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# Re: Spring Problem

• From: Leigh Palmer <palmer@sfu.ca>
• Date: Wed, 29 Oct 1997 17:16:00 -0800

I have a question that, on the surface, appears so simple, I'm
embarrased to ask it. But it's something that has me stumped. Maybe there is
something obvious that I'm overlooking.
We have a horizontal, unstretched spring connected at one end by a
rigid support. I want to apply a force at the other end to stretch the
spring.
By the 3rd law, the force I exert to the right (F=kx) will always be equal in
magnitude to the force that the spring exerts back to the left (F=-kx). In
this case, how can there ever be an acceleration of the end of the spring?

No. You must exert a force slightly greater than the elastic force to get
it started. Once it is expanding at uniform velocity every particle in it
is moving at uniform velocity and the force required will be equal to the
elastic force. When you reach the end of the motion you will decelerate
by reducing the force briefly below the elastic value.

Leigh