A physics problem for the third grade of junior high school, please help

This is a matter of reference.

Xiao Ming felt that he was using the ground as a reference as he was advancing, and he was moving forward at this time.

He felt like he was using the train as a reference as he retreated.

Because the speed of the train is much faster than the speed at which he rides.

So when he compares to the train, he feels like he's going backwards.

This is a different reference material.

When selecting the ground as a reference, Tom is moving forward.

The train is chosen as the reference, and the person is moving backwards relative to the train

If he uses the earth as a reference, he is advancing.

If he uses the train as a reference, he is going backwards.

I'll explain it to you:

Because the train runs by, the air velocity is large, the pressure near the side of the train is small, and the outside atmospheric pressure is greater than the air pressure on the side of the train, so that with the object being pressed in from the outside to the inside, Xiao Ming feels that moving forward is because his car is affected by inertia and wants to move forward, and he feels that the backward is because of the air pressure difference, and the outside air pressure squeezes him towards the train. Do you understand?

When the car passes by Xiao Ming's side, the flow rate between Xiao Ming and the car is relatively large and the pressure is small, while the external flow speed between Xiao Ming and the car is small and the pressure is strong, so there is a pressure difference, because the pressure difference pushes Xiao Ming to the direction of the car, so Xiao Ming will feel forward. And because the car moves faster than Xiao Ming, using the car as a reference, Xiao Ming will feel that he is going backwards.

Because the frame of reference is different, he feels like he's moving forward when he chooses the little tree on the side of the road as a reference, and when he uses the train as a reference, because the train is faster than him, he feels backward.

The two pulley sets are equal, raising the same object to the same height at a constant speed In the figure on the left, only two sections of rope share the weight of the moving pulley and the object, so f1=(1 2) The weight of the pulley and the object.

In the figure on the right, there are three sections of rope that share the weight of the moving pulley and the object, so f2=(1 3) The weight of the pulley and the object.

f1 f2 = (1 2) weight of pulley and object (1 3) weight of pulley and object = (1 2) (1 3).

f1>f2

g=2f1=3f2

Figure 1 has two ropes on the moving pulley and Figure 2 has three.

This one. Start by numbering the ropes from left to right as 123

In the first diagram, 3 is the resultant force of 1 and 2.

In the second diagram, 1 is the resultant force of 2 and 3.

So choose A instead of C

Ha ha!

When using the pulley block, there are several sections of rope load-bearing, and the pulling force is a fraction of the weight of the object, and when the figure on the right is connected, it is 3 sections of rope load-bearing, so it is more labor-saving.

Because the second tanker group is less labor-intensive than the first and has the same mechanical efficiency, f1 > f2, 1 = 2,

(1) The electric power is p=ui=1 10 8 2 10 4 w=2 10 12 w, and the work done by the current is q=pt=2 10 12 j=2 10 9 j= kw·h

2) The amount of electricity that can be saved per day is q1 = ((60-40) 1000) 1000 12 kw·h = 240 kw·h

The amount of electricity that can be saved in a year is 240 365 = 87600 kWh.

3) The power of the rice cooker is 450W, and the heat generated in one minute is Q2 = 450 60 J = 27000 J

d Multiple experiments are more convincing and reduce the error It is better to do it alone than two people Choose a The thunder I just heard is basically propagating in a straight line a, we seem to have done it too.

1)p=ui=2*10^4*1*10^8

w=2*10^12

w=2*10^9

kw time t=

s=1/3600000

H so w=pt=2*10 9

kw*1/3600000

h=2) 60 watt lamp consumes electricity in one day w1 = (60 1000kw) * 12h * 1000 = 720kw * h

The power consumption of a 40-watt lamp in one day w2 = (40 1000kw) * 12h * 1000 = 480kw * h

Save electricity every day w=w1-w2=240kw*h, save 365w=87600kw*h every year

3）t=1min=60s

w=pt=450*60j=27000j

1.The volume of water v1 can be calculated by h1 and d

2.Turn the bottle upside down and calculate the volume v2 of the part of the bottle without water by H2 and D

Calculate the volume.

The first Zheng Yubu: Weigh the mass m of the gold foil divided by the density p, and find the volume of Zhaoxian v

Step 2: Use a scale to measure the length l and width w of the gold foil, and find the area s

Step 3: Thickness d = v s = m (p*l*w).

1.Weigh the mass m of the gold foil divided by the density p, and find the volume of burn v

2.The length l and width of the gold leaf are closed, and the area of the old section is s

3.Thickness d = v s = m (p*l*w.)