1.2 Motion
Acceleration of Free Fall
When an object falls near the Earth, it experiences a nearly constant acceleration called the acceleration of free fall.
We write it as \$g\$ and it is approximately \$9.8\,\text{m/s}^2\$ for most everyday situations.
This means that every second the speed of the falling object increases by about 9.8 m/s, regardless of its mass (ignoring air resistance).
| Symbol | Meaning | Typical Value |
|---|
| \$g\$ | Acceleration due to gravity | \$9.8\,\text{m/s}^2\$ |
| \$s\$ | Displacement (downward distance) | Depends on time |
| \$t\$ | Time elapsed | Measured in seconds |
Key Formula (ignoring air resistance):
\$s = \frac{1}{2}gt^2\$
This shows that the distance fallen is proportional to the square of the time.
Analogy & Everyday Example
- Imagine a ball dropped from a balcony. Every second it’s speed increases by roughly 9.8 m/s – like a skateboard gaining speed on a downhill slope that keeps getting steeper at the same rate.
- Think of a free‑fall elevator: if it suddenly stops, the passengers feel a brief “weightlessness” because they’re still accelerating downward at \$g\$.
- In a playground, when you jump off a small step, you feel that “quick lift” because your body is briefly accelerating upward before gravity pulls you back down.
Exam Tips for IGCSE
- When asked to calculate the distance a ball falls in a given time, use the formula \$s = \tfrac{1}{2}gt^2\$ and remember to square the time.
- For velocity after a certain time, use \$v = gt\$. This is a straight‑line increase.
- Always state the value of \$g\$ you are using (9.8 m/s²) unless the question specifies a different value.
- When comparing two objects of different masses, remember that in a vacuum they fall at the same rate – a good point to mention if the question tests your understanding of mass independence.
- Use clear, labelled diagrams: draw a vertical line for the path, arrows for velocity and acceleration, and include units.