Centre of mass and gravity problem, confused and

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SUMMARY

The discussion focuses on a physics problem involving a uniform plank of length 6 meters and mass 90 kg, supported by sawhorses 1.5 meters apart. The key question is determining the maximum mass Throckmorton can have while standing at the right end of the plank without causing it to tip. The center of mass of the plank is located at its midpoint (3.0 meters), while Throckmorton's center of mass is at 6.0 meters. The center of mass of the system is calculated using the formula (90*3 + M*6) / (M + 90), where M represents Throckmorton's mass.

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Homework Statement



A uniform plank of length l = 6m and mass M= 90kg rests
on sawhorses separated by D = 1.5m and equidistant from the
center of the plank. Cousin Throckmorton wants to stand on the
right-hand end of the plank. If the plank is to remain at rest, how
massive can Throckmorton be?

Diagram:
throcky.jpg


Homework Equations





The Attempt at a Solution



I am confused at the diagram. Why is the centre of gravity not at the centre of mass of the plank which should be in the middle ?
 
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It is the centre of mass of the system which includes all the masses.
 
paisiello2 said:
It is the centre of mass of the system which includes all the masses.

Okay, then how do we find it ?, or what is the explanation that says it should be there ? (referring to the cg in the image)
 
The center of mass of the plank is at the middle of the plank. Taking x to be the distance from the left end of the plank, that would be x= 3.0. Throckmorton's center of mass is where he is standing at x= 6.0. The center of mass of the system is a weighted average, weighted by their masses: (90*3+ M*6)/(M+ 90) where M is Throckmorton's weight.
 
HallsofIvy said:
The center of mass of the plank is at the middle of the plank. Taking x to be the distance from the left end of the plank, that would be x= 3.0. Throckmorton's center of mass is where he is standing at x= 6.0. The center of mass of the system is a weighted average, weighted by their masses: (90*3+ M*6)/(M+ 90) where M is Throckmorton's weight.

Why is it m*6 and not m*3 ? and how do you know where the centre of gravity is ?( cg in the diagram)
 
He's taking moments about the left end.

He's giving you the formula for the location of CG
 

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