Homework Help: Statics, determine the force required to rotate a body

1. Mar 11, 2013

jonjacson

1. The problem statement, all variables and given/known data

I´ll use a picture to show the problem:

2. Relevant equations

Sum forces=0

Sum Moments=0

3. The attempt at a solution

Basically I´m not sure about the forces on the free body diagram and I´d like to talk about them.

Assuming that P is not acting, and the body is masless these are the only three forces acting on it: Fb, reaction at O on the x axis Rox, and reaction at C on the x axis too Rcx, Am I right?

I know that is not what the problem is asking, it´s only to make this clear, solving the equations it´s pretty easy:

Sum Fy= there are not any forces on this axis

Sum Fx=> Rcx + Nox =Fb

Sum Mc= Fb*40=Nox*80----> Nox=1750*10/1000*2=17.5N

Rcx=8.75N

Is everything right until here?

Now, assuming a force P must be acting on the body to start rotation, I´m a bit lost with forces, basically on the y axis.

I thought that at the point C a force on the y axis cannot act, but then I thought that maybe it was possible. For example, if you push an object against a wall it doesn´t fall, you are applying a force on the x axis, there is a reaction force at that axis, but there must be a friction force on the y axis with the same modulus as the gravity, well I think that´s the only option to guarantee that the statics equations are correctly used.

So, let´s suppose that we have a general force at the point C, and we don´t know its direction, we have three unknown quantities: P, Rcx, Rcy.

Now If I look to point O, I think that due to Fk there must be a reaction on the x axis, but I´m not sure if there could be a reaction on the y axis.

ANyway I have more unknown quantities Rox, Roy.

I have a total of 5 unknown quantities and three equations, so I can´t solve the system, but the book gives me a numerical answer, so I think that I should make two assumtions, maybe that Rcy is 0, and Roy is 0 too, but in that case for the y axis I have:

SUm Forces Y axis= P=0, which doesn´t make any sense.

So, Is the system able to create reaction forces on the Y axis? At what point? Why?

2. Mar 11, 2013

voko

When "breaking contact at C", what is the reaction at C?

3. Mar 11, 2013

jonjacson

Is cero and gives me these set of equations:

x---> Rox=Fk=17,5 N

y--> P=Roy

Moments--> -Fk*40 + P*120=0---> P=5,83N