218SCIENCE
What is
terminal
velocity and how
does it affect a
skydiver? Find out
on pages 234—235The word science comes from the Latin scientia, meaning knowledge.
Scientific ideas were recorded by the philosopher Aristotle about 2,350 years ago.
Sound waves with a frequency of 20,000 Hz or more are known as ultrasound.
The universe is about 13.7 billion years old and is expanding at an increasing rate.
The Earth’s core is a solid iron sphere surrounded by a deep layer of molten iron.
How
does a
nutcracker crush a
walnut? Find out
on page 233SCIENCE