Hooke’s Law

For question 7 I get 3.33 but the book says 33.3 but I don’t see where I’ve gone wrong?

I did (125*0.2^2)/(2*0.75)=3.33. Where have I gone wrong?
https://imgur.com/a/s0O6bLH

You worked out the energy, not the tension./

I thought that tension was the force that was opposing the force compressing the spring?

It is... I'm saying you worked out the wrong quantity.

I’m a bit confused now. What’s the quantity that I need to find then? I know the formula I’m meant to use is (lamda*x)/L but that’s for tension and I thought I didn’t want tension?

Why wouldn't you want tension?

The spring is in equilibrium with some force pressing down on it and the spring reciprocates it with equal tension... which we know how to find.

Well that would make sense but how do you know it’s in equilibrium?

This is the type of thing that should be intuitive. We are not told explicitly that it's in equilibrium, but just think about pushing down on a spring.

If you push down on it with constant force, then it will compress and at some point you won't be able to push any more because the tension builds up the more it compresses. It is at that point when the tension is equal to your hand's force, and hence equilibrium is achieved.

Increase your hand's force, and you can compress further but you will stop at some point again as the tension will build up again to make up for it.