Orbits
An orbit is the path an object in space travels around another. Lots of objects in the Universe orbit others. The Moon orbits the Earth.
- Why do orbits happen?
Orbits happen because of gravity and something called momentum. The Moon's momentum wants to carry it off into space in a straight line. The Earth's gravity pulls the Moon back towards the Earth. The constant tug of war between these forces creates a curved path. The Moon orbits the Earth because the gravity and momentum balance out.
Momentum
If an object is moving, it has momentum. This is what keeps an object moving in the same direction. The more momentum something has, the harder it is to change its direction or bring it to a stop.
So we can work out an object's momentum using the equation:
Momentum = mass × velocity
During an explosion or collision, the total momentum is conserved. This means the total momentum before the event is equal to the total momentum after the event. This is useful for studying collisions in space or stellar explosions.
Magnetism
Since ancient times, people have known about some rocks that could attract iron. When these rocks were rubbed on other types of metal, they had the power to make them attractive too. These items could also push each other away, repelling each other.
- Magnetic Poles
Ancient Chinese thinkers figured out that a magnetic needle in water would always move to the same position. It would line up from north to south. This discovery led to the invention of the compass, which has been used ever since to help people find their way. The reason this works is that the Earth itself is a giant magnet.
Gravitational Lensing
Space is not flat. It is 3D, and we say that everything in it is held together on an imaginary surface we call spacetime. The idea of spacetime was put forward in Einstein's theory of relativity.
- How does Gravitational Lensing happen?
Lensing happens when light from directly behind a massive object, from our line of sight, can be bent around it. We call this gravitational lensing, and it is a way to see an example of the theory of relativity.
Gravity
Gravity is the name of the force that pulls objects together. It is a non-contact force. This means it acts on objects that are not touching each other.
- Gravitational Field Strength
All objects with mass have a gravitational field. We measure the gravitational field strength (g) in N/kg. The more mass an object has, the greater its gravitational field strength will be.
On Earth, g = 9.8 N/kg.
- Weight
Weight is the force acting on an object due to gravity. It is measured in Newtons (N). You can calculate weight using this equation.
Weight = Mass x Gravitational Field Strength
Understanding Forces
We know today of 4 different forces which rule the world around us.
Newton's work seemed to work most of the time, but not always. For example, he could not explain the orbit of the planet Mercury. So Einstein came up with a new theory called 'general relativity' in 1915. This linked space and time to gravity. In 1919 Eddington took observations during a solar eclipse to test Einstein's theory. His results showed that light from stars was bent by the Sun on its way to Earth. This supported Einstein's work.
Johannes Kepler
Occupation
Astronomer, Mathematician, Scientist, Teacher
Year born
1571
Research Areas
Planetary motion, Optics, Geometry
- Early Life
Johannes was born in Weil der Stadt (now part of Germany). He had an interest in astronomy from a very young age. When he was 6, his mother took him up to a high place to see the Great Comet of 1577. When he was 9, he was called outdoors to see a lunar eclipse. His family were poor, but Johannes got a scholarship to study at the University of Tübingen.
Make Your Own Impact Crater
How Do Satellites Stay In Orbit?
A satellite is an object which orbits another. An orbit is the curved path that an object in space takes around another object.