The Role of Gravity

Gravity is the force that pulls objects toward the Earth. Near the Earth’s surface, all objects experience the same acceleration due to gravity, which is approximately 9.8 m/s². This means that, in ideal conditions, all objects speed up at the same rate as they fall.

Ignoring Air Resistance

If we ignore air resistance (friction from the air), both the 10 kg and 50 kg objects will fall at exactly the same speed and hit the ground at the same time. This is because gravity affects all masses equally.

This idea was famously demonstrated by Galileo, who showed that objects of different masses fall at the same rate when air resistance is negligible.

The Effect of Air Resistance

In real-world conditions, air resistance plays a role. Air pushes against falling objects and slows them down. Lighter objects are usually affected more because they have less weight compared to the force of the air.

In this case:

  • The 50 kg object experiences a stronger gravitational force.
  • Air resistance has less impact on it relative to its weight.
  • The 10 kg object may slow down slightly more.

As a result, the heavier object might reach the ground just a tiny bit earlier.

Real-Life Example

If both objects are compact and similar in shape (like two metal balls), the difference in falling time from 100 meters is extremely small-almost unnoticeable.

However, if one object has a much larger surface area (like a feather or a sheet of paper), air resistance becomes significant, and it will fall much slower.

How do i experiment air resistance so as to find it?

Experimenting with air resistance lets you see how it affects falling objects. Since you’re interested in finding or measuring air resistance, here are a few hands-on experiments you can try, ranging from simple observations to more quantitative methods.

1. Simple qualitative experiment (see the effect)

What you need:

  • Two sheets of paper (same size)
  • One flat sheet, one crumpled into a tight ball

Steps:

  1. Drop the flat sheet and the crumpled ball from the same height at the same time.
  2. Observe which lands first.

Why it works:
Both have the same mass, but the flat sheet has more surface area facing downward → more air resistance → falls slower.
The crumpled ball has less area → less air resistance → falls faster.

What you “find”:
You see that air resistance depends on the object’s shape and cross-sectional area.

2. Measuring drag force with a spring scale (static method)

This measures air resistance indirectly at terminal velocity.

What you need:

- Spring scale (0–5 N or more)

- Small object (e.g., a plastic ball or coffee filter)

- Fan (to create airflow)

Steps:

  1. Hang the object from the spring scale in still air → record weight (force due to gravity).
  2. Turn on a fan so air flows upward past the object.
  3. Read the spring scale again. It will show **less** force because air resistance pushes upward.
  4. Air resistance \( F_{\text{air}} = \text{Weight} - \text{Scale reading in airflow} \).

What you “find”:

The upward air resistance force at a given airspeed.

3. Coffee filter drop experiment (quantitative with timing)

What you need:

- Several coffee filters (identical, nestable)

- Stopwatch

- Meter stick or measuring tape (height ~2 m)

- Scale

Steps:

  1. Measure mass of 1 filter, 2 stacked, 3 stacked, etc.
  2. Drop each stack from the same height (e.g., 2 m) and time the fall.
  3. Calculate average fall speed: \( v = \frac{\text{height}}{\text{time}} \).

Analysis:

At terminal velocity (constant speed), air resistance \( F_{\text{air}} = m g \).

So for each stack:

\[F_{\text{air}} = (\text{mass of stack}) \times g\]

And since \( F_{\text{air}} \) depends on speed \( v \), you can plot \( F_{\text{air}} \) vs. \( v \) to see if air resistance is proportional to \( v \) (low speeds) or \( v^2 \) (higher speeds).

4. Drop from a height with video analysis (more accurate)

What you need:

- Smartphone with slow-motion video (240 fps or higher)

- Meter stick in frame for scale

- Object of known mass (tennis ball, small dense ball, and a light large object like a balloon or paper plate)

Steps:

  1. Record the fall from ~2 m height.
  2. Use video analysis software (e.g., Tracker, Vernier Video Physics, or even manual frame-by-frame) to measure position every few frames.
  3. Plot position vs. time → get velocity vs. time.
  4. Compare acceleration to \( g \). The difference is due to air resistance.

Finding air resistance:

From Newton’s 2nd law:

\[mg - F_{\text{air}} = m a\]

\[F_{\text{air}} = m(g - a)\]

At each moment, \( a \) is from the velocity-time graph slope.

5. Using a fan and motion sensor (lab setup)

What you need:

- Motion sensor (like from a physics lab kit)

- Fan with adjustable speed

- Light object on a low-friction track or hanging vertically

Steps:

  1. Mount object so motion sensor can measure its acceleration under airflow.
  2. Measure acceleration with fan off (just gravity, or zero if horizontal).
  3. Turn fan on to apply air resistance force.
  4. Use \( F_{\text{air}} = m a_{\text{total}} \) (accounting for other forces).