A physics problem in the third year of junior high school is helped

The definition of mechanical efficiency is the ratio of the useful work done by the machinery to the total work, and the useful work accounts for 80% of the total work by the title, so the extra work accounts for 1-80% = 20%, and the total work w = 40 20% = 200J

So w useful = w total -w extra = 160j

It is also possible to multiply the total work by the mechanical efficiency, ie.

200*80%=160j)

The extra power is 1-80%=20%;

Then the total common is 4 20%=200j;

The useful work is 200-40=160j;

The extra work is the part of that 20%, so the total work is 40 20%=200j, useful work 200*80%=160j

If the mechanical efficiency is 80%, then 80% of the total work is useful work. The remaining 20% is extra work. So find the total work w=40 20%=200j. The useful work is used to subtract the additional work from the total work, w (useful) = 200-40 = 160j

Mechanical efficiency = useful work Total work, the extra work is 40J, then the total work is 200J

w=w extra (1-80%).

w=200j

w useful = w 80% know.

w useful = 160j

There are two equations that can be listed in this problem

w total -w useful = 40j

w useful w total = 80%.

Can be solved to get:

W total = 200j

w useful = 160j

Divided into three groups. Three in each group.

1.Take out two sets of scales.

The two groups are the same weight, and the Shen Xun ball is wide in the other group.

If the two groups are not the same weight, they are in the lighter group.

The lighter group was chosen).

2.Take out the two small balls in the lighter group and weigh them.

The same weight is another small ball.

is not the same weight, it can be seen that it is the one that is light on the scale.

Pick out the lighter balls).

Divided into pieces. First, take two groups with balance scales, if equal, the other group of sails in the light one.

If it's not equal, it's a good choice.

Brother Erxunque, take the group that must have a light ball, and repeat it again according to the method just now, and you can find the light ball. acres raided.

This device is called a floating sinker.

There is an opening at the lower end of the floater, through which water can enter and exit the bottle. When water flows into the bottle, the gravity of the floater (the total weight of the bottle and the water in the bottle) increases; As the water flows out of the bottle, the gravity of the floater decreases. When the bottle is squeezed by hand, the volume of air enclosed in the upper part of the caraval decreases, and the pressure increases at the same temperature (Boehr's law). This increased pressure is transferred to the water, which is pressed into the vial, and the air in the vial is compressed (Pascal's principle).

After entering some water in the floater, the gravity of the floater increases, and when the gravity of the floater is greater than the buoyancy it receives, by.

The unbalanced force will change the motion state of the object, and the floater will sink downward.

When there is no extrusion at the beginning, the bottom of the small glass bottle is subject to the air pressure of mineral water and the pressure of the air inside the small glass bottle below, which is in equilibrium at this time, when the mineral water bottle is squeezed by hand, the pressure above becomes larger, and the pressure on it also becomes larger, and at this time, the pressure below does not change, and the force on the top is greater than that below, so it will sink, and when the force on the top and bottom is the same, it will be in equilibrium again. When the hand is released, just the opposite, it will go up.

When squeezing by hand reaches a certain level, the volume of gas in the bottle decreases, the pressure increases, and the small glass bottle containing oral liquid is pressed into the water, and when the hand is released, the small glass bottle returns to its original state and begins to float.

When the hand squeezes the bottle, the volume of the gas decreases, the pressure increases, and the pressure on the oral liquid bottle is greater than the buoyancy of the water, so it sinks, and on the contrary, the oral liquid bottle floats up again because the buoyancy is greater than the pressure.