# Mechinical vibration - simple

1. Feb 22, 2009

so u = -cos(t) + sqrt(3)sin(t)

so the Wo = 1, and R =2, but how do i find delta?

tan(del) = sqrt(3)/-1, but the answer has it as

u = 2cos(t-2pi/3)

i can't get the 2pi/3 no matter what i do...

thanks

2. Feb 22, 2009

### Gregg

If you follow the posting guidelines I think it will be easier for someone to answer you. The question doesn't seem to be very clear.

3. Feb 22, 2009

acutally, i found my mistake!

4. Feb 22, 2009

um one more question, what should be the unit for metric?

should i do it in kg, m, seconds, always?

thanks

5. Feb 22, 2009

### Gregg

yeah always use SI units

6. Feb 22, 2009

haha

ok, ONE LAST QUESTION (of this thread)..

how do i know when it's "weight" or "mass"? is it in the wordings of the problem?

thanks

7. Feb 22, 2009

### Gregg

Weight is a vector, a force. You can see that the dimensions of a force would be kg m/s^2 or Newtons. Mass is a scalar quantity and will be measured, if in SI units, in kg only.

F=MA

Weight = mg

where m is mass and g is the acceleration due to gravity

8. Feb 22, 2009