Forces & Motion: Mastering Terminal Velocity for IGCSE Physics

In  IGCSE PHYSICS (Cambridge 0625 or Edexcel 4PH1), few scenarios are as famous or as frequently tested—as the skydiver problem. But for students in Dubai, this isn't just a diagram in a textbook; it’s a real-life scene happening every day right above the Palm Jumeirah.

Imagine you are at Skydive Dubai, leaping out of a plane 13,000 feet above the city. To an examiner, this adrenaline-filled journey is actually a complex story of changing forces, fluctuating acceleration, and Newton's Laws of Motion. This is often a 5 or 6-mark "describe and explain" question.

This guide breaks down the physics of your jump over the Palm, phase by phase.

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Phase 1: The Jump Over the Palm (Start)

The Moment of Release (t = 0)

You step out of the plane with the Atlantis The Royal in view below. At this exact instant, your vertical velocity is zero.

• Weight (W): This acts downwards. It is constant throughout the entire fall (calculated by W = mg).
• Air Resistance (Drag): This is zero because you haven't started moving yet.
• Resultant Force: The only force acting is your Weight (downwards).
• Motion: According to Newton's Second Law (F = ma), there is a large unbalanced force downwards, causing maximum acceleration (approx 10 m/s²).

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Phase 2: Acceleration Decreases (The Tricky Part)

As Speed Increases

Gravity pulls you towards the drop zone, and your speed increases rapidly. This is where physics gets interesting.

• Air Resistance Increases: As you speed up, you collide with more air particles per second. Drag builds up acting upwards (opposing motion).
• Resultant Force Decreases: Since Resultant Force = Weight - Air Resistance, and Air Resistance is getting bigger, the net force downwards gets smaller.
• Acceleration Decreases: Because F = ma, if the resultant force drops, the acceleration drops.

⚠️ Common Pitfall:
Students often say "the skydiver slows down" here. Wrong. You are still speeding up towards the Dubai Marina skyline, but the rate at which you are speeding up is getting less. You are still accelerating, just not as fast as at the start.

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Phase 3: Terminal Velocity (Balanced Forces)

Constant Speed (approx. 190 km/h)

Eventually, you are falling so fast that the upward push of the air becomes exactly equal to your downward weight.

• Forces: Balanced (Weight = Air Resistance).
• Resultant Force: Zero.
• Acceleration: Zero (Newton's First Law).
• Motion: You fall at a constant maximum speed. This is Terminal Velocity. You are now cruising steadily above the Palm.

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Phase 4: The Parachute Opens

Rapid Deceleration

You pull the cord. The canopy opens up wide.

• Air Resistance Spikes: The huge surface area of the parachute causes a massive increase in air resistance, which is now much larger than your Weight.
• Resultant Force: There is now a large upward resultant force.
• Motion: You decelerate (slow down) rapidly. Note: You do not shoot upwards back towards the plane; you just slow down while continuing to fall downwards.

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Phase 5: New (Lower) Terminal Velocity

Safe Landing Speed

As you slow down, the air resistance decreases again (remember: less speed = less drag).

• Eventually, the Air Resistance drops until it equals Weight again.
• The forces are balanced once more.
• You glide down at a new, much slower constant speed, ready for a safe landing on the grass runway at Skydive Dubai.

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Exam Application: The Velocity-Time Graph

In your IGCSE exam, you will likely be asked to sketch or interpret a velocity-time graph of this journey. Here is what you must look for:

1. Steep Gradient at Start: Represents high acceleration.
2. Gradient Curves and Flattens: Shows acceleration decreasing to zero.
3. Horizontal Line (High): The first terminal velocity (freefall).
4. Sharp Drop: The parachute opening (rapid deceleration).
5. Horizontal Line (Low): The second, safer terminal velocity (gliding).

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Conclusion

Mastering Terminal Velocity is about understanding the relationship between Speed and Drag. If speed goes up, drag goes up. If drag goes up, resultant force goes down. If resultant force goes down, acceleration goes down.

If you can chain that logic together, you will master this topic. Struggling to visualize the forces? Our expert IGCSE Physics tutors can help you draw the free-body diagrams and master the graphs needed for an A*.