Section 1 of 7What is a resultant force?
One force replacing many
- One force replacing many When several forces act on an object they can be added (as vectors) into a single resultant force, also called the net force.
- The resultant has exactly the same effect on the object as all the original forces together.
What it tells you
- What it tells you Its direction shows which way the object will accelerate.
- Its size shows how strongly the motion is being changed.
Higher tier link (HT only)
- Higher tier link (HT only) Resultant force connects to Newton's second law, F = m × a: the larger the resultant, the larger the acceleration of a given mass.
Section 2 of 7Balanced or unbalanced
| Type | Resultant force | Effect on motion | Example |
|---|---|---|---|
| Balanced | Zero | A stationary object stays still; a moving object keeps its speed and direction | A book resting on a desk; a car cruising at steady speed |
| Unbalanced | Not zero | The object accelerates, decelerates or changes direction | A car braking; a rocket lifting off |
Section 3 of 7Working out a resultant on a straight line
1
Choose one direction as positive, for example to the right.
2
Give each force a sign: forces to the right are positive, forces to the left are negative.
3
Add the signed values together.
4
A positive answer means the resultant acts to the right; a negative answer means it acts to the left; zero means balanced.
Section 4 of 7Worked case: tug of war
1Person A
- Person A Pulls 80 N to the left.
2Person B
- Person B Pulls 100 N to the right.
3Subtract
- Subtract 100 N - 80 N = 20 N, because the forces oppose each other.
4Resultant
- Resultant 20 N to the right, so the forces are unbalanced and the rope accelerates towards B.
Section 5 of 7Balanced forces in everyday objects
Book on a table
- Book on a table Its weight acts downwards and the contact force from the table acts upwards.
- They are equal and opposite, so the resultant is zero and the book stays at rest.
Constant speed
- Constant speed A cyclist moving steadily has a driving force equal to the resistive forces.
- No resultant force does not mean no motion - only no change in motion.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Resultant force | The single force equivalent to all the forces acting on an object; also called the net force. |
| Balanced forces | Forces that cancel exactly, giving a resultant of zero. |
| Unbalanced forces | Forces that do not cancel, so there is a non-zero resultant force. |
| Newton (N) | The unit of force. |
Section 7 of 7Exam technique
1
Believing an object must have a resultant force to keep moving - at constant velocity the forces are balanced.
2
Adding opposing forces instead of subtracting them.
3
Leaving out the direction of the resultant when the question asks for it.
4
Saying balanced forces mean the object is stationary - it may be moving at constant velocity.