Estimate acceleration; draw and interpret velocity–time graphs to find acceleration and, at higher tier, displacement; explain terminal velocity using forces.
BEEZYREVISOR
GCSE Physics Revision
Learn it. Recall it. Revise it.
GCSE Physics revision
Acceleration
Describing motion along a line
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Apply acceleration knowledge to a relevant numerical or graphical problem and show the working.
Analyse an unfamiliar problem involving acceleration and justify the method or conclusion.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
What Happens the Moment You Step Out of a Plane
- At the instant a person steps out of a plane and is stationary, the only force acting on them is their weight, which acts downwards due to gravity.
- Because weight is the only force at this moment, the resultant force equals the weight, causing the person to accelerate downwards.
- Weight depends on mass, and since mass does not change, the magnitude of the weight force remains constant throughout the fall.
What Is Air Resistance (Drag)?
- Air resistance, also called drag, is an upward force that acts in opposition to the direction of motion of a falling object.
- Air resistance is caused by collisions between the falling object and the tiny air particles that make up the atmosphere.
- The size of the air resistance force depends on two key factors: the surface area of the object and its velocity.
How Surface Area and Velocity Affect Air Resistance
- A larger surface area means collisions with air particles can occur over a greater area, increasing the air resistance force.
- A higher velocity means the object collides with more air particles per second, also increasing the air resistance force.
- At the start of the fall, velocity is low, so air resistance is small and represented by a small upward force arrow.
Resultant Force During the Fall
- The resultant force is the vector sum of all forces acting on an object — in this case, weight (downwards) and air resistance (upwards).
- Because weight and air resistance act in opposite directions, air resistance partially cancels out the weight, reducing the resultant force.
- As long as the resultant force is still directed downwards, the object continues to accelerate downwards, just at a decreasing rate.
The Velocity–Time Graph for a Falling Object
- On a velocity–time graph, the gradient (steepness) of the line represents the rate of acceleration of the falling object.
- As velocity increases and air resistance grows, the resultant force decreases, so the gradient of the graph becomes less steep over time.
- The curve on the velocity–time graph gradually flattens out as the object approaches terminal velocity.
Reaching Terminal Velocity
- Terminal velocity is reached when the air resistance force increases until it equals the weight force acting downwards.
- At this point, the two forces balance, giving a resultant force of zero, so there is no longer any acceleration.
- With zero resultant force, the velocity remains constant — this constant velocity is called the terminal velocity.
Opening the Parachute
- When a parachute opens, the surface area of the falling person increases dramatically, causing a sudden large increase in air resistance.
- This means air resistance now exceeds weight, so the resultant force acts upwards, causing the person to decelerate (slow down).
- As the person slows down, their velocity decreases, which reduces the air resistance until it once again equals the weight.
- At this new balance point, the resultant force returns to zero and the person reaches a new, lower terminal velocity.
Why the New Terminal Velocity Is Lower
- The second terminal velocity (with parachute open) is lower than the first because the person has slowed down before the forces rebalance.
- A lower terminal velocity is safer for landing, which is the purpose of using a parachute.
Key Equations and Concepts to Remember
- Resultant force is calculated as: Fresultant = W − Fairresistance , where weight acts downwards and air resistance acts upwards.
- When Fresultant = 0, acceleration a = 0 (Newton's Second Law: F = ma), so velocity is constant at terminal velocity.
- The greater the surface area and velocity of a falling object, the greater the air resistance acting upon it.
Summary: The Stages of a Skydive
- Stage 1 — Large resultant force downwards: weight >> air resistance, so the object accelerates rapidly.
- Stage 2 — Decreasing resultant force: as velocity increases, air resistance increases, reducing the resultant force and the rate of acceleration.
- Stage 3 — Terminal velocity reached: air resistance equals weight, resultant force is zero, velocity is constant.
- Stage 4 — Parachute opens: air resistance exceeds weight, resultant force acts upwards, object decelerates to a new, lower terminal velocity.