GCSE · Physics · AQA · Spec 8463
Resultant forces and resolving forces
When several forces pull on one object, the resultant is the one force that can replace them all.
Physics · Resultant forces
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Switch case ↑
Two equal and opposite forces give a resultant of 0 N — yet both forces are still acting on the box.
Switch between the three cases and watch how the single resultant force changes.
Find the single force with the same effect
Problem
Find the resultant force in two situations: (1) two horizontal forces on a box, 12 N to the right and 5 N to the left; (2) two forces, 4 N and 6 N, both acting to the right.
Predict, then check
A resultant of zero is not the same as no forces.
A box has an 8 N force acting left and an 8 N force acting right. Which statement is correct?
Physics · Resolving forces
Explore
Tap a part below to see which piece of the force it names.
The two perpendicular components together have the same effect as the original force.
AQA Higher tier only.
Physics · Resolving forces
Use the scale to construct Force B as 3.0 cm north of A. When the two forces are correctly drawn tip-to-tail, measure the single resultant from Start to the final tip and convert its length using the scale.
1 cm represents 1 N. Force A is already drawn as 4 N east. Complete the 3 N north vector, then measure the resultant.
AQA Higher tier only. Use the scale-drawing method for this specification.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What you need to know
- A resultant force is one force that has the same overall effect as all the individual forces acting together.
- Straight-line forces in the same direction: add their magnitudes.
- Straight-line forces in opposite directions: subtract their magnitudes and give the resultant the direction of the larger force.
- A force can be resolved into two component forces acting at right angles; together they have the same effect as the original force.
- A scale vector / free-body diagram can be used for non-collinear forces and to show equilibrium; equilibrium means the resultant is zero.
- For AQA Physics, the resolving and non-collinear scale-vector/free-body-diagram work above is Higher tier. Use the specified scale-drawing method; trigonometric sine/cosine resolution is not required on this page.
The big picture
The resultant force is a single force with the same overall effect as all the forces acting together. For two forces on the same straight line, add magnitudes when they act in the same direction and subtract when they oppose; the resultant points in the direction of the larger opposing force. A single force can also be resolved into two perpendicular components. For non-collinear forces, a scale vector or free-body diagram can be used to find the resultant or show equilibrium.
Key points
Worked example
Problem
Two horizontal forces act on an object: 14 N to the right and 9 N to the left. Find the resultant force.
⚠ Watch out
Adding two opposite forces as if they point the same way, or giving only a magnitude and forgetting the resultant's direction.
★ Exam tip
For AQA Physics, resolving a force and finding non-collinear resultants by scale-vector/free-body diagrams are Higher-tier skills. Use the scale-drawing method specified for this topic; do not introduce sine/cosine resolution here.
Memory hook
Same way: add. Opposite ways: subtract. Angled forces: keep the directions and draw the vectors to scale.
Check yourself
Two horizontal forces act on an object: 18 N to the left and 11 N to the right. What is the resultant force?
Flashcards
(12)What is a resultant force?
Two forces act in the same direction: 8 N and 5 N. What is the resultant?
Two opposite forces are 12 N right and 5 N left. What is the resultant?
Two opposite forces are both 20 N. What is their resultant?
When finding a straight-line resultant, what should you do with forces in opposite directions?
What does it mean to resolve a force?
How are the two components used when a force is resolved in this topic?
What can a scale vector diagram be used to find?
What does equilibrium mean for the resultant force?
What information should a force vector show in a scale drawing?
For this AQA Higher-tier force work, what method should be used for non-collinear resultants?
Does this AQA lesson require resolving forces with sine and cosine?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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