Edexcel IGCSE Physics · Spec 1.6
Resultant Forces & Newton's First Law
Resultant forces and Newton's first law of motion.
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Resultant Forces & Newton's First Law
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Explained
Adding forces up, and what happens when they cancel
Objects rarely have just one force acting on them. The resultant force is the single force that would have the same effect as all of them together, and working it out is the first step in almost every mechanics question.
Finding the resultant
For forces along the same line, add those acting in the same direction and subtract those acting in opposite directions.
A car with 3000 N of driving force and 1200 N of resistance has a resultant of 1800 N forwards. If the resistance rises to 3000 N, the resultant is zero.
Always state a direction as well as a size, because force is a vector. A resultant of 1800 N means nothing without saying which way it points.
Newton's first law
An object stays at rest, or keeps moving at constant velocity, unless a resultant force acts on it.
The counterintuitive half is the second one. Constant velocity does not require a force. Something moving steadily has zero resultant force, exactly like something standing still. A force is needed to change motion, not to maintain it.
So a car travelling at a steady 30 m/s has a resultant force of zero, even though its engine is working hard. The driving force and the resistive forces are equal and opposite, and the engine is there to balance the friction, not to sustain the motion.
Balanced and unbalanced
Balanced forces mean a zero resultant, so velocity does not change: at rest it stays at rest, and moving it keeps the same speed in the same direction.
Unbalanced forces mean a resultant, so the object accelerates in the direction of that resultant. Accelerating includes speeding up, slowing down, and changing direction at constant speed, since all three change the velocity.
Terminal velocity
This is the best illustration of the whole idea. A skydiver leaves the aircraft with weight acting downwards and almost no air resistance, so the resultant is large and downwards and they accelerate.
As speed increases, air resistance increases. The resultant force therefore gets smaller, so the acceleration gets smaller, and the skydiver is still speeding up but less rapidly.
Eventually air resistance equals weight. The resultant is zero, the acceleration is zero, and the speed stays constant. That constant speed is the terminal velocity.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme takes exactly this situation and credits four separate ideas: that air resistance increases with increasing speed, that the resultant force decreases, that the acceleration decreases, and that the gradient of the graph decreases. It allows drag or friction in place of air resistance and condones slope for gradient.
Then it draws a distinction so fine it is worth reading twice. For the acceleration mark it allows the phrase velocity increases at a decreasing rate, and instructs the examiner to ignore accelerates at a decreasing rate.
The reason is that acceleration already means the rate at which velocity increases, so accelerating at a decreasing rate describes the rate of change of the acceleration, which is a different quantity and not what is happening. Saying the acceleration decreases is correct; saying it accelerates at a decreasing rate is not.
This is the kind of detail that separates a full mark answer from a nearly correct one, and it comes from thinking about what each word actually denotes rather than how it sounds.
Answering a forces question
Identify every force acting, work out the resultant with a direction, then say what that means for the motion. Zero resultant means constant velocity; a resultant means acceleration in its direction.
Drawing the forces as labelled arrows, with lengths roughly in proportion, makes this much harder to get wrong, and mark schemes credit force diagrams drawn with a ruler.
Spec 1.6
What you need to know
- Work out a resultant force
- State Newton's first law
- Explain balanced and unbalanced forces
Active recall
Quick check
Answer each question before opening the answer.
State Newton's first law of motion.
An object stays at rest, or moves at constant velocity, unless acted on by a resultant force.
What does a zero resultant force mean for an object's motion?
Its motion does not change — it stays still or keeps moving at constant velocity (forces are balanced).
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