In physics, you have to answer in the least significant figure in the question, or ONE better. This means that if in your question, the maximum your quantities are stated is upto 2 sf, you can answer in 2 sf or 3 sf, but not more or less. Teachers often tell students to use 3 sf in physics as well, and it works well for them because in most cases you can answer in 3 sf, thanks to this rule, BUT there will be exceptions. The mark scheme doesn't state all the possible forms of answers sometimes, so don't be daunted by the majority of them being 2 sf--they have a set standard for each exam and follow it, but their rules apply nonetheless.
