# 2.5 Exercises of Feyman lectures on Physics

## 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 dont why

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

the diagram

Yes the "free body" diagram.

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 thats something important

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

• berkeman
I finish it

so thw W goes down and have an opposite reaction

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

so the total forces are 5?

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

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

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.   like this?

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). 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!) ?

sorry Im in middle school

but the number of forces are correct

sorry what do you mean only one arrow

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)