# Homework Help: Rotation of body in stregth of materials problem

1. Aug 26, 2014

### Chacabucogod

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

A uniform bar of length 2l is rotated in a horizontal plane about its mid-point as center with an angular velocity ω. This case must be reduced to a problem in statics by the application of d'Alembert's principle, the "forces" acting being "centrifugal" forces.

a) Plot the stress distribution and the elastic displacement against the radial distance from the center, and derive formulae for the maximum stress and the maximum displacement.

ANS:$\frac{γω^2}{2g}(l^2-r^2)$
2. Relevant equations

$F=m*\frac{v^2}{r}=m*ω^2*r$
P=$\frac{F}{A}$

3. The attempt at a solution

I really have no idea where to begin or how to continue

2. Aug 27, 2014

### Staff: Mentor

Let ρ represent the density of the bar. How much mass dm is contained in the region of the bar between r and r + dr? What is the tension in the bar at location r = l ?

Chet

3. Aug 27, 2014

### Chacabucogod

The mass between r and dr will be

$$dm=4πρlrdr$$

The force at l will be

$$F=mω^2l$$

I'll try to see if I can come up with something using what you just told me.

Thank you

4. Aug 27, 2014

### Staff: Mentor

Actually, the differential mass between r and r + dr will be
$$dm=ρAdr$$
where A is the cross sectional area of the rod and Adr is the differential volume of the rod contained between r and r + dr.

No. Actually, the tension in the rod at r = l will be zero, since the rod is not contacting anything at that location.

Draw a free body diagram of the differential section of rod situated between r and r + dr. Call F(r) the tension in the rod as a function of r. What is the force acting on the surface of the differential element at r + dr, and in which direction does it point? What is the force acting on the surface of the differential element at r, and in which direction does it point? What is the mass times acceleration of the differential element, and in which direction does it point? Write down your force balance equation for the differential element.

Chet

5. Aug 27, 2014

### Chacabucogod

2 Things:

The cross sectional area would be: 2*pi*r*2l no?

If a point mass is rotating about a point a distance l wouldn't the centripetal force be the F that I just mentioned?

Thank you

6. Aug 27, 2014

### Staff: Mentor

No. In this problem, r is not the distance from the axis of the cylinder. See the problem statement. r is measured along the length of the cylinder from its midpoint.

Regarding the force, your equation would be correct only if all the mass is concentrated at a point. However, in this problem, the mass is distributed along the length, and different parts of the rod are experiencing different accelerations. You need to take this into account.

Chet

7. Aug 28, 2014

### Chacabucogod

So you are telling me that the rod is rotating over a plane and not on its own axis? That's not how I understood it.

Thank you.

NVM now I get it.

8. Aug 28, 2014

### Chacabucogod

So from the side nearest to the midpoint the force is $mω^2r$ and farther there is $mω^2(r+dr)$ right?

9. Aug 28, 2014

### Staff: Mentor

No. From the side nearest the midpoint, it's F(r), and farther there is F(r+dr). Now, what is the differential mass times acceleration?

Chet

10. Aug 28, 2014

### Chacabucogod

Rho*a*dr*centripetal acceleration right?

11. Aug 28, 2014

### Chacabucogod

Would this be the sum of forces on an element dr?

$$-ρAdr(ω^2r)+ρAdr(ω^2(r+dr))$$

you end up with a Rho*...*dr^2 which I imagine you won't take into account and it will work right?

12. Aug 28, 2014

### Staff: Mentor

Nope. The force balance I get is:

$F(r+dr)-F(r)=-ρAdr(ω^2r)$

or equivalently:

$$\frac{dF}{dr}=-ρAω^2r$$

See what you get when you integrate this between arbitrary r and r = l.

Chet

13. Aug 31, 2014

### Chacabucogod

Alright! Now I understand it! Thank you for your help and patience.