Statics, determine the force required to rotate a body

In summary, the student is trying to solve an equation system for a body rotating about an axis in space, but is having trouble understanding the forces acting on the body. The student believes that due to a force at the point C, there must be a reaction force on the y-axis. However, the student is not sure if there could be a reaction force on the y-axis. The student has five unknown quantities and three equations, but cannot solve the system.
  • #1
jonjacson
447
38

Homework Statement



I´ll use a picture to show the problem:

1zyh95e.jpg


Homework Equations



Sum forces=0

Sum Moments=0

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?
 
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  • #2
When "breaking contact at C", what is the reaction at C?
 
  • #3
voko said:
When "breaking contact at C", what is the reaction at C?

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

Which is the correct answer.

Thanks voko!
 

1. What is statics and why is it important?

Statics is a branch of mechanics that deals with the study of objects at rest or in equilibrium. It is important because it helps us understand the forces acting on a body and how they affect its motion.

2. How do you determine the force required to rotate a body?

The force required to rotate a body, also known as the torque, can be calculated by multiplying the force applied to the body by the distance from the point of rotation to the point where the force is applied.

3. What factors affect the force required to rotate a body?

The force required to rotate a body depends on the magnitude of the force applied, the distance from the point of rotation, and the angle at which the force is applied. The shape, size, and weight of the body can also affect the force required.

4. How do you calculate the direction of the force required to rotate a body?

The direction of the force required to rotate a body can be determined using the right-hand rule. This rule states that if you point your right thumb in the direction of the force, your fingers will curl in the direction of the torque.

5. What are some real-life applications of calculating the force required to rotate a body?

Calculating the force required to rotate a body is important in many fields, such as engineering, architecture, and physics. It is used to design structures and machines, understand the stability of objects, and analyze the forces acting on different parts of a system.

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