Part 3

P60 Example2 Solve the problem from the perspective of Newton’s second law and equation of motion

Some of them seem to have the same speed, that is, they seem to have experienced a collision (completely inelastic collision). From this point of view, the momentum theorem is more useful:) But how is the way: Newton’s second law to solve it? Actually when I deal with it, I found that I couldn’t write it anymore. Momentum conservation is a more basic physical law than Newton’s second law. The topic that can’t be solved by Newton’s second law may be solved by The momentum theorem. The two are not just simple in terms of quality. The difference between the two theorems is not just one of them multiplied by the quality.

P64 Why is the rocket’s momentum equation derivation not as simple as the conservation of momentum in high school?

Because the rocket can not ignore the external force while flying, and the direct effect of the external force is changing the momentum. So from this point of view, there is a subsequent derivation.

P66 Why is the quality of the fuel container not included in each stage of the rocket?

P65 For the rocket formula: v is the final speed

Because v is the result already considering all the momentum and external forces given by the fuel.

P69 What’s interesting is why $F·dr$ is not written as $dF·r

It is allowed to do this on the calculation, but it seems to have no meaning.

P72 The triangular relationship between dr and dr

Between dr and dr Interesting!!!

P73 In All elastic systems, the elasticity is conservative

P75 The direction of the result of the gradient operation is from small to large. Potential energy function is the same. But the problem of the direction of force has to undergo a certain dirction conversion.

Potential energy represents the energy that is accumulated, and ** field power tends to release energy**. The gradient of the potential function is the direction of energy increase.

P78 It is worth noting that the calculation of the functional principle includes the work of the internal force.

Different from momentum calculation The calculation of functional principle is scalar operation, and the internal force cannot be eliminated.

P79 The calculation of mechanical energy increment of the particle system does not include conservative internal forces.

Conservative internal forces are systematic, and potential energy is formed by the interaction of conservative forces between the particles in the particle system. Energy is not transmitted to the outside.

P93 To calculate the quality of the upper and lower chains to calculate the centroid

Otherwise, we must consider the change in the momentum distribution caused by the collision, which is not as simple as calculating the momentum theorem directly.

P94 Invariance of the laws of physics

It look like cool. But what is: the invariance of spatial translation, the invariance of time translation?