1. When a child with a mass of 30kg sits on a 10kg sled, the adult pulls the sled with a pulling force of 100N inclined upward at 37 ° to the horizontal direction to make the sled move at a uniform speed along the horizontal bottom, sin37 ° = 0.6, cos37 ° = 0.8. Find: (1) The pressure of the sled on the ground; (2) The dynamic friction coefficient between sled and horizontal bottom 2. Object a moves in a straight line at a constant speed, with a speed of 2m / s. After a starts for 4S, object B starts from the same place and starts to move in a straight line at a constant acceleration, with an acceleration of 2m / s ², And a and B move in the same direction. Find out (1) how long after B starts to catch up with a (2) the maximum distance before a and B meet 3. There is an object with a mass of 2kg on the horizontal bottom, and the dynamic friction coefficient between it and the bottom is 0.25. Under the action of thrust f inclined downward at 30 ° to the horizontal direction, as shown in the figure, the object makes uniform acceleration from standstill and moves forward 4m within 2S. What is the thrust f? 4. A small block with a mass of M = 2.0kg rushes up the inclined plane with an inclination of 37 ° at a certain initial speed. A student uses a sensor to measure the instantaneous speed of the small block at multiple times in the process of rushing up the inclined plane, and uses a computer to make the V-T diagram line of the sliding process of the small block, as shown in the figure (sin37 = 0.6 cos37 = 0.8 g = 10m / s) ²） Requirements: (1) The acceleration of small blocks when they rush up the inclined plane; (2) Dynamic friction coefficient between small block and inclined plane

1. When a child with a mass of 30kg sits on a 10kg sled, the adult pulls the sled with a pulling force of 100N inclined upward at 37 ° to the horizontal direction to make the sled move at a uniform speed along the horizontal bottom, sin37 ° = 0.6, cos37 ° = 0.8. Find: (1) The pressure of the sled on the ground; (2) The dynamic friction coefficient between sled and horizontal bottom 2. Object a moves in a straight line at a constant speed, with a speed of 2m / s. After a starts for 4S, object B starts from the same place and starts to move in a straight line at a constant acceleration, with an acceleration of 2m / s ², And a and B move in the same direction. Find out (1) how long after B starts to catch up with a (2) the maximum distance before a and B meet 3. There is an object with a mass of 2kg on the horizontal bottom, and the dynamic friction coefficient between it and the bottom is 0.25. Under the action of thrust f inclined downward at 30 ° to the horizontal direction, as shown in the figure, the object makes uniform acceleration from standstill and moves forward 4m within 2S. What is the thrust f? 4. A small block with a mass of M = 2.0kg rushes up the inclined plane with an inclination of 37 ° at a certain initial speed. A student uses a sensor to measure the instantaneous speed of the small block at multiple times in the process of rushing up the inclined plane, and uses a computer to make the V-T diagram line of the sliding process of the small block, as shown in the figure (sin37 = 0.6 cos37 = 0.8 g = 10m / s) ²） Requirements: (1) The acceleration of small blocks when they rush up the inclined plane; (2) Dynamic friction coefficient between small block and inclined plane

1. Calculate the upward force F = f pull * sin37 = 60N, pressure: F = Mg-F pull = 400n-60n = 340n, horizontal direction: fcos θ— F = 0 f friction= μ FN is obtained from the solution: μ= 4/17=0.24
2 use the same distance to list the equation, VT = 1 / 2at ^ 2m / s * (4 + T) = 1 \ 2 * 2m / S ^ * t connect t = 4S, the maximum distance between a and B before they meet, that is, when they have the same speed, that is, 2m / s * (4 + 1) - 1 / 2 * 2 * 1 = 9m. 3 according to moving forward for 4m in 2S, you can calculate the acceleration a = 2m / S ^, fcos30 MGU fsin30u = ma, which is a little big. Calculate it yourself, If you can't figure it out, you won't write the answer
4 calculate a = 8m / S / 1s = 8m / s according to V / T = a ^ according to the force analysis, it can be known that it is subject to friction and downward force mgcos37 * U + mgsin37 = 2 * 8, and calculate u = 0.25

