36. Xiao Ming, who weighs about 480 cattle, is doing push-ups. As shown in the figure, the solid point is the center of gravity. (1) Please calculate the force on your hands. (2) If Xiaoming's knees hit the ground while doing push-ups, what changes will be made to the thrust used by Xiaoming? Brief analysis.
V. Application and Design (5 points+2 points+3 points =10)
37. When an interest group was studying the ups and downs of potatoes in water, they finally made them float on the water by adding salt to the water. Because of the time, they didn't tidy up the experimental equipment in time. When they came to tidy up a few days later, they were surprised to find that the potatoes that had floated on the water were all sunk at the bottom of the container. They decided to study this phenomenon. They put forward the following conjectures about this phenomenon:
Conjecture 1: The density of salt water may increase due to the evaporation of water, leading to the sinking of potatoes;
Conjecture 2: It may be that the quality of potatoes becomes bigger after soaking in salt water for a few days, which leads to the sinking of potatoes;
Guess 3: It may be that the volume of potatoes becomes smaller after soaking in salt water for a few days, which leads to the sinking of potatoes;
After some discussion, they immediately denied the conjecture 1. Do you think the reasons for their denial are: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.
Then they guessed that 2 and 3 carried out the following experimental operations:
(1) Take three pieces of potatoes, number them A, B and C, and measure their mass and volume respectively;
(2) prepare a large glass of salt water;
(3) Put three potatoes in salt water and make them float. After a few days, they are found to have sunk at the bottom of the container. Take them out and dry them, and measure their mass and volume respectively.
The experimental data are as follows:
Please analyze the above experimental data and answer:
Guess 2 is _ _ _ (choose "correct" or "wrong"); What do you think caused the potato to sink is _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.
On a quiet night, Xiao Ming wandered in the bright moonlight, looking up at the starry sky and imagining that the moon is not a light source, but I can see her because the moon reflects sunlight. Similarly, the earth is not a light source, and I can infer that the earth can also reflect sunlight.
Please carefully observe the next picture and write down Xiao Ming's reasoning process. (Text description or sketch can be used)
39. Can you measure the weight of a classmate with a spring dynamometer with a range of 5N?
Please write down the auxiliary equipment you need:
Applied physical knowledge:
The source that inspired you to be so creative:
Exercise
I. True or false questions
1.The athlete held the barbell still, and at this time the athlete did work on the barbell. （ ）
2. A classmate kicked the shuttlecock, and during his stay in air movement, this classmate did something for the shuttlecock. （ ）
3. A schoolbag rolls down the stairs, because the rolling direction is obliquely downward, so gravity does not do work. （）
4. A small ball rolls on the horizontal ground, and gravity does no work at this time. （ ）
A child pushed the box hard, but the box didn't push, but because the child used a lot of force, the child did work on the box. （ ）
6. 1 0 An egg is lifted1m, and the work done is about 5 joules. （ ）
7. The force of cart A is greater than that of cart B, so cart A does more work than cart B.. （ ）
8. The magnitude of work is determined by the magnitude of the force and the distance the object passes in the direction of the force. （ ）
Second, fill in the questions
1.A man pulls a block of wood with a weight of 150 cattle along a straight line and advances at a constant speed for 20 meters on the horizontal ground. The horizontal pulling force of a man is 90 cattle. Then, the friction force suffered by the block in the progress is _ _ _ _ cattle, and the work done by a man on the block is _ _ _ _ _.
2. A passenger weighing 700 cows stood on a bus driving at a constant speed with a bag weighing 60 cows on his back. When the car passes a distance of 200 meters, the work done by the passengers to the bag is _ _ _ _ coke, and the work done by the support of the car floor to the passengers is _ _ _ _ coke.
Third, multiple choice questions
1.A football player kicks out a football with a weight of 4.5 cattle with the force of 150 cattle, and the maximum distance that the football moves forward along the horizontal field is 20 meters, then the work done by the player to the football is []
A．3000 coke B．90 coke
C．3090 J. D. can't be sure
2. Two objects, A and B, move in a straight line at a uniform speed under the action of horizontal tension. If the mass of A is 4 times that of B, the horizontal pulling force of A is 2 times that of B, and the speed of A is 3 times that of B, then at the same time, the ratio of work done by pulling force on A and B is []
A．24∶ 1 B． 12∶ 1
C．6∶ 1 D．2∶ 1
Fourth, the calculation problem
There is a block of wood on the smooth horizontal desktop, the mass of which is 2kg. At present, the horizontal force of F= 12 Niu is pulling the object forward 0.5m. Q: (1) How many forces are acting on the block? How big are they? Draw a schematic diagram of the force (2) What is the work done by each force?
Answer to exercise questions
I.1.× 2 .× 3 .× 4. √ 5 .× 6. √ 7 .× 8. √
Second,1.90,1800 2.0,0
Iii.1.d2.c
(1) gravity, supporting force and pulling force, G= 19.6 Niu, N= 19.6 Niu, F= 12 Niu (2)WG=0, WN=0 and WF=6 Jiao.