Originally Posted by

**Harold14370**
. The velocity in the direction of the original trajectory does not change, as there is no component of force in that direction. Therefore the final **velocity is found by the pythagorean theorem**, and the KE calculated accordingly.

**1)I had to think a little bit about the planet **in orbit, because** it does not gain any energy **(relative to earth) after an acceleration at right angles to its trajectory. But this KE calculation only works relative to an inertial frame of reference. After some acceleration at an instant in time, then some time later there is a velocity, and KE, relative to the inertial reference frame, which continues along the tangent line after that instant.

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1)That is the point I am making, and probably you are missing. There is no applying to frame of reference, because KE is an absolute value, and velocity is its sqrt, so the sqrt of an absolute value is an absolute value. you cannot apply vector-sum, you do not do it for planets, electrons,

*why poor sinkers*?

I hope sometime you'll begin to admit "you have a point there, logic"

sinker, asteroid, planet makes no difference if you say normal force does zero work, it must apply for every body.

2) the pickup example was not clear, you have one ball 1)flying over your head at v=5, one ball 2) at rest on a pickup running at same v=5, one ball 3) at rest on the ground, frame of reference is the ground, you hit them with a racket or push them as you like, from the ground or from pickup.Explain why you should make less effort in any of these instances

3)angular momentum cannot be a

**poltergeis**t, but only force because it has what you call the

** math definition **of a force, Harold, kg*m²/s, M*L²/T

4) when do you use cross product, in physics, what does it mean concretely in applied physics, in engineering?

why can you use it for L and torque?