# Need help :( I dun understand

• Nov 1st 2012, 07:50 AM
weijing85
Need help :( I dun understand
Hi guys,

Pls refer to the attachment.

I dun understand the following,
a) For the Damping Force, R1, why is it ''i'' but not ''-i''?
b) For the Damping Force, R2, why is it ''-i'' but not ''i''?
b) For the Spring Force, H, why is it ''i'' and not ''-i''?
c) Lastly, for Spring Force, H, why is it -k(x - lo)i?

Please explain to me in simple english, i asked my lecturer but i still don't understand :(

Rgds,
WeiJing
• Nov 1st 2012, 03:28 PM
HallsofIvy
Re: Need help :( I dun understand
The "damping force" is proportional to the velocity and always opposes it. The crucial point is NOT that R1 is "i" or "-i" but that it is "-x' i". It is horizontal, so "i" and it is opposite to the velocity, x', so -x' i. Similarly, R2 is opposite to the speed relative to position "B" which is x'- y'. "H" is the spring force. If the spring is compressed, that is if x is less than $l_0$, the force tends to "uncompress" and if the spring is stretched, that is if x is larger than [tex]l_0[tex] the force tends to "unstretch". In either case it is opposite to $x- l_0$ so proportional to $-(x- l_0)= l_0- x$