Resultant ForcesOxford AQA IGCSE Physics: Revision notes
Section 1
What is Newton's Third Law?
Newton's Third Law states that when two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction.
- These forces act on different objects, not the same one
- They occur simultaneously as a pair — you cannot have one without the other
- Example: when you push against a wall, the wall pushes back on you with an equal and opposite force
Newton's Third Law pairs act on two different objects — they never cancel each other out on the same object.
Section 2
What is a resultant force?
A resultant force is a single force that has the same effect as all of the individual forces acting on an object combined.
- Forces acting in a straight line: add forces in the same direction, subtract forces in opposite directions
- Two coplanar forces not in a straight line: the resultant can be found by scale drawing (drawing the forces to scale and measuring the diagonal of the resulting parallelogram)
- If the resultant force is zero, the forces are said to be balanced
A force of 10 N to the right and a force of 4 N to the left gives a resultant force of 6 N to the right.
Section 3
How does resultant force cause acceleration?
A non-zero resultant force causes an object to accelerate. Acceleration is the rate of change of velocity:
a = Δv ÷ t
where Δv is the change in velocity and t is time. A deceleration is simply a negative acceleration (slowing down).
On a velocity-time graph:
- The gradient gives the acceleration
- The area under the graph gives the distance travelled
State clearly whether you are reading gradient (acceleration) or area (distance) from a velocity-time graph — examiners penalise mixing these up.
Section 4
What are Newton's First and Second Laws?
Newton's First Law: if the resultant force on an object is zero, a moving object continues at the same velocity (constant speed, same direction) and a stationary object remains at rest.
Newton's Second Law: the resultant force on an object equals its mass multiplied by its acceleration:
F = m × a
where F is resultant force (N), m is mass (kg), and a is acceleration (m/s²). For a fixed mass, greater resultant force produces greater acceleration.
A resultant force of 20 N acting on a 4 kg object gives an acceleration of 20 ÷ 4 = 5 m/s².
Must Know
- Newton's Third Law: interaction forces are equal and opposite, acting on different objects
- Resultant force = single force with the same effect as all forces combined
- A zero resultant force means balanced forces (no change in velocity)
- Acceleration a = Δv ÷ t; gradient of a velocity-time graph = acceleration; area under it = distance
- Newton's First Law: zero resultant force means no change in velocity
- Newton's Second Law: F = m × a
That's the notes covered.
Carry on to the next subtopic.