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# Momemnts -Maths Mechanics watch

1. The total mass of two children is 60kg. They are sitting on a seesaw one at each end.
Find their separate masses.
-Child 1 is 1.6 metres from the centre and child 2 is 1.4 metres from the centre?

2. (Original post by joyoustele)
The total mass of two children is 60kg. They are sitting on a seesaw
Find their separate masses.
-Child 1 is 1.6 metres from the centre and child 2 is 1.4 metres from the centre?

Draw a diagram.
3. (Original post by RDKGames)
Draw a diagram.
I did.
-Should I do trial and error to find their separate masses?
As their combined moment should be equal to 0?
4. Take moments about the pivot point. That will give you an equation that relates m1 and m2. The question also tells you that m1 + m2 = 60. Solve simultaneously or by substitution.
5. (Original post by joyoustele)
I did.
-Should I do trial and error to find their separate masses?
As their combined moment should be =0
Trial and error is a waste of time if there is a solid systematic way of getting the answers.

Say their masses are and respectively.

You know that

Mark on the reaction force in the middle of the seesaw, what would it be?

Then use one kid as a pivot, and calculate moments from there and work out the mass of the other kid.

There isn't much to the context from what you've said so I just assume the seesaw is perfectly horizontal with the system in equilibrium.
6. (Original post by RDKGames)
Trial and error is a waste of time if there is a solid systematic way of getting the answers.

Say their masses are and respectively.

You know that

Mark on the reaction force in the middle of the seesaw, what would it be?

Then use one kid as a pivot, and calculate moments from there and work out the mass of the other kid.

There isn't much to the context from what you've said so I just assume the seesaw is perfectly horizontal with the system in equilibrium.
- is this the right equation?
7. (Original post by old_engineer)
Take moments about the pivot point. That will give you an equation that relates m1 and m2. The question also tells you that m1 + m2 = 60. Solve simultaneously or by substitution.

is this right?
8. (Original post by joyoustele)
is this right?
No. Where did the right hand side come from?
9. (Original post by old_engineer)
No. Where did the right hand side come from?
Reaction force for the total mass
10. I haven't been taught moments yet, im trying to learn it before we start
11. (Original post by joyoustele)
Reaction force for the total mass
OK, moments gives you 1.6(m1)g = 1.4(m2)g. Clockwise moment equals anti-clockwise moment, as the system is in equilibrium. Now proceed from there with the substitution you already worked out from m1 + m2 = 60.
12. (Original post by old_engineer)
OK, moments gives you 1.6(m1)g = 1.4(m2)g. Clockwise moment equals anti-clockwise moment, as the system is in equilibrium. Now proceed from there with the substitution you already worked out from m1 + m2 = 60.
Thank you very much
I get m1 is 35kg and m2=25kg
13. (Original post by joyoustele)
Thank you very much
I get m1 is 35kg and m2=25kg
That's not right. You need to check those numbers in the moments equation then check your arithmetic.
14. (Original post by joyoustele)
The total mass of two children is 60kg. They are sitting on a seesaw one at each end.
Find their separate masses.
-Child 1 is 1.6 metres from the centre and child 2 is 1.4 metres from the centre?

Set the mass of child 1 to be x and the mass of child 2 to be 60-x. The moment is equal to the force times the distance from the pivot, so find the moments for each child. The total moment should be 0, and the two moments are in opposite directions, so set them equal to each other!

Don't try to do it with two variables. If you can easily eliminate one (as I did with the 60-x bit), do so.
15. (Original post by old_engineer)
That's not right. You need to check those numbers in the moments equation then check your arithmetic.
oops I meant 28Kg for M1 and 32 for M2

m1 should be 84g/3g =28
16. (Original post by TheMindGarage)
Set the mass of child 1 to be x and the mass of child 2 to be 60-x. The moment is equal to the force times the distance from the pivot, so find the moments for each child. The total moment should be 0, and the two moments are in opposite directions, so set them equal to each other!

Don't try to do it with two variables. If you can easily eliminate one (as I did with the 60-x bit), do so.
that's the first method I tried, I couldn't solve it. Maybe I went wrong in my calculations.
17. (Original post by joyoustele)
oops I meant 28Kg for M1 and 32 for M2

m1 should be 84g/3g =28
Yes that's it. You should always be looking for quick consistency checks you can apply to your answers.
18. (Original post by joyoustele)
that's the first method I tried, I couldn't solve it. Maybe I went wrong in my calculations.
1.6x = 1.4(60-x)
1.6x = 84 - 1.4x
3x = 84
x = 28

Should be 28kg and 32kg, as you managed to get earlier.

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