Edexcel IGCSE Physics 复习笔记 1.2.7 Terminal Velocity

• Falling objects experience two forces:
  • Weight (due to gravity)
  • Air resistance (due to friction)

   

• The force of air resistance increases with speed. This is illustrated in the image below:

Debbie initially accelerates downwards due to her weight. The upwards air resistance increases as she falls until it eventually grows big enough to balance the weight force

• Initially, the upwards air resistance is very small because the skydiver isn't falling very quickly
  • Therefore, there are unbalanced forces on the skydiver initially

   

• As the skydiver speeds up, air resistance increases, eventually growing large enough to balance the downwards weight force
• Once air resistance equals weight, the forces are balanced
  • This means there is no longer any resultant force

   

• Therefore, the skydiver's acceleration is zero - they now travel at a constant speed
• This speed is called their terminal velocity

Part (a)

The weight of an object is the force of gravity which acts on it.

• • The weight force is due to the Earth's gravitational pull on the object, so weight is due to gravity

   

Part (b)

When something falls, initially it accelerates.

• • The resultant force on the object is very large initially, so it accelerates
  • This is because there is a large unbalanced force downwards (its weight) - the upward force of air resistance is very small to begin with

   

Part (c)

The faster it falls, the larger the force of friction which acts on it.

• • The force of air resistance is due to friction between the object's motion and collisions with air particles
  • Air particles try to slow the object down, so air itself produces a frictional force, called air resistance (sometimes called drag)

   

Part (d)

Eventually it falls at a steady speed when the force of friction equals the force of gravity acting on it.

• • When the upwards air resistance grows enough to balance the downwards weight force, the resultant force on the object is zero
  • This means the object isn't accelerating - rather, it is moving at a steady (terminal) speed

   

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