Several questions······················· 1. The filament of an incandescent lamp usually operates at a temperature above 2000 ℃. According to the analysis of the melting point table, what material should be selected to make the filament? Why? 2. What factors do you think affect the speed of evaporation? Please design an experiment to explore one of the factors Requirements: specify the selected equipment, main steps, phenomena and conclusions 3. How to make wet clothes dry as soon as possible, write down the three measures you take 4. According to the Beijing Evening News, on May 20, 2001, it was cloudy in Lop Nur, Xinjiang. The temperature was about - 15 ℃. The sand dunes in the desert were covered with about 5cm ~ 10cm thick snow. However, it disappeared after about 20min, but the foot was dry sand. This phenomenon made the captain of the scientific investigation team tongue tied. Please use your knowledge to answer why the snow is missing?

1. The melting point of tungsten wire is higher than 3000
2. With the temperature, the area of the object, and the flow velocity around the object, the same wet clothes are placed at 20 degrees and 60 degrees to see which is faster
3. Raise the temperature, spread the clothes and put them in a ventilated place
4. Snow evaporates instantaneously, because only evaporation can be carried out at any temperature

(1) Use of spring dynamometer: ① see () and (); ② Before use (); ③ The direction of the measured force is consistent with that of (2) 9.8n/kg (2) Physical meaning of 9.8n/kg () (3) The center of gravity of an object with regular shape and uniform mass distribution is at its () (4) Methods to reduce friction: ① reduce (); ② Decrease (); ③ Replace () with () action; ④ Add () (5) The clockwork has () energy; The river water in the upper reaches of the Yangtze River dam has () energy compared with the river water in the lower reaches of the dam (6) When swimming, people will move forward when their hands and feet paddle backward. The force applying object that pushes people forward is (). This phenomenon shows that the function of force is ()

1. Range division value pull spring extension 2. Gravity of 1kg object 3 geometric center 4 plane smoothness pressure sliding rolling lubricating oil 5 do not know potential energy 6 water interaction

Throw an object horizontally. After throwing it for one second, the speed direction forms an angle of 45 ° with the horizontal direction, and the angle is 60 ° when landing. Find 1.2.3. Start throwing, and the height from the ground is 4. Horizontal range One person rode to the east at the speed of 4m / s and felt the wind blowing from the south. When he drove at the speed of 4 times root 3, he felt the wind coming out from the West and the actual wind speed and direction

The horizontal throwing motion makes a free fall in the vertical direction. After one second, the vertical speed is 10m / s. at this time, the horizontal speed is equal to the speed in the vertical direction, that is, the initial speed is 10m / s. when landing, the initial speed is 60 degrees. If the initial speed remains unchanged, it can be seen that the landing speed is 20m / s. from the second question, it can be seen that the speed in the vertical direction is 10 root sign 3m / s, and the height can be obtained from 2ah = V ^ for 3 seconds, The horizontal range X = V initial velocity * t can be found. The second question is that the vector sum of the wind's velocity to man and man's velocity is equal to the wind's velocity to the ground, which can form a closed triangle. Is it really blowing from the west?

When the water in the kettle is boiled, the "white gas" can be seen only at a certain height above the small hole in the lid, while the "white gas" can not be seen close to the small hole. Why?

When the water in the kettle is boiled, the water will vaporize, and the water will change from liquid to gaseous;
When water vapor is cold, it will liquefy;
Because the temperature of the air at a certain height above the small hole of the pot cover is significantly lower than that of the water vapor compared with the temperature at the pot mouth;
That is, the temperature near the small hole of the pot cover is high, and the water vapor cannot be liquefied;
The place higher from the hole has a lower temperature, and the water vapor liquefies to form small water droplets, that is, white gas

There is a flat car on a smooth horizontal plane. A man stands on the car and hits the left end of the car with a hammer. Can the car move continuously to the right under continuous beating? Please explain the reasons

Can't move, because force = reaction force, people push to the right, and the car pushes to the left, but people don't move, so people are subject to the right friction of the car, so the car is also subject to the left friction of people, so the force on the car is balanced at this time, so it doesn't move