Freefall (CIE IGCSE Physics)

• In the absence of air resistance, all objects fall with the same acceleration
• This is called the acceleration of freefall (this is also sometimes called acceleration due to gravity)

In the absence of air resistance, Galileo discovered that all objects (near Earth's surface) fall with an acceleration of about 9.8 m/s²

• This means that for every second an object falls, its velocity will increase by 9.8 m/s
• The symbol g also stands for the gravitational field strength, and can be used to calculate the weight of an object using its mass:

weight = mass × gravitational field strength

W = mg

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Falling Objects without Air Resistance

• In the absence of air resistance, all objects falling in a uniform gravitational field, fall with the same acceleration, regardless of their mass

• So long as air resistance remains insignificant, the speed of a falling object will increase at a steady rate, getting larger the longer it falls for.

 

In the absence of air resistance objects fall with constant acceleration

 

Falling Objects with Air Resistance

 

• Objects falling through fluids (fluids are liquids or gases) in a uniform gravitational field, experience two forces:
  • Weight (due to gravity)
  • Friction (such as air resistance)

• A skydiver jumping from a plane will experience:
  • A downward acting force of weight (mass × acceleration of freefall)
  • An upward acting force of air resistance (frictional forces always oppose the direction of motion)

• 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

• When the skydiver opens the parachute, the air resistance increases
  • This is due to the increased surface area of the parachute opening
• The upward force of air resistance on the skydiver increases, slowing the acceleration of the skydivers fall
  • The skydiver decelerates 
• Eventually, the forces balance out again, and a new slower terminal velocity is reached

Graph showing how the velocity of a skydiver changes during the descent

Part (a)

The weight of an object is the product of the object's mass and the gravitational field strength.

• • The weight force is due to the Earth's gravitational pull on the object's mass as it falls through a uniform gravitational field

Part (b)

When an object 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)

As the object falls faster, the force of friction acting upon the object increases.

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

Part (d)

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

• • When the upwards air resistance increases 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