Resultant forces (AQA GCSE Physics): Revision Notes
Resultant forces
What is a resultant force?
A resultant force is the single force that has the same effect as all the other forces acting on an object combined together. Think of it as the overall force after you've combined all the individual forces.
Understanding resultant forces is fundamental to predicting motion. Instead of tracking multiple individual forces, we can replace them all with one equivalent force that produces the same effect.
Resultant forces tell us what will happen to an object:
- Will it stay still?
- Will it move at a steady speed?
- Will it speed up or slow down?
Forces as vectors
Forces are vectors. This means they have two important features:
- Size (how strong the force is, measured in Newtons)
- Direction (which way the force is pointing)
Both the size and direction matter when working out the resultant force. You can't just add up the numbers - you must consider which way each force is pointing!
How to calculate resultant forces
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 a 20N force both pushing to the right:
- Resultant force = 10N + 20N = 30N to the right
Forces in opposite directions
When forces act in opposite directions, you subtract the smaller force from the larger force.
Worked Example: Forces in Opposite Directions
A 5N force pushing left and a 3N force pushing right:
- Resultant force = 5N - 3N = 2N to the left
Zero resultant force
When the resultant force equals zero, the forces are balanced. This means:
- If the object is stationary, it stays stationary
- If the object is moving, it keeps moving at the same speed in the same direction
Worked Example: Balanced Forces
A car with 200N engine force forwards and 200N drag force backwards has a resultant force of zero. The car travels at constant speed.
Calculation: 200N - 200N = 0N (no resultant force)
Unbalanced forces
When there is a resultant force (not zero), the forces are unbalanced. This means:
- The object will accelerate (speed up, slow down, or change direction)
- A bigger resultant force causes more acceleration
Worked Example: Unbalanced Forces
A falling ball has weight pulling it down and air resistance pushing up. If weight is greater than air resistance, there's a downward resultant force, so the ball accelerates downward.
For instance: Weight = 8N down, Air resistance = 3N up Resultant force = 8N - 3N = 5N downward
Critical Concept: Objects need an unbalanced force to change their motion. Without a resultant force, an object's motion will not change - it will either remain stationary or continue moving at constant velocity.
Key principles to remember
Essential Physics Principles:
- Objects need an unbalanced force to change their motion
- Balanced forces (zero resultant force) mean no change in motion
- The direction of the resultant force shows which way the object will accelerate
- Forces in the same direction get added together
- Forces in opposite directions get subtracted
Key Points to Remember:
- Resultant force = the single force that replaces all other forces acting on an object
- Same direction forces = add them together
- Opposite direction forces = subtract the smaller from the larger
- Zero resultant force = object stays at rest or moves at constant speed
- Non-zero resultant force = object accelerates in the direction of the resultant force