Forces & Motion
Explore how forces like gravity, friction, and air resistance affect the way objects move, speed up, slow down, and change direction.
What Is a Force?
A force is a push or a pull that acts on an object. Forces can make things start moving, stop moving, speed up, slow down, or change direction. Forces can also change the shape of an object (like squashing a ball).
Forces are measured in Newtons (N), named after the famous scientist Sir Isaac Newton.
Key Idea: You cannot see forces, but you can see their effects. When you kick a football, you apply a force that makes it move. When you catch it, you apply a force that makes it stop.
Types of Forces
There are several important forces you need to know about:
Gravity
A force that pulls objects toward each other. Earth's gravity pulls everything toward its centre. It is why objects fall when you drop them, and why we stay on the ground. The bigger an object, the stronger its gravitational pull.
Friction
A force that opposes movement when two surfaces rub together. Friction slows things down. Rough surfaces (sandpaper) create more friction than smooth surfaces (ice). Friction also produces heat.
Air Resistance (Drag)
A type of friction caused by air pushing against a moving object. The faster or larger an object, the more air resistance it experiences. Parachutes work by using air resistance to slow a fall.
Applied Force
A force that a person or object applies directly. When you push a shopping trolley or pull open a door, you are applying a force. Muscles, engines, and magnets can all apply forces.
Balanced and Unbalanced Forces
When more than one force acts on an object, the forces can be balanced or unbalanced.
Balanced Forces
Forces are equal in size but act in opposite directions. They cancel each other out.
Result: The object stays still or keeps moving at the same speed. A book sitting on a table has balanced forces (gravity pulling down, table pushing up).
Unbalanced Forces
One force is bigger than the other. The object will move in the direction of the larger force.
Result: The object moves to the right (net force = 10 N right). Kicking a ball creates an unbalanced force that makes it move.
Australian Example: When a surfer paddles onto a wave at Bondi Beach, the wave pushes them forward (unbalanced force). Once riding the wave at a steady speed, the forces become balanced.
Key Vocabulary
Force
A push or pull that can change an object's speed, direction, or shape. Measured in Newtons (N).
Gravity
A pulling force that attracts objects toward each other. Earth's gravity pulls everything toward its centre.
Friction
A force that opposes motion when two surfaces are in contact. It slows objects down and produces heat.
Air Resistance
A type of friction caused by air pushing against a moving object. Also called drag.
Worked Examples
Why does a ball eventually stop rolling on grass?
Step 1: When you roll a ball, you apply an applied force to start it moving.
Step 2: As it rolls, friction between the ball and the grass acts in the opposite direction.
Answer: Friction gradually slows the ball down until it stops. Grass creates more friction than a smooth floor, so the ball stops faster on grass.
A skydiver jumps from a plane. Why do they open a parachute?
Step 1: After jumping, gravity pulls the skydiver down, and they speed up.
Step 2: Opening the parachute creates a large surface area that increases air resistance.
Answer: The parachute dramatically increases air resistance (drag), which slows the fall to a safe landing speed.
Two teams are playing tug-of-war. Team A pulls with 200 N and Team B pulls with 200 N. Who wins?
Step 1: Team A pulls left with 200 N. Team B pulls right with 200 N.
Step 2: The forces are equal and opposite, so they are balanced.
Answer: Nobody wins -- the rope does not move. The forces are balanced, so there is no net force in either direction.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
What force pulls objects toward the centre of the Earth?
Question 2
A car is driving at a constant speed on a straight road. The forces acting on it are:
Question 3
Which surface would create the most friction on a sliding box?
Question 4
A feather falls more slowly than a rock because it has more:
Question 5
Forces are measured in which unit?
Key Concepts Summary
- ●A force is a push or pull that can change an object's motion or shape, measured in Newtons (N).
- ●Gravity pulls objects toward Earth; friction opposes movement between surfaces; air resistance slows moving objects.
- ●Balanced forces cancel each other out and produce no change in motion.
- ●Unbalanced forces cause an object to speed up, slow down, or change direction.
- ●Rough surfaces create more friction; larger surface areas create more air resistance.