I am anxious to solve this 8th grade physics problem

The buoyancy is directly proportional to the displacement volume, and if all the wooden blocks are to enter the water, the new drainage volume is 1 (1-2 5) = 5 3 times of the original, so the gravity of the object with a new total downward pressure of 60 * 5 3 = 100 g is 40 g of iron

If the iron is hung underneath the block, the block will not be able to enter the water in its entirety because the block also occupies the volume of drainage, forming a new buoyant.

It is explained that the volume of the wooden block immersed in water is three-fifths of the volume, and the volume of the water discharged is also three-fifths of the wooden block, and the gravity of this part of the water is, to be immersed in the water, the volume of the water discharged should be increased by two-fifths, and the volume of the two-fifths of the row should be the gravity of the iron block, so the iron block that needs to be placed on the wooden block

1.Let the two mirrors be 1 and 2, then you draw a good diagram first, the angle between the reflected ray 1 and the reflected ray 2 = the angle of reflection 2 - the angle between the reflected ray 1 and the mirror 2, note that the angle of incidence is equal to the angle of reflection, so the angle of incidence 1 = angle of reflection 1 = 30, the angle of incidence 2 = angle 2 = 45, the angle between mirror 1 and mirror 2 is 15, so the angle between reflected ray 1 and mirror 2 = angle 1 - angle between mirror 1 and mirror 2 = 15, so 45-15 = 30 is sought.

2.You draw the four lines of incoming rays, reflective lines, plane mirrors, and horizontal lines, and the inscription shows that the horizontal line and the reflected line are perpendicular, and the incoming rays and the horizontal line are 30, because it is impossible for sunlight to come under the horizontal lineTherefore, the entrance ray is above the horizontal line and becomes an angle of 30, then the incident angle bridge first accompaniment = reflection angle = x, then x+x+90+30=180 (the four angles together are a flat angle), x=30, and the angle between the plane mirror and the horizontal line is x+30=60.

3 Both of your teachers mean correctly, that is, the drip time interval is equal, so the first drop, the second drop, the second drop, the third drop, the third drop, the fourth drop... The interval between them is equal, then you only need to observe the distance between the ink, because the distance of the ink is actually the moving distance of the trolley, because the ink distance is very small (the dripping time is very fast), and the trolley moving distance is also very small (in a very short drip time), so it can be considered that the trolley is doing uniform motion in each small distance, but the uniform velocity of this small segment and the next small segment is not necessarily the same, if the distance is equal, because each small segment does uniform motion, v=s t, Min stupid so each section s s are the same, t is also the same, so v is also the same, which shows that not only each small section is at a constant speed, but also between the small segments, so the whole process is also uniform, so as to judge that the car moves in a straight line at a uniform speed. This is actually a micro-element idea, so the two of you teachers roughly mean this, which is more appropriate than the latter.

4.This place you have to understand the definition of proportionality and inverse proportionality, the ratio of two positive numbers is a normal number, one increases and the other increases, this is proportional, the product of two positive numbers is a normal number, one increases and the other decreases, this is inverse proportional, here is a uniform linear motion, only v can be regarded as a constant, and for s and t are both variables, the ratio of s to v is t, t is a variable, not proportional, the product of v and t is s, s is a variable, and it is not inversely proportional, and the uniform motion means that v is unchanged, It is only related to the object itself, and has nothing to do with s and t, and of course there is no positive and inverse proportional relationship, so both statements are wrong, and it can only be said that s is proportional to t.

The first 16 seconds is the echo of the formation of the cloud, the time multiplied by the speed, is in the air, 340*16 2 is the distance from the cloud to the ground (except 2 you understand), which is equal to 2720m.

The thickness of the cloud, the time difference is two seconds, which means that the sound has walked in the cloud for two seconds, and the speed of the sound in the cloud is 1000m per second, so 1000*2 gets the sound to go 2000m in the cloud, because the sound is transmitted from below and then back, so it is also a round trip from bottom to top in the cloud, so it is also divided by 2 to get the thickness of the cloud is 1000m.

