Falling safely

The aim of these revision cards are to summarize the following things-

  • Motion of falling objects
  • How air resistance slows down falling objects
  • Terminal speed
  • Created by: Pinkypops
  • Created on: 23-12-10 11:56

Falling objects

  • When objects are dropped they fall due to the gravity which is a pull from the centre of the Earth.
  • All objects accelerate at the same rate no matter what their mass is...we know this by Galileo Galilei's demonstration at the leaning tower of Pisa.(http://www.endex.com/gf/buildings/ltpisa/ltpnews/ltpdrop.gif)
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Galileo Galilei's demonstration at the leaning tow

Here is a wonderful link to Galileo's summary...the white man is Galileo Galilei and the tower is of course the Leaning tower of Pisa. And if you see at the bottom of the tower are the two cannon balls...


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Air resistance

  • Objects that have a larger area of cross section fall more slowly due to air resistance or drag.
  • Objects with larger area are parachutes, shuttlecocks, feathers...
  • Therefore, objects with a smaller area of cross section such as a ball will hit the ground first compared to a feather.
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  • The upward air resistance force changes on them when a free fall parachutists make their way on Earth.
  • When the forces acting on an object are balanced the falling object reaches its maximum speed. This speed is called terminal speed.


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Graph to show the speed of a free fall parachutist


1. Mr. Bob jumps of the plane

2. His weight is greater than air resistance...Therefore, Mr. Bob accelerates

3. Weight of Mr. Bob = air resistance                              4. Constant speed

5. Air resistance larger than weight                                 6. Mr. Bob decelerates

7. As Mr. Bob slows down, air resistance decreases...slower terminal speed

8. Mr. Bobs weight = air resistance

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Summary of Terminal speed

  •  As the speed of a free fall parachutists increases they displace more air molecules every second so the air resistance force increases. This reduces the acceleration.
  • When their weight is equal to the air resistant force, the forces on them are balanced so they travel at a constant speed- the terminal speed.
  • When the parachute opens the upward force on them increases suddenly as there is a much larger surface area, displacing more molecules every second.
  • The air resistant force decreases because they decelerate which displaces fewer air molecules each second.
  • Eventually, they reach a new slower terminal speed when their weight is equal to the air resistance speed once more.
  • This means they can land safely...
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Frictional force

  • All frictional force act on opposite directions persuading the object to stop.
  • To reduce frictional force we use lubricating oil between the moving parts of a machine and use vehicles with streamlined body shape.
  • Streamlining is designed to reduce the drag force acting on the vehicle, which allows its top speed to increase. Air passes over a streamlined vehicle more easily than if i had hard sharp corners and a square shape.


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Falling objects

  • All objects fall with the same acceleration but only if the air resistance is very small.
  • When falling towards earth through the atmosphere there is always a drag force as the falling objects displaces air molecules.
  • The size of he air force on a falling object depends on:

Its cross sectional area
Its speed

  • Air resistance only has a significant effect on motion when it is large compared to the weight of the falling object...
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Free- fall

A true free- falling object falls under the influence of gravity

  • Free falling object do not experience air resistance
  • Free falling objects, irrespective of mass or shape, accelerate downwards at the same rate...

Examples of free- falling are:

  • Objects falling above Earth's atmosphere
  • Objects falling on the Moon
  • The moon itself.

The moon is in free- fall towards Earth. It is falling with an acceleration that is equal to gravity, g, at the height of its orbit (about 0.003 m/s^2). It never gets closer to Earth because of Earth's curvature. 

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