3.7 Free fall and g

What is acceleration due to gravity?

Objects with mass exert gravitational force on each other. The Earth is so massive that its gravitational pull is enough to keep us on its surface so an object released on Earth will accelerate vertically downwards towards the centre of the Earth.

What is free fall?

When an object is accelerating under gravity with no other force acting on it.

What is the acceleration of free fall denoted by?

g

The unit is ms^-2

What is the value for g close to the Earth's surface?

It varies depending on different factors such as altitude and geology of an area. 9.81ms^-2 is generally used.

Describe the electromagnet and trapdoor method of finding g?

An electromagnet holds a small steel ball above a trapdoor. When the current is switched off, a timer id triggered, the electromagnet demagnatises and the ball falls. When it hits the trapdoor, the electrical contact is broken and the timer stops. The value for g is calculated from the hight of the fall and the time taken.

Describe the light gate method of determining g:

The electromagnet and trapdoor introduce tiny delays into the timing. Instead we can use light gates, two light beams, one above the other with detectors connected to a timer. When the ball falls through the first beam, it interrupts the light and the timer starts. When the ball falls through the second beam a known distance further down, the timer stops.

Describe the taking pictures method of determining g:

A small metal ball is dropped from rest to a metre rule, and its fall is recorded on video or with a camera in rapid fire repeating mode. Alternatively, a stroboscope illuminates the scene with rapid flashes. The camera shutter is held open, producing a photograph with multiple images of the falling ball. The position of the ball at regular intervals is then determined by examining the recording.