IGCSE Physics / Forces and movement
Falling objects and terminal velocity
Edexcel International GCSE Physics (4PH1), chapter 3: Forces and movement, specification point 1.21. Free lesson.
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A falling object speeds up as gravity pulls it down, but increasing air resistance reduces its acceleration. It reaches terminal velocity when air resistance balances its weight, so it continues falling at a steady speed.
How the forces change as an object falls
A falling object has weight acting downwards and air resistance acting upwards. Weight stays the same, but air resistance grows as the object’s speed rises. This makes the resultant force, the overall force after combining the forces, get smaller. When the object is released, it is not moving, so there is no air resistance. The resultant force is its weight and it accelerates downwards. As it speeds up, air resistance increases. The resultant force and the object’s acceleration decrease, although it is still speeding up.
What happens at terminal velocity
Terminal velocity is the steady speed reached when air resistance equals weight. The forces are balanced, so the resultant force is zero. With no resultant force, there is no acceleration, and the object continues at a constant velocity. The weight and air resistance are both still acting. Balanced forces are not the same as absent forces.
How to compare the forces
Force diagrams show the size and direction of each force. A larger force should have a longer arrow. At terminal velocity, the arrows must be equal in length because the forces are equal and opposite.
How to read the velocity-time graph
Time is on the horizontal axis and velocity is on the vertical axis. The gradient (the steepness of the line) shows acceleration. The curve starts at zero, rises steeply, becomes less steep, then levels off at 40 m/s. A steep line means a large acceleration. As the curve gets less steep, acceleration decreases because air resistance is increasing. The flat section shows terminal velocity: velocity is constant and acceleration is zero. A flattening curve means the object is accelerating less, not slowing down.
Explain why acceleration decreases
For a three-mark explanation, link the changing air resistance to the resultant force and then to acceleration.