A force is a push or a pull that can change the speed, direction, or shape of an object. Forces are measured in newtons (N). Friction is a contact force that opposes motion between surfaces in contact. Understanding forces and friction is a core part of KS3 physics and the foundation of Newton's laws of motion.
What is a force?
A force is an interaction between two objects that can:
- Change the speed of an object (accelerate or decelerate it)
- Change the direction of an object
- Change the shape of an object (squash, stretch, bend)
Forces are vector quantities — they have both magnitude (size) and direction. They are represented on diagrams by arrows: the length of the arrow shows the size of the force, and the direction of the arrow shows which way it acts.
Forces are measured in newtons (N), named after Sir Isaac Newton. A force of 1 N is roughly the weight of a small apple.
Contact forces vs non-contact forces
Forces are classified based on whether the objects need to touch:
Contact forces
These only act when objects are touching:
- Friction — opposes relative motion between surfaces
- Air resistance (drag) — the friction-like force between an object and the air
- Tension — the pulling force in a rope, string, or cable
- Normal contact force — the force a surface exerts perpendicular to an object resting on it (also called the normal reaction)
- Upthrust — the upward force a fluid exerts on a submerged or floating object
Non-contact forces
These act across a distance, with no touching required:
- Gravity (gravitational force) — attraction between any two objects with mass
- Magnetic force — attraction or repulsion between magnetic poles
- Electrostatic force — attraction or repulsion between electrically charged objects
What is friction?
Friction is the contact force that resists or opposes the relative motion (or tendency to move) of two surfaces in contact. It always acts in the opposite direction to motion.
Friction arises because no surface is perfectly smooth — at a microscopic level, even polished surfaces have tiny ridges and bumps that interlock. When one surface moves across another, these microscopic irregularities resist the motion.
When is friction useful?
- Walking and running — friction between shoe soles and the ground stops your feet from sliding.
- Car brakes — friction between brake pads and disc rotor slows the wheel.
- Writing — friction between a pen nib and paper allows the pen to deposit ink rather than glide over the surface.
- Holding objects — friction between your fingers and an object prevents it from slipping.
When is friction a problem?
- Machinery — friction between moving parts generates heat, wastes energy, and causes wear. Engineers use lubricants (oil, grease) to reduce friction.
- Vehicle tyres — friction causes tyre wear; worn tyres provide less friction, increasing braking distance.
What is the resultant force?
When more than one force acts on an object, the resultant force is the single force that has the same effect as all the individual forces combined.
Balanced and unbalanced forces
- If the resultant force is zero (forces are balanced), the object will remain stationary or continue moving at a constant speed in a straight line (Newton's First Law).
- If the resultant force is not zero (forces are unbalanced), the object will accelerate in the direction of the resultant force (Newton's Second Law).
A worked example: forces on a car
A car travels at constant speed along a motorway. Its engine produces a driving force of 3,000 N forwards. Friction and air resistance produce a total drag of 3,000 N backwards.
- Resultant force = 3,000 N − 3,000 N = 0 N
- Because the resultant force is zero, the car continues at constant speed (no acceleration).
Now suppose the driver accelerates: the engine produces 4,500 N, but drag is still 3,000 N.
- Resultant force = 4,500 N − 3,000 N = 1,500 N forwards
- The car accelerates in the direction of the resultant force.
Newton's Three Laws of Motion (KS3 overview)
Newton's First Law
An object will remain stationary or continue to move in a straight line at a constant speed unless acted upon by a resultant (unbalanced) force. This is called inertia.
Example: A book on a table stays still because gravity (downward) and the normal contact force (upward) are balanced — the resultant force is zero.
Newton's Second Law
The acceleration of an object is directly proportional to the resultant force and inversely proportional to the mass:
Force (N) = mass (kg) × acceleration (m/s²)
F = ma
Example: A resultant force of 20 N acts on a 4 kg trolley.
Acceleration = 20 ÷ 4 = 5 m/s²
Newton's Third Law
When object A exerts a force on object B, object B exerts an equal and opposite force on object A. Forces always come in pairs.
Example: A rocket burns fuel and pushes gases backwards (action). The gases push the rocket forwards (reaction). This is why rockets work in the vacuum of space, where there is nothing else to push against.
Measuring forces: the newton meter
A newton meter (sometimes called a force meter or spring balance) measures force. When a force is applied, a spring inside the meter stretches by an amount proportional to the force (Hooke's Law). The extension is read off a calibrated scale in newtons.
According to the Department for Education's Science Programmes of Study for Key Stage 3, pupils should be taught to use force diagrams, describe contact and non-contact forces, and explore the relationship between force, mass, and acceleration. BBC Bitesize KS3 Physics explains resultant forces and Newton's First and Second Laws with interactive examples.
Frequently asked questions
What is the difference between contact and non-contact forces?
Contact forces require objects to be touching — examples include friction, tension, and the normal contact force. Non-contact forces can act across empty space — examples include gravity, magnetic force, and electrostatic force.
What happens when the resultant force on an object is zero?
When the resultant force is zero, the forces are balanced. The object either remains stationary or continues moving at a constant speed in a straight line. This is Newton's First Law of Motion.
How does friction affect the stopping distance of a car?
Friction between the brake pads and wheels, and between tyres and road, slows the car. Greater friction means a shorter braking distance. Factors that reduce friction — wet roads, worn tyres, or ice — increase braking distance, which is why stopping distances are much longer in poor conditions.
What is the equation linking force, mass, and acceleration?
Force (N) = mass (kg) × acceleration (m/s²), or F = ma. To find acceleration, rearrange to a = F ÷ m. For example, a resultant force of 10 N on a 2 kg object produces an acceleration of 5 m/s².
What is Newton's Third Law in simple terms?
Newton's Third Law states that every action has an equal and opposite reaction. If you push against a wall, the wall pushes back against you with the same force. These pairs of forces always act on different objects — they do not cancel each other out.
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