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#1
Question: This my attempt to draw it out. Basically a man of length 1.56m and mass 62kg lies on a wooden plank of 1.56m and mass 15kg. The wooden plank rests on a brick and some scales. The scales get a reading of 30kg, use this to determine how far the centre of gravity of the person is from the man's toes.

--------1.56m--------

0-----l---------------< <man (62kg/608.22N)
----------------------- <wooden plank (15kg)
^ _______________^ Scales (30kg)
Brick

Sorry about the awful drawing, thought it may help. Any help is appreciated
0
5 years ago
#2
(Original post by luke5675)
Question: This my attempt to draw it out. Basically a man of length 1.56m and mass 62kg lies on a wooden plank of 1.56m and mass 15kg. The wooden plank rests on a brick and some scales. The scales get a reading of 30kg, use this to determine how far the centre of gravity of the person is from the man's toes.

--------1.56m--------

0-----l---------------< <man (62kg/608.22N)
----------------------- <wooden plank (15kg)
^ _______________^ Scales (30kg)
Brick

Sorry about the awful drawing, thought it may help. Any help is appreciated
How far have you got with this problem so far? Show us your working / thoughts.

To get you started thinking about what's going on, what would the reading on the scales be if the centre of gravity for both the plank and the man was all at one extreme end?

What would the reading on the scales be if the c of g for both the man and plank is exactly in the middle between the two ends?
0
#3
(Original post by uberteknik)
How far have you got with this problem so far? Show us your working / thoughts.

To get you started thinking about what's going on, what would the reading on the scales be if the centre of gravity for both the plank and the man was all at one extreme end?

What would the reading on the scales be if the c of g for both the man and plank is exactly in the middle between the two ends?
Honestly I did not get very far at all, I didn't understand the question very well.

Assuming the centre of mass was extreme at one end, the reading would be (15+62) 77kg?

For the second question I think it could be (77/2) 38.5kg? This is from thinking if you go half the distance away from the scales, the scale will read half the mass.
0
5 years ago
#4
(Original post by luke5675)
Honestly I did not get very far at all, I didn't understand the question very well.

Assuming the centre of mass was extreme at one end, the reading would be (15+62) 77kg?

For the second question I think it could be (77/2) 38.5kg? This is from thinking if you go half the distance away from the scales, the scale will read half the mass.
Correct and correct.

With the c of g in the middle, the force on the scale at one end and the force on the brick at the other are identical and therefore exactly half of the total mass.

So if the scale is reading 30kg, what is the kgf on the brick?

Can you see where this is leading?
0
#5
(Original post by uberteknik)
Correct and correct.

With the c of g in the middle, the force on the scale at one end and the force on the brick at the other are identical and therefore exactly half of the total mass.

So if the scale is reading 30kg, what is the kgf on the brick?

Can you see where this is leading?
The kgf would be the same so 30kg? I think I understand this, however I am struggling to relate this to the distance the centre of gravity is from the person's toes.

Edit: If the scale is 30kg , does that mean the kgf would be 47kg on the brick and the c of g is more towards the brick?
0
5 years ago
#6
(Original post by luke5675)
Edit: If the scale is 30kg , does that mean the kgf would be 47kg on the brick and the c of g is more towards the brick?
Correct.

So, if we placed a knife edge fulcrum at the c of g and removed the brick and the scale, what would happen?

Spoiler:
Show
The plank and man would be perfectly balanced and in a horizontal position.
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#7

Edit: I misunderstood the question, understand it now I see the answer
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#8
Just had another go at the actual question. Is 30/47 x 1.56 correct? Giving an answer of 0.99m or rounded to 1m.
0
5 years ago
#9
(Original post by luke5675)
Just had another go at the actual question. Is 30/47 x 1.56 correct? Giving an answer of 0.99m or rounded to 1m.
Correct. Don't forget to state which end the 1m is measured from.
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