You are Here: Home >< Physics

# Physics, moments, lifting supports? Help? Watch

4)b) A uniform plank of weight 150N and of length 4m rests horizontally on two bricks. One of the bricks is at the end of the plank. The other brick is 1m from the other end of the plank.

A child stands on the free end of the plank and just causes the other end to lift off its support. Calculate the weight of the child.
2. (Original post by Konnichiwa)

4)b) A uniform plank of weight 150N and of length 4m rests horizontally on two bricks. One of the bricks is at the end of the plank. The other brick is 1m from the other end of the plank.

A child stands on the free end of the plank and just causes the other end to lift off its support. Calculate the weight of the child.
Have you attempted this yourself?
What are you stuck on?
I presume you have studied moments.
Start by drawing a diagram, marking the forces and their positions along the plank. Can you do this?
3. Yes, I've completed that, I'll show you what I've done soon. One second, let me complete it again since I've seemed to have lost it.

The answer to b) was 150N but I did not know how this was done.
4. Question a) was "Sketch the arrangement and calculate the support force on the plank from each brick.".

I think this is correct.
CALCULATING THIS WITHOUT THE CHILD...
3X = 1*150;X=50

3Y = 150*2
Y = 100.
5. Yes, the support forces are 100N and 50N
Now for the second part...
The key to doing this is a correct diagram.
Without this you can't write down the moments and balance the clockwise with the anticlockwise.
The clue is that there is no force from the support at the end when "A child stands on the free end of the plank and just causes the other end to lift off its support."
This simplifies the problem.
6. Oops, I meant the answer was 150N.

Is this the correct working out?
Force Of Child = 1(150)?

I really have no idea. I guess I may have to revise moments.
7. (Original post by Konnichiwa)
Oops, I meant the answer was 150N.

Is this the correct working out?
Force Of Child = 1(150)?

I really have no idea. I guess I may have to revise moments.
Yes the child is 150N but it's not clear, without the diagram, how you got the answer.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 19, 2013
Today on TSR

### Is this person a genius?

...with these A Level results?

### I think I'm transgender AMA

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.