By Thread Academy · 21 September 2026 · Physics
A force is simply a push or a pull. Every time you kick a football, push open a door or lift your school bag, you are applying a force. Forces are everywhere in physics, and understanding them is the key to understanding motion itself. This guide breaks down what forces are, the different types, and how to calculate them.
What is a force?
A force is a push or a pull that acts on an object. It can change the object's motion: it can start it moving, stop it, speed it up, slow it down or change its direction. A force can also change an object's shape, such as when you squash a piece of dough.
Forces are measured in newtons, named after Sir Isaac Newton. The unit symbol is N. To give you a feel for the scale, the weight of a medium-sized apple is about 1 newton.
Forces are vectors, which means they have both a size (magnitude) and a direction. Saying "a force of 10 newtons" is incomplete — you also need to say which way it pushes or pulls, for example "10 newtons to the right".
Contact and non-contact forces
Forces fall into two broad families.
Contact forces
A contact force acts only when two objects touch. Common examples include:
- Friction — the force that opposes sliding between two surfaces, such as tyres gripping a road.
- Air resistance — the force opposing motion through air, which grows as speed increases.
- Tension — the pull through a stretched rope, string or cable.
- Normal contact force — the upward push from a surface that supports an object, such as a table holding up a book.
Non-contact forces
A non-contact force acts at a distance, without the objects touching. The main ones are:
- Gravitational force — the pull between masses. On Earth, gravity pulls every object downward and gives it weight.
- Electrostatic force — the attraction or repulsion between charged objects.
- Magnetic force — the attraction or repulsion between magnets.
Resultant force
When several forces act on one object at the same time, they combine into a single overall effect called the resultant force. Forces pulling in the same direction add together; forces pulling in opposite directions partially cancel.
Suppose you push a box to the right with 20 N while friction pushes left with 8 N. The resultant force is 20 minus 8, which equals 12 N to the right. If two equal forces push in opposite directions, they cancel completely and the resultant force is zero — the object stays still or keeps moving at the same speed.
Worked example 1: Finding the resultant force
Question: A car engine provides a forward force of 3,000 N. Air resistance and friction together produce a backward force of 800 N. What is the resultant force on the car, and in which direction does it act?
Step 1: Identify the forces and their directions. Forward: 3,000 N. Backward: 800 N.
Step 2: Subtract the opposing force from the driving force, since they act in opposite directions: 3,000 minus 800 equals 2,200.
Step 3: The larger force decides the direction, so the resultant acts forward.
Answer: The resultant force is 2,200 N forward, so the car will accelerate forward.
Linking force and acceleration: F = ma
Newton's second law connects force to motion with the famous equation:
F = ma
Here F is the resultant force in newtons, m is the mass of the object in kilograms, and a is the acceleration in metres per second squared (m/s^2).
The equation tells us three useful things: a bigger force produces a bigger acceleration; a heavier object needs a bigger force to reach the same acceleration; and the acceleration is always in the same direction as the resultant force.
Worked example 2: Calculating force from mass and acceleration
Question: A 1,200 kg car accelerates at 2.5 m/s^2. What is the resultant force acting on it?
Step 1: Write down what you know. Mass m = 1,200 kg, acceleration a = 2.5 m/s^2.
Step 2: Choose the equation F = ma.
Step 3: Substitute the values: F = 1,200 times 2.5 = 3,000.
Step 4: State the answer with units.
Answer: The resultant force is 3,000 N in the direction of motion.
Worked example 3: Rearranging to find mass
Question: A resultant force of 500 N accelerates a trolley at 4 m/s^2. What is the mass of the trolley?
Step 1: Rearrange F = ma to make mass the subject: m = F / a.
Step 2: Substitute: m = 500 / 4 = 125.
Answer: The mass of the trolley is 125 kg.
A common mistake to avoid
Students often confuse mass and weight. Mass is the amount of matter in an object, measured in kilograms, and it is the same everywhere. Weight is the gravitational force acting on that mass, measured in newtons, and it changes if gravity changes. On Earth, weight equals mass times the gravitational field strength: W = mg, where g is about 10 N/kg on Earth.
Key takeaways
- A force is a push or a pull, measured in newtons (N), and it has both size and direction.
- Contact forces (friction, air resistance, tension) need touching; non-contact forces (gravity, electrostatic, magnetic) act at a distance.
- The resultant force is the single force that has the same effect as all the forces combined.
- Newton's second law, F = ma, links resultant force, mass and acceleration.
- Mass is measured in kilograms and stays constant; weight is a force measured in newtons and depends on gravity.