Q6.10 Spot the difference (S)

What are the units of linear momentum? What are the units of angular momentum?

Under testing circumstances a physics undergraduate once wrote: ‘In the collision process linear momentum is turned into angular momentum.’ Comment, critically (…and make a mental note never to write this yourself!)

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Hint

Remember the key things about units! (Key Point 0.1).
 

Solution

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Solution

$\vec{p}=m\vec{v}$ : units kg m s-1.

$\vec{L}=m\vec{r}\times\vec{v}$ : units kg m2 s-1.

Since they have different units, it is quite wrong to talk of one “turning into” the other: linear and angular momentum are different kinds of quantities; each has its own distinct conservation law. (Key Point 0.2).