A vacuum is a space from which all the air has been removed.
Ultrasound is sound waves with a frequency beyond the upper limit of hearing for a human, which is about 20 000 hertz.
Terminal velocity is reached when an object is travelling at a constant speed because the weight of the falling object is exactly balanced by the air resistance on the object.
SI units (Systéme International d’Unités) are the units in which scientific measurements are made, there are 7 base units.
Radioactivity is the decay of an unstable nuclei. Click for more.
The pascal is a unit of pressure it is equal to 1 newton per square metre.
The electrical current in a conductor is proportional to the potential difference applied to it provided the temperature remains the same. Click for more.
The National Grid is a network of cables and transformers which link power stations with towns and cities. Click for more.
