energy and spring constant

[Flinthill84]

I need a little help on this one:

One end of a massless spring is welded to a flat surface, the other points upward. A mass of 1.0kg is gently set down on top of the spring until the spring is compressed by 17cm to a new equilibrium position. What is the spring constant?


I know that you can find the spring constant by using F=-kx but I don't know how to find it without knowing the force. If anyone could help me I would greatly appreciate it. Thankyou!!

[Tom Mattson]

Erm...you do know the force. The spring is compressed 17cm when a 1.0kg mass is placed on it. How much does 1.0kg weigh?

[Flinthill84]

I thought that force was mass times acceleration though?

[Tom Mattson]

Yes, and if you multiply the given mass by the acceleration of gravity, then what physical quantity does that represent?

[Flinthill84]

Force...so if I multiply the 1.0kg times 9.8m/s^2 I will get the force and therefore be able to calculate the spring constant

[Doc Al]

I thought that force was mass times acceleration though?
Newton's 2nd law says that the net force on an object equals mass times acceleration. The acceleration of the mass is zero, of course, so the net force on it is zero. The two forces acting on the mass--its weigh (pulling down) and the spring force (pushing up)--must exactly balance. So the spring force must equal the weight of that 1.0kg mass.

[Flinthill84]

alright I hope I am understanding this correctly...from what I understand that your telling me the force is just the 1.0kg? if that is correct I though force had to be in newtons?

[Doc Al]

The force is the weight of the 1.0kg mass. You can calculate the weight as Tom explained, by multiplying the mass by the acceleration of gravity. And it will be in Newtons.

[Flinthill84]

ok I understand now...thankyou very much!!!