ACARA v9 CONTENT DESCRIPTION “investigate and represent balanced and unbalanced forces, including gravitational force, acting on objects, and relate changes in an object’s motion to its mass and the magnitude and direction of forces acting on it”
Builds on earlier work with pushes and pulls as forces with a strength and a direction. Here those ideas become quantitative: forces add up, and the leftover, the net force, decides whether and how the motion changes.
Forces come in pairs of strength and direction
A force is a push or a pull. Every force has a size and a direction, so we draw it as an arrow: longer for a stronger force, pointing the way it acts. Several forces can act on one object at once. To predict the motion, we add them up, taking direction into account, to find the single net force that is left over.
Balanced or unbalanced?
Two pushes act on the box. When they cancel, the motion does not change; when they do not, it does.
Equal and opposite pushes balance. A balanced object stays at rest, or keeps moving at a steady speed in a straight line.
Balanced forces keep motion steady
When the forces on an object cancel out, the net force is zero and we say they are balanced. A balanced object does not change its motion: if it was still it stays still, and if it was already moving it keeps the same speed in the same straight line. Only an unbalanced force, a net force that is not zero, can change motion.
More mass, less acceleration
The same push moves a light object more than a heavy one. Acceleration is force shared out over mass.
The push (force) is the same, but the green line, the acceleration, shrinks as the mass grows. Double the mass and the same force gives half the acceleration.
The same force, a different result
An unbalanced force changes the motion of an object, but by how much depends on its mass. The greater the mass, the harder it is to change its motion, so the smaller the acceleration from the same force. Acceleration is the force shared out over the mass.
Gravity is a force toward Earth
Gravity pulls every object down. The more mass an object has, the larger that pull, its weight.
Weight is the pull of gravity on an object, measured in newtons. Tripling the mass triples the weight, but the direction never changes: always straight down, toward the centre of the Earth.
Friction, the force that fights motion
Not every force is a deliberate push. Wherever two surfaces rub together, friction acts. It always points opposite to the way the object is moving, slowing it down. A rough surface grips more strongly than a smooth one, so it produces a larger friction force and brings a sliding object to rest sooner.
Friction works against motion
Friction is a force that pushes back on a sliding object. A rougher surface gives more friction, so the object does not travel as far.
The smooth surface barely drags. The gold friction arrow is small, so the same push carries the box much further before it stops. Friction always points opposite to the way an object moves.
Balanced does not mean nothing is happening
In a tug-of-war both teams pull as hard as they can, yet if their pulls match, the rope does not move. The forces are balanced, the net force is zero, and the motion stays the same. The moment one team pulls a little harder, the forces become unbalanced and the rope finally moves toward the stronger side. Balanced forces can be large; what matters is whether they cancel.
Tug-of-war: when forces balance
Two teams pull a rope in opposite directions. Equal pulls cancel and nothing moves; a stronger pull drags the knot its way.
The two teams pull equally hard, so the forces are balanced. The net force is zero and the knot does not move from the line, even though both teams are straining.
Why this matters
Forces explain why things start, stop, speed up, slow down and turn, from a kicked ball to a falling apple to a car braking. Seeing forces as arrows that add to a net force, and relating that net force to mass, is the foundation for all of mechanics in later years.
Quick self-check
1. Two equal forces push a still box in opposite directions. What happens?
2. A box is pushed harder to the right than to the left. The forces are...
3. The same push acts on a light box and a heavy box. Which accelerates more?
4. In which direction does gravity pull an object on Earth?