What makes glue sticky ? James

Glues like the ones used at school are made up of long chains called polymers, which are a bit like spaghetti strands. When you glue 2 pieces of paper together the polymers get into all the microscopic cracks and crevices in the paper, (a bit like ink soaking into the porous surface of paper). As it dries and the water evaporates the polymer molecules are left in the cracks to hold the paper together. But not all glues work like this. Sometimes the glue contains a substance that temporarily melts the 2 surfaces you're trying to stick together, effectively welding them together when the glue dries. Superglue contains cyanoacrylate. When this substance comes into contact with water it forms chains which bond to the ends of the 2 things you're trying to stick together. As the glue dries the chains harden until they can't move.

March 2003

Why don't rockets leave big holes in the ground when they launch ? Lucy

When the rocket motors ignite, huge amounts of hot gas are forced from the back of the rocket. As the famous Cambridge physicist Isaac Newton observed in his 3rd law of motion, for every action there is an equal and opposite reaction, meaning that in the context of the rocket, the hot gas coming out of the back exerts the same force on the rocket, but in the opposite direction, pushing it forwards. The rocket isn't actually pushing on the ground to make it go along and although it doesn't produce big holes in the ground, things can get a bit scorched !

March 2003