Line A: Decelerate first, then accelerate (or understood as, kinetic energy is converted into potential energy, and then into kinetic energy, the kinetic energy is the same at the beginning and end, the average kinetic energy is less than the kinetic energy at the beginning and the end position, and the average velocity is less than the velocity at the beginning and the end), the average speed is small, and the time is more.

Line B: first accelerate, then decelerate (or understood as, potential energy is converted into kinetic energy, and then into potential energy, the average kinetic energy is greater than the kinetic energy of the beginning and end position, and the average velocity is greater than the speed of the beginning and end position), the average speed is large, and the time is small.

Obviously, A moves upwards first, and then moves downward after reaching the apex, that is to say, decelerates first, then accelerates, and the velocity of points A and C is the same, that is to say, in the whole process, the velocity is less than the initial velocity, so the average velocity is naturally less than the initial velocity;

On the other hand, B accelerates first and then decelerates, and the velocity of the whole process is greater than the initial velocity, so the average velocity is greater than the initial velocity.

Time is naturally tA》 tB.

Sure, one increases first and then decreases, one decreases and then increases, and the average value of the former is large, and the displacement is the same, and B takes less time.

Analysis: by velocity formula v=st

It can be seen that since the motion distance of two objects is the same, to compare the motion time of the object, as long as the relationship between the motion speed of the two objects is compared, the relationship of time can be compared, that is, the distance is fixed, and time is inversely proportional to the velocity

Solution: In the process of moving to the highest point, the kinetic energy should be converted into gravitational potential energy, and the velocity of the motion is less than the initial velocity; In the process of falling, regardless of the friction of the slideway, the mechanical energy is conserved, and the gravitational potential energy is converted into kinetic energy, and the velocity is equal to the initial velocity;

In the process of moving to the lowest point, the gravitational potential energy should be converted into kinetic energy, and the motion speed is greater than the initial velocity. In the process of falling, regardless of the friction of the slideway, the mechanical energy is conserved, and the kinetic energy is all converted into gravitational potential energy, and the velocity is equal to the initial velocity;

Therefore, the average velocity of the moving object in the convex slideway is less than the velocity of the moving object in the concave slideway, and it can be seen from t=s v that the time t A takes to pass through the A slideway is greater than the time t B to pass through the B slideway, so t A >> t B.

This is the elementary school math bar.

It's a total divided by speed, and if you're in a good mood, the second row is converted into adulthood.

According to the real situation, at normal atmospheric pressure, such an assumption is impossible to realize, let alone calculate. The people who can come up with this kind of brain-dead problem must be from ancient times!

The first thing to know is the formula fkx

kelastic coefficient

x amount of spring change )

Then column the first equation

K multiplication. Because there is no such symbol

So I barely used the text to see the =

And so it goes. k can be calculated without the = that can be reduced later

After all, I'm lazy)

Then list the second equation again:

Multiply. k into. x has three hooks, so it's multiplied by three

x is the amount of spring change after hanging).

So combined. k

Calculate it =. =

Finally. xcm) due to.

x is the amount of spring change.

So you have to add the original chief to get the answer. so

Length. cm) Finally, answer the question.

The length of the spring dynamometer at this time is. 16cm。That's it

This question is selected from the 2010 Shanghai 24th Junior High School Physics Competition semi-final paper (Datong Middle School Cup) In the resistance network shown in Figure (a), the resistance value of each resistance is r A student uses a device shown in Figure (b) to measure the current between any two nodes in the network If the voltage of the power supply is always e, then the maximum current is.

9e4r9e4r

The minimum current is.

9e10r9e10r

For details, please refer to Jingyou.com.

r；rac=109r；rae=56r；rbf=49r；

It can be seen that the minimum equivalent resistance of the resistor network is rmin=rbf=49r;

Circuit maximum current i max = er min = e4r9 = 9e4r;

The maximum resistance of the resistor network r max=rac=109r;

Circuit minimum current i min = er max = e10r9 = 9e10r;

So the answer is: 9e4r; 9e10r

The speed of sound v=340m s was already transmitted by the time he heard it.

5100/340s=15s

So s=250*15=3750 meters.