Terminal Velocity and Newton's Laws of Motion — AQA GCSE Physics
Newton's laws describe how forces affect motion, and explain why falling objects reach a terminal velocity.
Newton's laws of motion
- First law: if the resultant force on an object is zero, it stays at rest or moves at a constant velocity. A change in velocity (speeding up, slowing down, or changing direction) requires a resultant force.
- Second law: the acceleration is proportional to the resultant force and inversely proportional to the mass: $$F = m \times a$$
- Third law: when two objects interact, they exert equal and opposite forces on each other.
Inertia
Inertia is the tendency of an object to continue in its state of rest or uniform motion. Inertial mass measures how difficult it is to change an object's velocity (mass = force ÷ acceleration).
Terminal velocity
When an object falls through a fluid (e.g. air):
- At first it accelerates due to its weight (gravity).
- As it speeds up, air resistance (drag) increases.
- Eventually, air resistance equals the weight, so the resultant force is zero.
- The object then falls at a constant maximum speed — the terminal velocity (no further acceleration).
Exam tips
- First law: constant velocity needs zero resultant force.
- Learn F = ma (second law) and practise rearranging.
- Third law: forces are equal and opposite.
- Explain terminal velocity using balanced forces (weight = air resistance).