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[O] Pure Physics Smart Guides

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    1. Physical Quantities, Units & Measurement
    7 Topics
    2. Kinematics
    8 Topics
  3. 3. Dynamics (Forces)
    7 Topics
  4. 4. Mass, Weight & Density
    4 Topics
  5. 5. Turning Effect of Forces
    4 Topics
  6. 6. Energy, Work & Power
    4 Topics
  7. 7. Pressure
    3 Topics
    8. Temperature
    2 Topics
  9. 9. Kinetic Model of Matter
    2 Topics
  10. 10. Transfer of Thermal Energy
    5 Topics
  11. 11. Thermal Properties of Matter
    3 Topics
    12. General Wave Properties
    3 Topics
  13. 13. Light
    7 Topics
  14. 14. Electromagnetic Spectrum
    2 Topics
  15. 15. Sound
    5 Topics
    16. Static Electricity
    5 Topics
  17. 17. Current of Electricity
    3 Topics
  18. 18. D.C. Circuits
    2 Topics
  19. 19. Practical Electricity
    4 Topics
  20. 20. Magnetism
    4 Topics
  21. 21. Electromagnetism
    3 Topics
  22. 22. Electromagnetic Induction
    4 Topics
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What is free fall?
It is when an object is falling without air resistance.

All free-falling objects experience:
Constant acceleration of 10m s⁻²

Technique to Answer Free Fall Questions

When the skydiver jumps out of the plane:

  • Air resistance less than the weight of the skydiver
  • The resultant force is positive
  • Since F(resultant force)=ma, the acceleration is positive
  • As the velocity increases the air resistance increases
  • Therefore the skydiver is falling with a decreasing acceleration.

When the skydiver is falling in the air:

  • As the speed of the falling skydiver increases, the air resistance increases
  • the air resistance increases until it is equal to the weight of the skydiver.
  • Resultant force decreases to zero.
  • Since F(resultant force)=ma, the acceleration is 0 m s⁻² and the skydiver is falling at constant velocity, also known as terminal velocity.

When the skydiver opens his parachute:

  • When the parachute opens, the air resistance is greater than the weight of the skydiver.
  • Resultant force becomes negative.
  • Since F(resultant force)=ma, the acceleration is negative and the skydiver is falling at a deceleration until he reaches another terminal velocity with much lower value and lands safely.

Classic Questions of Free Fall

1. Describe how an object released from rest (subjected to air resistance) will eventually fall with constant velocity.

  • Immediately upon release, gravitational force acting on the object causes it to accelerate at 10 m s⁻².
  • As the object gains speed, the upward air resistance increases. This will cancel out the downward gravitational force and decrease the acceleration.
  • Eventually, the upward air resistance increases to a point where it equals the downward gravitational force.
  • The resultant force acting on the object is now zero. Therefore the acceleration is zero and it continues falling with constant velocity (known as terminal velocity).

2a. Which parachutist will reach terminal velocity faster, the parachutist weighing 100kg or 150kg? Why?

  • The 100kg parachutist.
  • This is because it will be faster for the force of air resistance to be equal to his weight of 100kg than the 150kg parachutist.

2b. Which parachutist will reach the ground faster? Why?

  • The 150kg parachutist.
  • As the 150 kg parachutist is heavier than the 100kg parachutist, there will be more force acting on the 150 kg parachutist, causing him to fall at a higher speed and thus, reach the ground faster than the 100 kg parachutist.
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