Energy Equation of two masses and spring

[pepster]

Hi all,

I'm having a problem with the following question.
I have attached the question and my working for the solution however, I was told that the force summation solution which I have used and which many others have used is incorrect - an energy equation needs to be used.

Basically we are given two masses joint by a spring with coefficients of spring stiffness and coefficient of friction. We need to find the force applied to the second mass to move the first mass.

Any help would be much appreciated.

Thanks,
pepster

 

[PeterO]

Hi all,

I'm having a problem with the following question.
I have attached the question and my working for the solution however, I was told that the force summation solution which I have used and which many others have used is incorrect - an energy equation needs to be used.

Basically we are given two masses joint by a spring with coefficients of spring stiffness and coefficient of friction. We need to find the force applied to the second mass to move the first mass.

Any help would be much appreciated.

Thanks,
pepster

I don't see a problem with your work?
The only difficulty I could see with your calculation is that Block B would have to be sliding - or at least have slid a little, before A will begin to move since the spring has to extended beffore it exerts a Force on Block A. As such we may have to use the dynamic friction for Block B rather than the static friction. However, if we were to pause with the spring stretched so that A is on the point of moving, and the net force on the now stationary Block B wa at the points of again moving it, then your answer looks good to me.
I can give you an example of what I was trying to describe there if necessary. Just ask.

ps: I can't see any need for an energy equation to be used.