Why is W = 3/√2 kg for equilibrium in Feynman lecture 2.5?

[new90]

Homework Statement

A uniform plank 1.5m long and weighting 3kg is pivotedat one end.The plank is held in equilibrium in a horizontal position by a weight and pulley arrangement.Find th e W needed to balance th plank.Neglect friction

Relevant Equations

no equation

I think that the W should be 3kg to be in equilibrium but in the book it said that id 3/√(2 I don't why

 

[berkeman]

@new90 -- how about you go to your other thread where you have left a lot of us hanging, waiting for your reply so we can finish off that thread? Why are you starting new threads and not posting in your existing thread?

https://www.physicsforums.com/threa...di-term-in-this-equation.986369/#post-6318989

 

[hutchphd]

The pulley rope is attached to the end of the plank I assume. So the first thing you do for any force problem is ___________ ?

 

[new90]

the diagram

 

[hutchphd]

Yes the "free body" diagram.

 

[new90]

ok well I know that the the pulley is aright triangle so the part that is conected to the plank forms a 45 angle and i think that's something important

 

[hutchphd]

Draw the free body diagram and show us the details .
Also first please see @berkeman comment and finish that up.

 

[new90]

I finish it

 

[new90]

so thw W goes down and have an opposite reaction

 

[hutchphd]

You need to draw a good complete free body diagram.

1. It will include three forces acting on the plank, two of unknown magnitude.
2. From the fact that the plank is stationary in two different ways two equations obtain.
3. solve for the unknowns

If you desire further help you must show us your work.

 

[new90]

so the total forces are 5?

 

[hutchphd]

please list the five. This is not a game of "twenty questions"

 

[new90]

the Wx,Wy the gravity that affract the plank the opposite reaction of the gravity and the opposite reaction of Wx

 

[hutchphd]

Please draw and submit "free body" diagram: Draw the plank with no external forces ("free body") and one by one add all the labelled vector forces acting ON the plank. Then show your result. Then we can discuss.

 

[new90]

 

[new90]

 

[new90]

 

[new90]

like this?

 

[hutchphd]

Close. But on the first drawing make a labelled arrow representing each force and its position of application. Doesn't have to be pretty (you don't really need the pulley but you do need the angle).

 

[new90]

 

[hutchphd]

Draw a single arrow representing each and every force ON the plank (if the direction is wrong the vector will turn out negative) .
These are the tools of the trade. Has no one taught this to you previously (review as necessary!) ?

 

[new90]

sorry I am in middle school

 

[new90]

but the number of forces are correct

 

[new90]

sorry what do you mean only one arrow

 

[hutchphd]

Ah, that's good to know. You are doing well then, but may need some basics . Do you have a text other than Feynman availible ? (don't get me wrong I adore RPF lectures but they are difficult to learn from at times)

 

[new90]

yes I have the volume 1 of feyman lectures on physics

 

[hutchphd]

Can you get a copy of Tipler or Thornton or 0Sears,Zemansky or even Halliday,Resneck . Library??

 

[new90]

Those are books

 

[new90]

I think I got it

 

[new90]

thanks I will try to get a copy