Materials

Structure and properties - Ripping paper

Imagine we're reading a newspaper. We then proceed to take out our magnifying glass and look at the fibres of the paper we're holding. What we would see is long, thin stands lined up in the same direction.

We then proceed to rip the paper in two directions:

    

Firstly, let's think about what would happen when we rip the paper vertically, in the first direction. When we rip the paper this way we will find that it is incredibly easy, and a relatively straight rip can be obtained. That's because all we're doing is separating fibres. Think of the paper as a pack of cards, and each fibre as a single, individual card. When you apply a force that acts to separate the cards, you'll find it incredibly easy. Now we need to think what happens when we tear the paper in the other direction. We should find this a lot harder, because in this case what we actually have to do is break molecules in order to rip the paper, which requires a lot more energy. We therefore seldom see straight rips when we tear paper in this direction. In our deck of cards analogy, what we're doing is trying to separate the cards into two groups by ripping through 52 cards - this is obviously a lot harder than just separating them without having to rip through them.