Resultant forces — AQA GCSE Physics
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Resultant forces explained
The single force that has the same effect as all the forces acting on an object is called the resultant force.
Read the full explanation
Its size and its direction together decide what the object does. If the resultant force is zero the object either stays at rest or carries on at constant velocity; that is the case for a hailstone at terminal velocity, where air resistance has grown until it equals the weight. If the resultant force is not zero the object accelerates in the direction of that resultant, which is why a hailstone accelerates at the start of its fall, when its weight is greater than the air resistance. Because the resultant is a vector, an answer is only complete when it gives both a size in newtons and a direction.
Your focus
- State what a zero resultant force implies about an object's motion, covering both possible cases.
- Explain why a falling hailstone first accelerates and then reaches terminal velocity, in terms of the resultant force.
- Give a resultant force as a complete vector answer, with size and direction.
Resultant forces exam tips
Marking Points
- one mark for naming the single equivalent force the resultant force
- one mark for a resultant force of zero meaning the object stays at rest or moves at constant velocity
- one mark for a non-zero resultant force meaning the object accelerates in the direction of the resultant
- one mark for applying this, such as a hailstone accelerating because there is a resultant force, its weight being greater than the air resistance
Examiner Tips
- 💡For 'why does it accelerate', the one-mark answer is that there is a resultant force acting; adding which force is larger makes it safe.
- 💡Use 'constant velocity' rather than 'not moving' when the resultant is zero, since both cases are allowed.
- 💡Quote a direction with every resultant, even when the arithmetic feels obvious.
Common Mistakes
- giving the name 'resultant force' when the question asks for its size and direction
- saying a resultant force of zero means no forces act on the object
- claiming an object at terminal velocity has stopped moving
- assuming the resultant force always points in the direction the object is travelling
- treating the resultant force as a separate physical force pushing the object in addition to the real ones