Put more than half a cup of rice in the glass, insert a chopstick in the middle, press the rice tightly to make the chopsticks stand upright, and then continue to add a little water into the glass. After a while, you can lift the glass with rice and talk about the truth

Put more than half a cup of rice in the glass, insert a chopstick in the middle, press the rice tightly to make the chopsticks stand upright, and then continue to add a little water into the glass. After a while, you can lift the glass with rice and talk about the truth

The rice has been pressed tightly, which means that the chopsticks have been in close contact with the rice. After adding water, the rice absorbs water, and the volume expands. The rice grains squeeze each other and bear force. The rice around the chopsticks has pressure on the chopsticks, so after a while, you can pick up the chopsticks and lift the rice glass

1. Use the repeater to record your reading voice. Listen to it. It doesn't sound like your own voice, but it doesn't sound much different from listening to yourself. Why?
2. A man is watching the carpenter on the ground from a high place with a telescope to drive a nail once a second. When he hears the sound, he just sees the action of hammering. When the carpenter stops hammering, he hears the sound of hammering twice. How far is the carpenter from him?

1. Because the repeater record is stereo, others listen to you speak is stereo, but you listen to yourself speak is not stereo, so you listen to the repeater recording is not like your own voice
2. The standard speed of sound in the air is 340 meters per second, so 340 times 2 equals 680 meters

The worker pushes the box onto the truck with an inclined plane. The box weighs 1600n. The worker uses 400N force and the inclined plane is 5m long
(1) Mechanical efficiency
(2) Friction

1)
Useful work w = GH = 1600n * 1m = 1600j
Total work Wo = FS = 400N * 5m = 2000j
Mechanical efficiency = w / Wo = 1600 / 2000 = 0.8 = 80%
2)
Additional work W '= wo-w = 2000-1600 = 400j
Because W '= FS
Therefore, the friction force F = w '/ S = 400j / 5m = 80N