Forces and interactions

1. A force is a push or a pull that one object exerts on another object.
2. Forces are measured in newtons, which have the symbol $N$.
3. A force can make an object speed up, slow down, change direction or change shape.
4. Forces never come from a single object on its own, because every force is produced by an interaction between two objects.
5. When you kick a football, the force on the ball comes from the interaction between your foot and the ball.
6. When an apple falls from a tree, the force that pulls it down comes from the interaction between the apple and the Earth.
7. Interactions are sorted into two groups depending on whether the objects need to touch.
   1. Non-contact interactions act across a gap, so the objects can be separated.
   2. Contact interactions act only while the surfaces of the objects are touching.

Non-contact forces

1. A non-contact force acts between objects that are not touching, so it can act across empty space.
2. There are three kinds of non-contact interaction that you need to recall: gravitational, electrostatic and magnetic.
3. Gravitational interactions happen between any two objects that have mass.
   1. The gravitational force is always attractive, so it pulls the two objects towards each other.
   2. The gravitational pull of the Earth on an object near its surface is the object's weight, which acts downwards towards the centre of the Earth.
   3. Gravity also holds the Moon in orbit around the Earth and the Earth in orbit around the Sun.
4. Electrostatic interactions happen between objects that carry an electric charge.
   1. Objects with opposite charges, one positive and one negative, attract each other.
   2. Objects with like charges, both positive or both negative, repel each other.
   3. A balloon rubbed on a woollen jumper becomes charged and can then attract small pieces of paper or stick to a wall.
5. Magnetic interactions happen between magnets, or between a magnet and a magnetic material such as iron, steel, nickel or cobalt.
   1. Unlike poles (a north pole and a south pole) attract, while like poles (two north poles or two south poles) repel.
   2. A magnet always attracts a magnetic material that is not itself a magnet, which is how a fridge magnet holds a shopping list in place.
6. Gravitational forces can only attract, while electrostatic and magnetic forces can either attract or repel.
7. Each non-contact force gets weaker as the distance between the objects increases.

Contact forces

1. A contact force acts only when the surfaces of two objects are touching each other.
2. The normal contact force is the push that a surface exerts on an object pressing on it.
   1. The word normal means perpendicular, so this force always acts at right angles to the surface.
   2. A mug resting on a desk is held up by an upward normal contact force from the desk.
   3. A ladder leaning against a wall feels a horizontal normal contact force from the wall, because the wall surface is vertical.
3. Friction is the force that opposes the movement, or the attempted movement, of one surface sliding over another.
   1. Friction acts parallel to the surfaces in contact, in the opposite direction to the sliding.
   2. Even surfaces that feel smooth have tiny bumps that catch on one another, and this is what produces friction.
   3. Friction between your shoes and the pavement lets you walk, and friction between a brake pad and a bicycle wheel slows the bike.
4. Air resistance is a type of friction that acts on an object moving through air, and it gets larger as the object moves faster.
5. A similar resistive force, called drag, acts on objects moving through water or any other liquid.
6. Two further contact forces are common in force problems.
   1. Tension is the pull exerted by a stretched string, rope or cable on the object attached to it.
   2. Upthrust is the upward push of a liquid or gas on an object that is partly or fully immersed in it.