Forces

Forces explain why things start, stop and change direction. This topic takes the three ideas that cause the most confusion. It begins with mass and weight. Mass is how much matter an object has and does not change from place to place, while weight is the force of gravity on that mass and does change. Next is Newton's second law, F = ma. The key word is net: you must add up all the forces, taking direction into account, before dividing by mass. Using just one of the forces is the usual slip. The topic ends with Newton's third law. Every force comes in a pair, equal in size and opposite in direction, but the two forces act on different objects, which is why they never cancel each other out. Drawing a simple force diagram for every question makes all three pages easier, so practise that habit here. Label each arrow with what is pushing or pulling, and on which object.