Is the swing a force
only silver coins were unearthed in history
I don't know if there is any gold coin
if it is genuine
it is priceless
solution: 29 + 21 + (21-15) = 56
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SAT is an examination sponsored by the College Board of the United States. Its score is an important reference for high school students from all over the world to apply for admission qualifications and scholarships to American universities. It and act (American College Test) are both called American college entrance examination
however, not all universities are required to provide this test result. Some top universities are not required to provide this test result in terms of admission conditions. Students can use other scores instead. When applying for a university, they need to check the admission conditions of the University carefully
after the reform in 2016, the total score of the new SAT is 1600, which is divided into three parts: reading, grammar and mathematics, and writing is changed to selective examination. ETS is responsible for its proposition and marking. However, they are only one of the materials for reference when enrolling students, and can not play a decisive role. Their grades are valid for two years.
1. Different Kapu (patron saint) will appear in the ruins of the four islands, which can be captured at fixed points. If you run away or kill yourself, you will be resurrected if you challenge in baokemeng League again
2. You can get the keegeld core in your own home, but it seems you can get it without customs clearance
3. When you get home, brother Hei will give you the special Z crystal of the other two royal families
4. The battle tree is open on Bonnie island
5. Indivial judgment is open, but it needs to win 20 consecutive battles in the battle tree
6. You can get "gene wedge", "punishment pot" and "water drop of heart" in the middle room of 2F underground of Ethereum
7. You can get a strange card from your home and go to the motel on route 8 to fight Lila. After the victory, UB series missions will take place
8. You can meet Gina on the road to the east of Hapu's home on Boni island. After winning the battle with the man, you can get the key stone and super Hudi stone. Mega evolution is open
When the swing swings from the lowest point to the highest point, gravity does negative work, and the kinetic energy of the system is transformed into the potential energy of the system. When the swing swings from the highest point to the lowest point, the potential energy of the system is transformed into the kinetic energy of the system. The mechanical energy is conserved in the whole process, and the swing will swing with equal amplitude
if you want the swing to swing higher and higher, you must rely on external forces. If you play on the swing yourself, you can't rely on external forces, and the system can't get energy from the outside. Only by doing work with the internal force of the rope, can the internal energy of the swing be transformed into the mechanical energy of the system, and the internal force of the swing can only be achieved by standing up or squatting down on the swing
It is obviously unreasonable to use the resonance condition of forced vibration to explain the process of swing. However, some people will ask the following question: man and the swing are a system, the force of man on the swing should be internal force, and the internal force of the system can not change the motion state of the center of mass. Why can man swing high? This problem itself mixes the law of motion of the center of mass, the momentum theorem of the system of particles and the functional principle of the system of particles to describe the mechanical law of the motion of the system of particles from different angles. It is easy to make people fall into a dilemma. In fact, as long as we make clear the differences and connections between the three laws, the problem will be solved The principle of function tells us that the increment of mechanical energy of particle system is equal to the algebraic sum of the work done by external force and the work of non conservative internal force. Although the external force does not work when swinging, the internal force does not work when the human body is deformed. As long as the non conservative internal force does positive work in one swing, the mechanism energy of the system will increase. The law of motion of the center of mass and the theorem of momentum of the particle system show that the generation of the acceleration of the center of mass and the change of the total momentum of the particle system must require that the combined external force on the system is not zero, and the internal force cannot change the motion state of the center of mass of the systemwe know that any mechanical process must go through a period of time. As long as the external force is not zero, the impulse of the external force must be generated. But in this process, the external force does not necessarily do work. When swinging, the energy increase of the system mechanism comes from the work done by the non conservative internal force when the human body is deformed, and the change of the motion state of the center of mass of the system is caused by the impulse of the external force, that is, the impulse of the constraint reaction at the suspension point
It can be seen that the change of the mechanical energy of the particle system does not necessarily require the work of external force, and the work of non conservative internal force can also change the mechanical energy of the system. The change of the motion state of the center of mass of the particle system must rely on the external force, but the external force does not have to do work, as long as there is a certain impulse. For example, when a car starts, the increase of kinetic energy comes from the combustion of fuel in the engine, the expansion of gas, the pushing of piston, the driving of the vehicle... In a word, it depends on the combustion of fuel, the work of non conservative internal force, and the conversion of chemical energy into mechanism energy However, the acceleration of the vehicle depends on the static friction between the driving wheel and the ground. It is a well-known fact that static friction does not workwork is the physical quantity describing the cumulative effect of force on space, and impulse is the physical quantity describing the cumulative effect of force on time. As long as we deeply understand the differences and relations between these two physical quantities and their effects, we will not have some vague ideas when analyzing practical problems