More M2 questions- moments!

Hi
I've been working through a sheet on moments and was wondering if someone could show me how to do these two questions. I've not been taught this, but was reading thorugh the book and I'm not sure whether I'm doing the right thing.

1) A child of mass 20kg sits on the end of a see saw. Her father of mass 70 kg sits at the other end. The see-saw consists of a uniform beam lenght 4m and mass 10kg. How far from the child should the support be if the see-saw is to balance horizontally?

2) Find the moment of the couple applied to a corkscrew by two equal and opposite forces of 9N acting along lines 5cm apart. (can someone please explain the concept of couples!)

Thanks :smile:

Reply 1

steve2005

darth_vader05

Hi
1) A child of mass 20kg sits on the end of a see saw. Her father of mass 70 kg sits at the other end. The see-saw consists of a uniform beam lenght 4m and mass 10kg. How far from the child should the support be if the see-saw is to balance horizontally?

Let x be the distance of support from the centre of the see-saw. See attachment. Child should be 3m from support.

Picture 192.png45.1KB

Reply 2

non-centrally acting

steve2005

Let x be the distance of support from the centre of the see-saw. See attachment. Child should be 3m from support.

Thanks for your answer, but could you please explain it to me? :smile:

Reply 3

steve2005

darth_vader05

Thanks for your answer, but could you please explain it to me? :smile:

Is this a bit clearer ?

Picture 211.png66.8KB

Reply 4

Pookie

darth_vader05

Thanks for your answer, but could you please explain it to me? :smile:

Long time since I did moments, so I had to look up the equation :wink:

It is defined as 'Moment is equal to the product of mass and distance (radius) from the center of rotation'

Therefore moment=mx (where m is mass, and x is the distance)

For the seesaw to balance the moments must be equal on either side.

First rule of thumb for Mechanics Draw a Picture, see the attachment someone has added above. Notice all the forces, and lengths have been marked on.

2nd rule of thumb = think about the logic
Now the father is heavier, therefore he needs to be sitting less further away from the centre of the pivot for the seesaw to balance. Therefore he needs to be 2m plus a little bit more, which is

2 - x

Now the child being lighter will need to be further away to the pivot, so they need to be less than 2m close to the seesaw. Therefore

2 + x

Also to consider is the weight of the seesaw, which is

x

distance away from the pivot.

third rule of thumb -resolve into mathematics, which I see someone has provided vua the use of a mathematical program

Hope that goes someway to understanding the thought required behind the algebra.

PS- Never forget the piccie.

Reply 5

steve2005

Pookie

Now the father is heavier, therefore he needs to be sitting further away from the centre of the pivot for the seesaw to balance. Therefore he needs to be 2m plus a little bit more, which is

2 + x

The father is heavier, therefore he needs to be sitting nearer the pivot.

This shows how levers/moments work

http://www.walter-fendt.de/ph14e/lever.htm