Springs in series and parallel : Problem for U

sss_1981

I came across an objective question on the topic 'Springs in series and parallel' .. Have i asnwered it correctly?

Q. A Force of 5Kg compresses 2 springs in series by 10 cm. What will be the compression when the same force acts on the same 2 springs but when the springs are in parallel?

Choices >>
1) 5 cm
2) 10 cm
3) 2.5 cm
4) 20 cm

MY ANSWER >> 2.5 cm

Please correct me if I am wrong..

pcdagr8

You are correct !
though it should have been mentioned that the springs are identical but as nothing is mentioned we can take them to be identical

sss_1981

Thanks buddy...

Oh yes, U r spot on.. I forgot to mention that the springs are identical..

The logic I followed was >>

In series,
* The compression is 10 cm
* This means each spring compresses by 5cm
* Since the springs are in series, both the springs experience the full 5 kg
force
* So, each spring compresses 1 cm for 1 kg (Spring constant=1)

In parallel,
* Now, the force is divided between the 2 springs
* So, force acting on each spring is 2.5 N
* So, in this case, the spring compresses to 2.5xSpring constant(1) = 2.5 cm

Hope I am right with my logic too.. Sorry, I should have mentioned this earlier

pcdagr8

most welcome

well....your logic seems correct but some problem in units.

u say spring constant is 1??
if its 1 it must be 1 cm/kg wt but in si it will be not be so

but later you say the force on each spring is 2.5 N it can't be its 2.5kg wt

otherwise your soln is correct

n btw ...
if you are preparing for some objective type examinatiom
then you can use
for series
1/k=1/k1+1/k2
and parallel
k=k1+k2
where k is equivalent spring constant

sss_1981

Thanx chap..

(Ooops, the 2nd force that I mentioned is 2.5 KG (Not Newton) ... by mistake I wrote it as 2.5N)