Acceleration due to gravity

  2. Mechanics
  3. Kinematics
  4. Acceleration due to gravity
    5. 10'

The force of gravity provides objects in freefall with a constant acceleration, g. On Earth, g has an average magnitude of 9.81 m s-2.

Provided an object is fairly close to the surface of the Earth, we assume that g is constant (and that the object has a constant weight). This means that we can use suvat equations for falling motion.

Key Concepts

Freefall is an example of constant negative acceleration. It occurs when an object has just one force acting: the weight.

The most common misconception about acceleration due to gravity is that it changes direction as an object travels up and down - it doesn´t. Acceleration is always down as the force acting is always towards the centre of the Earth. We generally define upwards as positive, and so acceleration due to gravity is negative.

Graphs of freefall

  • A graph of displacement-time is an upside-down parabola.
  • A graph of velocity-time is linear with negative gradient.

 

 

Essentials

Measuring the acceleration due to gravity is one of the suggested practicals in the Subject Guide.

Ball drop experiment

In this clip, a simulation is used to generate data. In order to find g, we need to produce a linear graph with a known gradient. This is a common way of processing data in physics experiments.

 

Video analysis

g can be measured by analysing a video of a falling ball. The value obtained was not very accurate, probably due to an error in the scaling. The ruler used to scale the video was a bit further away than the ball, this would give an over estimate for the scale, resulting in a high value for acceleration.

 

Other techniques

There are many equations in which g crops up, such as the time period for a pendulum \(T=2\pi \sqrt{l\over g}\). A picket fence (multiple interrupt card) can be used along with a light gate. Alternatively, a beaker of water on a turntable rotated until the water surface is 45° shows that weight = centripetal force.

 

 

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