Free fall

1. An object is in free fall when the only force acting on it is its weight.
2. With no other force acting, the weight is an unbalanced force, so the object accelerates towards the ground.
3. True free fall happens where there is no air, such as on the Moon or inside a tube with the air pumped out.
4. Close to the Earth's surface, a small, dense object such as a stone or a ball bearing dropped a short distance is very nearly in free fall, because the air resistance on it is tiny.

The acceleration in free fall

1. The acceleration of an object in free fall near the Earth's surface is about $10{m/s}^2$.
2. A more precise value is $9.8{m/s}^2$, but $10{m/s}^2$ is the value used in most calculations at GCSE.
3. The acceleration is directed vertically downwards, towards the centre of the Earth.
4. An acceleration of $10{m/s}^2$ means that the velocity of a falling object increases by $10m/s$ every second.
   1. A dropped object is falling at $10m/s$ after $1s$, at $20m/s$ after $2s$ and at $30m/s$ after $3s$.
5. The acceleration in free fall has the same value as the gravitational field strength, because $1N/kg$ is the same as $1{m/s}^2$.
   1. A field of $10N/kg$ pulls each kilogram with $10N$, and a resultant force of $10N$ on $1kg$ gives an acceleration of $10{m/s}^2$.
6. The symbol $g$ is used for both the acceleration in free fall and the gravitational field strength.

Mass does not affect free fall

1. In free fall, all objects accelerate at the same rate, whatever their mass.
2. A heavier object has a larger weight pulling it down, but it also has a larger mass, so it needs a larger force to accelerate it.
3. The two effects cancel exactly, because doubling the mass doubles the weight and also doubles the force needed for the same acceleration.
4. In 1971, the astronaut David Scott dropped a hammer and a falcon feather together on the Moon during the Apollo 15 mission, and they hit the ground at the same moment because there is no air on the Moon.
5. On Earth, a feather and a coin released together inside a long tube with the air pumped out also land together.

Air resistance in real falls

1. Most falling objects on Earth are not in true free fall, because air resistance acts upwards on them as they move through the air.
2. Air resistance reduces the resultant force on a falling object, so its acceleration is less than $10{m/s}^2$.
3. Air resistance has the largest effect on objects that are light and have a large surface area, such as a feather, a leaf or a flat sheet of paper.
4. A crumpled sheet of paper falls much faster than a flat sheet of the same mass, because its smaller surface area meets less air resistance.
5. You can treat a falling object as being in free fall whenever a question tells you that air resistance can be ignored.