We've been dabbling a lot in the mysterious world of quantum physics lately, so to get back down to Earth we thought we'd bring you reminder of good old classical physics.

Mathematics of Planet Earth 2013 is a year-­long international effort highlighting the contributions made by mathematics in the study of global planetary problems: migrations, climate change, sustainability, natural disasters, pandemics... and in the search for solutions.

Solving equations often involves taking square roots of numbers and if you're not careful you might accidentally take a square root of something that's negative. That isn't allowed of course, but if you hold your breath and just carry on, then you might eventually square the illegal entity again and end up with a negative number that's a perfectly valid solution to your equation.

Science advisors to government are an embattled lot. Remember the l'Alquila earthquake debacle or David Nutt's stance on drugs which cost him his job. Bridging the gap between politics and science isn't easy. Politicians like clear messages but science, and the reality it tries to describe, is rarely clear-cut.

Modelling the spread of disease is a difficult business. Epidemiologists use incredibly complex models involving huge amounts of transport, social contact and disease data to predict the spread of diseases. But is there a way to hide all this complexity and draw a simpler picture of how diseases spread, even in today's complex world?

Sequences of numbers can have limits. For example, the sequence 1, 1/2, 1/3, 1/4, ... has the limit 0 and the sequence 0, 1/2, 2/3, 3/4, 4/5, ... has the limit 1. But not all number sequences behave so nicely. Can we still discern some sort of limiting behaviour?

Yesterday we opened the Plus New York office, amidst snow covered streets at the foot of the Empire State building.