Freefall (Cambridge O Level 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)

acceleration of free fall = g = 9.81 m/s²

Galileo's free fall experiment

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

$W = mg$

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.

Free fall with constant acceleration

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:

Change in resultant force on a skydiver

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

Speed-time graph for a skydiver

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