Forces I

Pause for a second and look around. The chair pushes up on you. Gravity pulls everything downwards. A breeze nudges a curtain. Every one of these is a force — a push or a pull from one thing on another.

You cannot actually see a force. What you see is the effect of the force: something starts to move, stops, changes direction, or changes shape.

Force is measured in newtons (N), named after Sir Isaac Newton. We use a small spring scale called a newton meter (or force meter) to measure it. One newton is roughly the weight of a small apple held in your hand.

There are a few forces that appear in almost every MYP question and in everyday life:

• Gravity — a pulling force from a planet (like Earth). It pulls every object toward the centre of the Earth, which is why dropped things fall down. Gravity acts even without anything touching.
• Friction — the force that appears when two surfaces rub against each other. It always pushes against the direction of motion. Rough surfaces give more friction than smooth ones.
• Normal contact force (sometimes "reaction force") — the upward push a surface gives to anything resting on it. The floor pushes up on your feet exactly enough to stop you sinking through it.
• Upthrust — the upward push that a liquid (or gas) gives to anything in it. Upthrust is why a beach ball pops up when you let go of it underwater.

Forces can be split into contact forces (the objects must touch — friction, normal contact, upthrust) and non-contact forces (no touching needed — gravity, magnetism, electrostatic).

In MYP Sciences you'll often be asked to draw forces on an object. The rules are simple:

1. Draw the arrow starting from the object, pointing in the direction the force acts.
2. Make the length of the arrow match how big the force is — a bigger force = a longer arrow.
3. Label each arrow with the type of force (e.g. "weight", "friction") and, if you know it, the size in newtons.

Forces don't just move things — they can also change shape or change direction:

• A football is kicked: a force speeds it up and may change its direction.
• A car brakes: friction slows it down — a force changing speed.
• A sponge is squeezed: forces change its shape.
• A ball bounces off a wall: a force changes the ball's direction.

If two equal forces act on an object in opposite directions, they cancel out — we call this balanced. The object's motion doesn't change. If the forces are different sizes, they are unbalanced, and the object's motion does change.