What to write in discussion and conclusion?
Also how to answer how to make this experiment more accurate/reliable/valid questions?
In discussion write about how the experiment is accurate/reliable/valid
talk about each one, make a judgement to whether it was that or not, and make sure you reference how it could be improved
conclusion
talk about how the aim was fulfilled (dont literally write that)
eg "The experimental value of gravitational acceleration in this experiment was calculated (as stated in the aim) as g≈9.16 ms^(-2). From the results of the experiment it can be determined that in a pendulum motion the period squared is directly proportional to the length of the pendulum. In addition when the inverse gradient of that relationship is multiplied by the coefficient of 4π^2 an experimental value for gravitational acceleration can be obtained. It is therefore evident that if the experiment was repeated with a string smaller than 0.5m, a value for T2 smaller than 1.99 sec would be obtained."
Bascially sum up your results in relation to your aim, and then use the result to make an assertion (read: conclusion) regarding the experiment. as above i make the assertion that based off the results we can say that the square of the period is proportional to the length
about the theory, you probs wont need it to answer any questions BUT you should have a very thorough understanding of it anyways. it helps you spot mistakes and can help you predict specifically what they are looking for with each question
also knowing theory well can often mean you don't even need to do the experiment because you already know what should happen