Gravitational Force (Weight)

Forces and Motion

## Falling objects: a demonstration

Practical Activity for 14-16

**Demonstration**

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

Read our standard health & safety guidance

The objects should be dense enough and the length of fall small enough to make the effect of air resistance insignificant.

Procedure

- 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:

Teaching Notes

- 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*a*_{g}. - With more advanced students, you could point out that Newton’s second law refers to inertial mass
*m*, and write_{ i }*a = F/m*_{ i }. - Likewise, the force of gravity on an object depends on gravitational mass
*m*_{g}and*F = m*_{ g }*g.* - Einstein showed that inertial mass and gravitational mass are the same, i.e.
*m*_{i}=*m*_{g}. So*a = F/m*_{ i }=*m*_{g}*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*