Momemnts -Maths Mechanics

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?

Any hints? please help

(edited 6 years ago)

Reply 1

RDKGames

Study Forum Helper

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?

Any hints? please help

Draw a diagram.

Reply 2

joyoustele

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? :smile:

(edited 6 years ago)

Reply 3

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.

Reply 4

RDKGames

Study Forum Helper

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

m_1 \mathrm{kg}

and

m_2 \mathrm{kg}

respectively.

You know that

m_1+m_2=60

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.

(edited 6 years ago)

Reply 5

joyoustele

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

m_1 \mathrm{kg}

and

m_2 \mathrm{kg}

respectively.

You know that

m_1+m_2=60

m_1 g1.6 -(60-m_1)g1.4=60g

- is this the right equation?

Reply 6

joyoustele

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.

m_1 g1.6 -(60-m_1)g1.4=60g

is this right?

Reply 7

old_engineer

Original post by joyoustele

m_1 g1.6 -(60-m_1)g1.4=60g

is this right?

No. Where did the right hand side come from?

Reply 8

joyoustele

Original post by old_engineer

No. Where did the right hand side come from?

Reaction force for the total mass

Reply 9

joyoustele

I haven't been taught moments yet, im trying to learn it before we start

Reply 10

old_engineer

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.

Reply 11

joyoustele

Original post by old_engineer

Thank you very much :smile:

I get m1 is 35kg and m2=25kg

Reply 12

old_engineer

Original post by joyoustele

Thank you very much :smile:

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.

Reply 13

Dysf(x)al

Original post by joyoustele

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.