary to maintain a uniform motion at the end of the rope when lifting, and if it is not moving at a uniform speed, acceleration (acceleration or deceleration) will be generated, so that the force acting on the object will also change with the balance time and gravity, and the tension force will be greater than the gravity force or less than the gravity force, resulting in the wrong result of the analysis.

2.If the tension does not reflect gravity, then there will inevitably be inaccuracies in the calculation efficiency.

Hope it helps;

Constant speed: the spring dynamometer is good reading, vertical and easy to measure the distance, if it is not vertical, the oblique pull will make the tension arm of the tension force smaller, the force will become larger, and the mechanical efficiency of the entire pulley group will be lower.

There is a w by the formula =w

Total work = GHFS analysis, it can be seen that 1) (2) to measure the weight g and the tensile force f, a spring dynamometer is required; A scale is required to measure the height of the object raised and the distance the tension end moves;

3) If the object rises to the height h, the distance s that the tensile force moves, the magnitude of the tensile force f, and the gravitational force g of the object;

then the useful work w useful = gh, the total work w total work = fs, the mechanical efficiency of the pulley block = w useful w total work = ghfs;

4) As we know from the figure, n=4, then s=4h=4 10cm=40cm=,w useful=50n,w total work=fs=24n,mechanical efficiency of pulley block:

w has a total work of w.

Answer: (1) Spring dynamometer, scale;

2) the height of the object raised h, the distance of the tensile force moving s, the magnitude of the tensile force f, the gravitational force g of the object;

3) w has w

Total; ghfs（4）40；52%

However, if you really want to accelerate or decelerate the pull, the total work.

There are many kinds of values.

There is only one useful function.

Mechanical efficiency. It will not be different.

If it is the same, it will not be possible to determine the ballast, which will complicate the research problem, and the tensile force will change in a thousand ways due to different accelerations.

Pulley block. Mechanical efficiency.

Spring dynamometer.

Pull upwards at a constant speed.

But it doesn't correspond to reality, and what you call acceleration or deceleration is closer to reality.

Physics put. Constant motion.

As an ideal model? What does acceleration or deceleration cause?

Answer: The mechanical efficiency spring dynamometer of the first coarse wheel set should be pulled upwards at a uniform speed.

It's to make the problem more simplified and simpler. It is convenient to provide a theoretical basis for the transformation of pulley blocks.

1. Why the mechanical efficiency of the pulley block has nothing to do with the tensile force - when lifting the heavy object g, w has w total g object (g object g moves). The "g" in the formula is the weight of the moving pulley. As you can see from the formula, the mechanical efficiency of the block of pulleys is independent of the pulling force.

2. Why should the dynamometer be pulled at a uniform speed when measuring - because only by pulling the dynamometer at a uniform speed, the moving pulley and the object will move together in a uniform linear motion, and only in this case, the force on the moving pulley will be balanced, so that the total pulling force of the rope is exactly equal to the total weight of the moving pulley and the object.

3. What will be the consequences if the velocity is uneven - if the dynamometer is pulled at an accelerated rate, the measured value is too large; If you slow down and pull the dynamometer, the measured value is on the small side.

1. This is to figure out the definition of mechanical efficiency, which is the ratio of useful work to total work, and the size of the pulling force is determined by the weight, otherwise the pulling force is large, which can only increase the speed of lifting, and has nothing to do with the mechanical efficiency itself. Be sure to get the definition clear.

2. If the tensile force is not uniform, it will cause the indication of the dynamometer to be large and small, and it cannot be read. Too fast, and the indicator will be larger than the reading when pulled at a constant speed. Therefore, it is necessary to emphasize the uniform speed of pulling.