Resultant forces (AQA GCSE Physics Combined Science): Revision Notes
Resultant forces
What is a resultant force?
When multiple forces act on an object, we can work out their combined effect by finding the resultant force. The resultant force is like having just one single force that does the same job as all the individual forces put together.
Resultant forces are the key to understanding motion - they tell us exactly what will happen to any moving object.
Resultant forces are important because they tell us what will happen to an object's motion. They determine whether something will:
- Stay still (stationary)
- Keep moving at the same speed
- Speed up or slow down (accelerate)
Forces are vectors
Forces are vectors, which means they have two important features:
- Size (how strong the force is, measured in Newtons)
- Direction (which way the force is pointing)
Both size and direction matter when we work out the resultant force. You can't just ignore the direction - a 10N force pushing left is very different from a 10N force pushing right!
How to calculate resultant forces
The way you combine forces depends on their directions:
Forces in the same direction
When forces act in the same direction, you add them together.
Worked Example: Forces in Same Direction
A 10N force and another 10N force both pushing right:
- Both forces act in the same direction (right)
- Resultant force = 10N + 10N = 20N to the right
Forces in opposite directions
When forces act in opposite directions, you subtract the smaller from the larger.
Worked Example: Forces in Opposite Directions
A 5N force pushing right and a 3N force pushing left:
- Forces act in opposite directions
- Resultant force = 5N - 3N = 2N to the right
- The resultant force acts in the direction of the larger force
Zero resultant force
When the resultant force equals zero, this means:
- All the forces are balanced
- The object will either stay still OR keep moving at a constant speed
- There is no acceleration
Worked Example: Zero Resultant Force
A 5N force pushing left and a 5N force pushing right:
- Forces are equal in size but opposite in direction
- Resultant force = 5N - 5N = 0N
- The forces cancel each other out completely
Non-zero resultant force
When there is a non-zero resultant force, this means:
- The forces are unbalanced
- The object will accelerate (speed up, slow down, or change direction)
- A resultant force is needed to make anything accelerate
Remember: Any time there's a non-zero resultant force, the object MUST accelerate. No resultant force means no acceleration!
Real-life example: Moving car
Think about a car driving at a steady speed:
Real-Life Example: Car at Constant Speed
When a car travels at steady speed:
- The engine creates a forwards force
- Air resistance creates a backwards drag force
- If these forces are equal, the resultant force is zero
- The car moves at constant speed (no acceleration)
When accelerating:
- Engine force becomes stronger than drag
- There's a non-zero resultant force forwards
- The car accelerates
Real-life example: Falling object
When a ball is dropped:
Real-Life Example: Falling Ball
During a ball's fall:
- Gravity pulls it downwards (typically around 10N for a small ball)
- Air resistance (drag) pushes upwards
- If gravity is stronger than drag, there's a downward resultant force
- The ball accelerates downwards towards the ground
Key Points to Remember:
- Resultant force is the combined effect of all forces acting on an object
- Forces have both size and direction (they are vectors)
- Same direction forces: add them together
- Opposite direction forces: subtract to find the resultant
- Zero resultant force = no acceleration (object stays still or moves at constant speed)
- Non-zero resultant force = object will accelerate