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| 183_notes:examples:walking_in_a_boat [2014/10/01 05:46] – pwirving | 183_notes:examples:walking_in_a_boat [2014/10/01 05:51] (current) – pwirving | ||
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| Line 106: | Line 106: | ||
| (M+m)x=mL | (M+m)x=mL | ||
| + | |||
| + | Therefore: | ||
| x=(mM+m)L | x=(mM+m)L | ||
| Line 112: | Line 114: | ||
| Does this make sense? | Does this make sense? | ||
| + | |||
| + | If you want to check whether something makes sense a good start is to check the units: | ||
| Units (x)=m (mM+m) = unitless | Units (x)=m (mM+m) = unitless | ||
| - | If M is really big then x≡0, think oil thanker | + | Therefore m is the remaining unit. |
| + | |||
| + | Another check is that we know that if M is really big then x≡0, think of a similar scenario to one just discussed occurring on a oil thanker. | ||
| x=(mM+m)L≡mML≡0 when M>>m | x=(mM+m)L≡mML≡0 when M>>m | ||
| - | If M = 0 then x ≡L, no boat limit | + | Another check would be to check what happens when M = 0 then x ≡L, i.e. if there was no boat. |
| x=(mM+m)L≡mML≡L when m>>M | x=(mM+m)L≡mML≡L when m>>M | ||
| So the motion of the center of mass of a system is dictated by the net external force. | So the motion of the center of mass of a system is dictated by the net external force. | ||