Forces, Resultant Force and Newton’s Laws
A force is a push or a pull, measured in newtons (N). Force is a vector, so direction matters — and that is why forces can cancel out.
1. What a force can do
A force can change an object’s speed, its direction, or its shape.
Those three effects cover every force question at IGCSE. A force can start something moving, stop it, speed it up, slow it down, turn it, or deform it.
2. Resultant force
The resultant force is the single force that has the same effect as all the forces acting on an object.
Add forces along the same line using signs; use Pythagoras for forces at right angles.
Example: 10 N right and 8 N left → resultant = 2 N to the right.
A resultant is not “a force acting opposite to the original”. That was a recorded misconception. It is the net effect of everything acting — the one force you could replace them all with.
Always give the direction of a resultant force.
Balanced vs unbalanced:
| Resultant | Effect |
|---|---|
| Zero (balanced) | stays at rest, or carries on at constant velocity |
| Not zero (unbalanced) | accelerates in the direction of the resultant |
Zero resultant force does NOT mean the object stops. This was one of the most-recorded misconceptions in the topic. A car at a steady 30 m/s has zero resultant force. Balanced forces mean no change in motion, not no motion.
3. Newton’s first law
An object stays at rest, or moves at constant velocity in a straight line, unless acted on by a resultant force.
This is sometimes called the law of inertia — the tendency of an object to resist changes in its motion. Mass is a measure of inertia: a more massive object is harder to speed up, slow down or turn.
4. Newton’s second law
F = m a resultant force (N) = mass (kg) × acceleration (m/s²)
Example: a 1200 kg car accelerating at 2.5 m/s².
- F = 1200 × 2.5 = 3000 N
Example: a 5 kg mass with a 20 N resultant force.
- a = F/m = 20 ÷ 5 = 4 m/s²
F is the RESULTANT force, not one of the individual forces. If a 100 N push meets 40 N of friction, use 60 N in F = ma.
Mass must be in kilograms. Converting grams to kilograms was a recorded error, and it changes the answer by a factor of 1000.
A decelerating force comes out negative — that is correct, and it simply means the force opposes the motion. Confusion over using F = ma for deceleration was recorded; use the negative acceleration and interpret the sign.
F = ma is not Hooke’s law. Confusing the two was recorded — Hooke’s law is F = kx, about springs and extension.
5. Newton’s third law
For every action there is an equal and opposite reaction.
The two forces:
- are equal in size
- act in opposite directions
- are the same type of force
- act on two different objects
The two forces act on DIFFERENT objects — that is why they never cancel. If they acted on the same object nothing could ever accelerate. A book on a table pushes down on the table; the table pushes up on the book.
Don’t confuse the first and third laws. A recorded error: the first law is about a single object with balanced forces; the third is about a pair of objects exerting forces on each other.
6. Free body diagrams
A diagram showing all the forces acting on one object, drawn as arrows from the object.
The usual four:
- Weight — always straight down
- Normal contact force (reaction) — perpendicular to the surface, usually up
- Friction / drag — opposing motion
- Thrust / applied force — in the direction of the push
Drag and friction always act OPPOSITE to the direction of motion. Misunderstanding the direction of drag was a recorded error. If the object moves right, drag acts left.
Arrow length should show relative size, and each arrow must be labelled with the force’s name.
Check your assumptions about angles. A recorded error assumed a right angle where the geometry didn’t give one. If forces are at an angle other than 90°, you may need the cosine rule rather than Pythagoras — though most 0625 questions keep to right angles or straight lines.
7. Common situations
Terminal velocity. A falling object accelerates; as speed rises, air resistance increases; when air resistance equals weight, the resultant is zero and it falls at constant velocity.
At terminal velocity the object is still moving — steadily, not stopped.
A rising balloon. Removing ballast reduces the weight, so the upward resultant increases and the balloon accelerates upwards more.
Objects on a rope. The tension acts along the rope, pulling the object. If the rope is cut, the tension disappears — you don’t subtract it, it is simply gone, leaving weight (and drag) acting.
When a rope is untied, remove the tension entirely rather than subtracting it as an ongoing force. This exact confusion was recorded.
8. Mistakes that cost marks
Thinking zero resultant force means stationary.
Defining resultant as an opposing force.
Using an individual force in F = ma instead of the resultant.
Leaving mass in grams.
Confusing Newton’s first and third laws.
Saying an action–reaction pair acts on the same object.
Drawing drag in the direction of motion.
Unlabelled arrows on a free body diagram.
Confusing F = ma with F = kx.
Assuming a right angle in a force triangle.
Omitting units — force in N.
Frequently asked questions
What can a force do? Change an object’s speed, direction or shape.
What is the resultant force? The single force with the same effect as all forces acting.
What happens when the resultant force is zero? The object stays at rest or moves at constant velocity.
What is Newton’s first law? An object keeps its velocity unless acted on by a resultant force.
What is Newton’s second law? F = ma — resultant force equals mass times acceleration.
What is Newton’s third law? Every action has an equal and opposite reaction, on a different object.
Why don’t action and reaction cancel out? Because they act on two different objects.
Which way does friction act? Opposite to the direction of motion.
What is terminal velocity? The constant velocity when air resistance equals weight, giving zero resultant force.
What is inertia? An object’s resistance to a change in motion, measured by its mass.
Quick revision checklist
- I know the three effects of a force
- I can find a resultant force and give its direction
- I know zero resultant means rest or constant velocity
- I can state Newton’s first law
- I can use F = ma, with the resultant force and mass in kg
- I can handle deceleration and interpret a negative force
- I can state Newton’s third law and the four properties of the pair
- I know the pair acts on different objects
- I can draw a labelled free body diagram
- I draw drag opposing motion
- I can explain terminal velocity in terms of forces
- I don’t confuse F = ma with Hooke’s law
- I give units in newtons
These notes cover forces, resultant force and Newton’s laws in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
Finished this topic?
Saved on this device — no account needed.