- #126

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If you do that, then the reference point won't be stationary.Or around any other reference point you choose to describe its motion.

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- Thread starter John Mcrain
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- #126

- 52

- 12

If you do that, then the reference point won't be stationary.Or around any other reference point you choose to describe its motion.

- #127

A.T.

Science Advisor

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Turn off the boat's engine and lift it out of the water. Now fling it into the air. It will then rotate around its center of mass.

Or around any other reference point you choose to describe its motion.

Neither is the center of mass, if you fling something it into the air.If you do that, then the reference point won't be stationary.

- #128

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It is said that rocket,plane rotate about center of gravity ,why this is is not case for boats?

Boat pivot point is not in center of gravity.

When you have multiple variables, placing one of them at the origin of your frame of reference makes the calculations easier. Some of the videos you posted, that I watched, are simply placing the CoM as the origin of the grid space used to conduct calculations because zeroes are easy to deal with in calculations. It is an unconscious decision for most people to define their grid space in this way so when people see these explanations, they come to incorrect conclusions.

You see this, sometimes, when dealing with simple gravity problems because common language does not distinguish gravitational acceleration "downwards" as negative acceleration while altitude "above" a reference plane is considered positive. Without realizing that the origin is set to the "ground," a ball thrown "up" has a positive displacement, a positive velocity, and a negative acceleration. Once it hits the apex of its travel, its displacement is at its maximum positive displacement, its velocity is zero, and its negative acceleration remains constant (relatively speaking). As it begins to "fall", its displacement decreases, its velocity continues to decrease into negative values, and its acceleration remains constant. Just before it hits the "ground," its displacement is almost zero, its velocity has the same magnitude but negative of its launch velocity, and its negative acceleration remains constant.

However, if you chose the apex of flight as the origin, the calculations are different but give the same results. If you chose the center of the Earth as the origin, the calculations are more complicated still, but the results will still be the same.

Things rotate "around their CoM" because the calculations derived from that choice of origins for your grid space makes all the calculations easier. Basically, it is a short hand way of saying, "I am choosing the CoM" as the origin of my grid space to make my calculations easier." just like saying, "I am choosing the ground as zero altitude to make my calculations easier for the flight of a baseball."

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