Spring Mechanics homework problem

[aviv87]

There are N weights, each of mass M, connected as shown.
At the moment of cutting the top spring, what will be the acceleration of the top and bottom weights?

Thanks ahead

 

[aviv87]

anyone?
my guess is that the one on the top will have a=ng and the one on the bottom a=0.
is that right?

 

[Doc Al]

Since your attachment is not yet viewable, you'll have to describe the arrangement to get any help. What work have you done?

 

[aviv87]

there's a spring connected to a ceiling. a mass M is connected to it, to her another spring is connected, another mass etc' for N masses.
ceiling
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/
\
/
M
\
/
\
M
\
/
\
M

Does this help?

 

[Doc Al]

my guess is that the one on the top will have a=ng and the one on the bottom a=0.

Sounds right to me. But no need to guess. Consider the forces on each mass, and the fact that the springs won't change lengths instantly.

 

[aviv87]

Sounds right to me. But no need to guess. Consider the forces on each mass, and the fact that the springs won't change lengths instantly.

Well, that's what I got, but my physics teacher said he thinks it's a=0 for the bottom one, but that the one on top would have a<g, but he wasn't sure.
That made me think I did something wrong...

 

[Doc Al]

Well, that's what I got, but my physics teacher said he thinks it's a=0 for the bottom one, but that the one on top would have a<g, but he wasn't sure.

I don't see how he could think that the one on top could have an acceleration less than g. If the only force on it were gravity, a = g. But you've also got a stretched spring pulling it down, so a > g. Ask him to explain his reasoning.

 

[aviv87]

I don't see how he could think that the one on top could have an acceleration less than g. If the only force on it were gravity, a = g. But you've also got a stretched spring pulling it down, so a > g. Ask him to explain his reasoning.

My thoughts exactly
Well, I guess he wasn't paying enough attention or something...
Thanks!