Falling objects: a demonstration
Practical Activity for 14-16
It may seem surprising that the motion of all objects falling freely under gravity is the same. Click here for a video showing free fall from which you can make measurements (please note this runs only in Internet Explorer 4+).
Apparatus and Materials
- Objects, 2, larger and smaller (e.g. pair of smallish stones, or sets of keys
- Vacuum pump
- Hoffman clip
Health & Safety and Technical Notes
The objects should be dense enough and the length of fall small enough to make the effect of air resistance insignificant.
- Simultaneously release the two objects side by side and watch what happens.
- A multiflash photograph could be taken of the falling objects. See the guidance note:
- Your accompanying chat might run like this: "Why doesn’t the heavier object fall faster? I can feel the Earth pulling it with a bigger force."
- The gravitational force acting on each object is found as F = mg, where g is the gravitational field strength. In other words, the pull of gravity on an object is proportional to its mass.
- Ask: "What happens to an object when an unbalanced force acts on it?" Newton’s Second Law will apply and the object’s acceleration will be a = F/m. In other words, with a bigger mass, a greater force must be applied to cause the same acceleration.
- Putting the two equations together, a = F/m = mg/m
- As a result, the acceleration of free fall a = g, is independent of an object’s mass. All masses fall in the same way. The units of acceleration and gravitational field strength look different but are really the same.
- This means that you can measure the strength of the gravity field by finding the acceleration of free fall.
- Many students will puzzle over this result because the same symbol g is used for both acceleration of free fall and gravitational field strength. Some students will understand the argument here better if you use a different symbol for the acceleration of free fall, perhaps ag .
- With more advanced students, you could point out that Newton’s second law refers to inertial mass m i , and write a = F/m i .
- Likewise, the force of gravity on an object depends on gravitational mass mg and F = m g g.
- Einstein showed that inertial mass and gravitational mass are the same, i.e. mi = mg . So a = F/m i = mg g/m i = g . In other words, mass as measured by accelerations is exactly the same as mass measured by gravitational forces.
This experiment was safety-tested in May 2005