Why does a ball roll to a stop? Why are you pushed back into your seat when a car speeds up? The answers come from a few simple ideas about forces. A force is just a push or a pull, and once you understand how forces add up and change motion, a huge amount of everyday physics falls into place.
A force is a push or a pull on an object, measured in newtons (N). Forces have a size and a direction — pushing a door and pulling it are different because the direction is different. Gravity pulling you down, friction slowing a slide, and your hand pushing a trolley are all forces.
Forces are drawn as arrows: longer arrow means bigger force, and the arrow points the way the force acts. Getting into the habit of drawing the forces on an object is the single best way to understand a physics problem.
Usually more than one force acts on an object at once, and what matters is how they add up. When forces are balanced — equal and opposite — they cancel out, and the object either stays still or keeps moving steadily. A book resting on a table has gravity pulling down and the table pushing up, exactly balanced, so it stays put.
When forces are unbalanced, there's a leftover force in one direction, and *that* changes the motion: it speeds the object up, slows it down, or changes its direction. No change in motion means the forces are balanced; any change means they're not.
First: an object keeps doing what it's doing — staying still, or moving steadily in a straight line — unless an unbalanced force acts on it. This is why a rolling ball only stops because friction pushes against it, and why you lurch forward when a car brakes: your body 'wants' to keep moving.
Second: the bigger the unbalanced force, the more an object speeds up — and heavier objects are harder to get moving. A hard push accelerates a trolley more than a gentle one, and an empty trolley is easier to accelerate than a full one.
Third: every force comes in a pair. When you push on something, it pushes back on you just as hard, the opposite way. A swimmer pushes water backwards, and the water pushes the swimmer forwards; a rocket pushes gas down, and the gas pushes the rocket up.
Friction is a force that acts whenever surfaces rub, always pushing *against* motion. It's why a ball rolls to a stop and why you can walk without slipping. Friction can be a nuisance (slowing machines) or essential (brakes, grip) — it's not 'bad', just a force to account for.
Gravity is the pull between masses; on Earth it pulls everything toward the ground, giving objects their weight. Weight is a force (in newtons) and is different from mass (how much stuff is in an object). The same object has the same mass on the Moon but weighs less there, because the Moon's gravity is weaker.
Try these. Pick an answer to see whether it's right and why.