BS: help with this please. (science)

Subject: RE: BS: help with this please. (science)
From: Blackcatter
Date: 01 Apr 04 - 09:39 PM

Just how thick are you planks in Britain? In the U.S. what is usually called a plank is probably only 2 inches thick (1 3/4 in. actually - from that dimensional lumber stupid measuring thing).

I can see thick as a beam or post - they're 4 inches or better.

Subject: RE: BS: help with this please. (science)
From: GUEST
Date: 01 Apr 04 - 05:26 AM

Outline:

Centre of pivot point (left hand side of plank) plus attatched plank, length = 80cm. Box on extreme right hand side of plank. Underneath plank there is a block 20cm from centre of the pivot which has an upwards force of 100N.

Question 1.

Calculate the moment (turning affect) of the 100N force about the pivot? Show your working?

Force of 100N acting upwards 0.2m from pivot
Resulting Moment is 100N x 0.2m = 20Nm anti-clockwise.

Question 2.
The box on the right hand side of the plank is just heavy enough to keep the plank balanced.
What is the moment of the box about the pivot. Give the units?

The plank is balanced, therefore the sum of the anti-clockwise moment must equal the sum of the clockwise moment. Therefore the moment exerted by the box on the right hand end of the plank must be 20Nm clockwise.

(NOTE: This is theoretical as the plank has no weight, in real life the moments exerted on the pivot point would be anti-clockwise exerted by the upward force of 100N with balancing clockwise moments exerted by the box at the right hand end of the plank plus the weight of the plank acting at 0.4m from the pivot point)

Question 3.
What is the weight of the box?

Sum of clockwise moment = Sum of anti-clockwise moment

Force exerted by box x 0.8m = 100N x 0.2m

Force exerted by box = (100N x 0.2m)/0.8 = 25N

Weight of Box = 25N x 0.981 = 24.525kg

Question 4.
The 100N force acts on a block with area 5cm squared.
Calculate the pressure on the block? Give the units?

Taking it as a surface area of 5 square centimetres as opposed to a square 5cm x 5cm and the plank is in balance

Pressure = Force/area = 100N/5sq.cm = 20N/sq.cm

Subject: RE: BS: help with this please. (science)
From: GUEST,Sooz(at work)
Date: 01 Apr 04 - 02:48 AM

Red Eye - distance in M so moment =100 x 0.6 Nm

Subject: RE: BS: help with this please. (science)
From: GUEST,Red Eye
Date: 31 Mar 04 - 11:43 PM

This is not my problem although I have taken it on. My son asked me to have a look for him a couple of days ago. Sooz from what you are saying 'Moment = force x distance from pivot' means that the box is 60cm from the pivot, therefore 100N X 60cm =?

Bobert, what is the weight of the box.

Blackcatter. English saying, 'You're as thick as two short planks'.

Subject: RE: BS: help with this please. (science)
From: Johnny in OKC
Date: 31 Mar 04 - 09:13 PM

Constantly amused.

Subject: RE: BS: help with this please. (science)
From: Blackcatter
Date: 31 Mar 04 - 08:50 PM

What's so funny about planks?

Subject: RE: BS: help with this please. (science)
From: Bobert
Date: 31 Mar 04 - 07:59 PM

Hey, this is easy stuff since the mechanical advantage of the fulcrum (pivot point) is exactly 1:1... This mean that given the fulcrum is place exactly in the middle of the plank and on level ground that two 100 pound objects will balence exactly the same distance from the point of the fulcrum...

Now lets say you have two objects. One weighs 100 pounds and the other 50 pounds. Since the mechanical advantage is still 1:1 you will need to place the 50 pound object exactly twice as far away from the fulcrum as you have placed the 100 object in order to equalize...

NOw convert all that stuff to metrics and everything should workout fine.

Bobert's Wes Ginny Slide Rule

Subject: RE: BS: help with this please. (science)
From: Amos
Date: 31 Mar 04 - 03:15 PM

Red Eye:

Draw it out and work through it -- you can do it, and you really do NOT want ot turn in someone else's working under your name.

A

Subject: RE: BS: help with this please. (science)
From: GUEST,Sooz(at work)
Date: 31 Mar 04 - 10:33 AM

Have you sorted your problem out?

Subject: RE: help with this please.
From: GUEST,Sooz(at work)
Date: 30 Mar 04 - 07:41 AM

force/area = pressure (N/cm2)
Your description of the plank is confusing me for the rest but:
Moment = force x distance from pivot
The plank is balanced when the clockwise and anticlockwise moments are equal
Does that help?

Subject: RE: help with this please.
From: The Fooles Troupe
Date: 30 Mar 04 - 04:14 AM

answered here

Subject: help with this please.
From: GUEST
Date: 30 Mar 04 - 03:51 AM

Help in working this problem out please. Diagram from left shows pivot point, plank, box on top of plank. Underneath a block with an upwards arrow.

No jokes about planks please!

Centre of pivot point (left hand side of plank) plus attatched plank, length = 80cm. Box on extreme right hand side of plank. Underneath plank there is a block 20cm from centre of the pivot which has an upwards force of 100N.

Questions

Calculate the moment (turning affect) of the 100N force about the pivot? Show your working?

The box on the right hand side of the plank is just heavy enough to keep the plank balanced.
What is the moment of the box about the pivot. Give the units?
What is the weight of the box?

The 100N force acts on a block with area 5cm squared.
Calculate the pressure on the block? Give the units?

If anyone can work this out please show workings as well?

Much appreciated

Red Eye.