# Single Degree of Freedom Equation of Motion(Newtons Second Law)

1. Jan 8, 2012

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

Hello guys. Really need your helps. When I construct an equation(moment about point O), do I have to include the rod's mass also? Ive heard that I can neglect the rod's mass due to equilibrium equation. On the other hand, Ive also heard that I can only neglect mass if the point of mass is supported. Appreciate if you could show me the equation. Thanks in advance. * For some reason I cannot attached the image.*

2. Relevant equations

3. The attempt at a solution

2. Jan 8, 2012

### Simon Bridge

You have not supplied the geometry.
Is this a mass on the end of a rod, with the pivot at the opposite end of the rod to the mass?

If you suspect you cannot neglect the rods mass, do the calculation both ways and see what difference it makes.

There are two ways to attach an image - you can use the attachment manager, or host the image elsewhere and use the image tags. The attachment manager can be temperamental ... the trick I've found is to submit the post as soon as the attachment is uploaded, then edit the post.

These sorts of problems are not about finding the right equation and then plugging in the numbers, they are about your understanding of the physics of the situation. You can find a lot of equations for moment of inertial in wikipedia as well as notes on how to use them.

3. Jan 8, 2012

### Redbelly98

Staff Emeritus
The OP had posted the image elsewhere, so I am reposting it here:

[STRIKE]https://www.physicsforums.com/attachment.php?attachmentid=42534&d=1325989363[/STRIKE]

You may neglect the moment of inertia of the rod if it is significantly smaller than the moment of inertia due to other objects in the system -- in this case, the M.O.I. due to the mass M. Usually, a problem statement will say either (1) the rod has negligible mass, in which case you can neglect it's mass and M.O.I., or (2) provide you with the mass or M.O.I. of the rod, in which case it should be included in the calculations.

Hope that helps.

EDIT: image is in attachment

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Last edited: Jan 8, 2012
4. Jan 8, 2012

5. Jan 8, 2012

### Redbelly98

Staff Emeritus
Weird, I guess only Mentors and Admins can see images from deleted posts.

A valid image is now attached to my previous Post #3.

6. Jan 8, 2012

### Simon Bridge

Oh I see now - it's a weird mass-on-a-spring ... probably they only care about the lever-arm effect for the rod.

7. Jan 8, 2012

Ive gone through every material that I have. Please correct me if Im wrong. Based on the rod(attached image), we can create and equation (Moment about point O = I$\alpha$). From my understanding, We can cancelled out Moment about point O of rod's mass (mg*l/2) due to mass*acceleration effect on the other side of the equation. Please enlighten me.

8. Jan 8, 2012