# Forced oscilation/position

• qasbrv

#### qasbrv

Homework Statement
A spring is stretched 2m by a mass of 2 kg. The body is immersed in a viscous fluid with the damping coefficient having 4 Ns / m which is driven by an external force 4cos (2t). Plot mass position versus time. g = 10 m / s2.

The attempt at a solution
I know first I have to find an equivalent form as that of forced oscilations
x⋅⋅+2γx02x = F×cos(ωt)/m
but I can't figure out because I don't have an elasticity const k or a frequency. I think I don't need one, but I don't know how to start.

Don't you need the spring constant to find the natural frequency?
Is there any other information supplied? Maybe in the context?

Don't you need the spring constant to find the natural frequency?
Is there any other information supplied? Maybe in the context?

I thought that too, but that's all; nothing else given. Maybe the problem has some bugs.

I will
Thank you anyway!

What do you suppose "a spring is stretched 2m by a mass of 2 kg" means?

Simon Bridge and qasbrv
It means that the maximum elongation x (amplitude) is exerted by the gravitational force =m*g ...so
-m*g=-kx
k=m*g/x= 10 N/m xD

Thank you very much!

Ooh... I missed it too, well done.

Poorly worded problem.
Is the mass subjected to the force 4cos (2t) directly, or is the force exerted on the mass indirectly by applying the force to a container containing the mass and viscous fluid? If the latter the mass of the fluid + containing vessel would also have to be specified.