P3aL4 Gravity and Orbits
Key Words
Centripetal acceleration - acceleration towards the centre of a circle.
Centripetal force - a force acting towards the centre of the circle.
Circular motion - going round and round in circles.
Ellipse - a squashed circle
Free fall - object falling to Earth under the influence of gravity with nothing opposing the fall.
Gravitational force - force due to gravitational attraction.
Gravity - a property of all objects that have mass that results in an attractive force.
Mass - amount of material in an object
Orbit - path of a planet around a star (or a satellite around a planet).
Weight - force due to gravity acting on a mass.
Grade E
Gravity is always an attractive force that exists between all objects which have mass. There is even a gravitational attraction between two students sitting next to each other, but it's so small it can't be felt. It can be felt only because planets and stars are massive objects. Although it's a tiny force, gravity has an infinite range.
On the Earth gravity can be felt because the Earth is a really big object. The weight of an object is due to the gravitational attraction on a mass. The direction of the gravitational force (which is a vector) is towards the centre of the Earth.
In Space, dust gets attracted together by gravity over millions of years to form stars and planets.
When solar systems form, the planets form from dust and rocks that are in a slowly spinning disc around the newly lit star. The sense of the rotation gives the planets their orbits.
Planets are held in their orbit by centripetal force provided by gravity. The combination of a constant forward velocity and a force that always acts at 90o means that the planets go in circular paths, and obey the rules of circular motion.
Actually, the orbit tends to be a slightly squashed circle called an ellipse. At some points in the orbit the planet is slightly close to the sun than at others. The amount may be small, but there can be climatic effects on the planet.
Gravity has nothing whatever to do with magnetism.
Grade C
Weight is worked out using the simple formula:
weight (N) = mass (kg) × acceleration due to gravity (m s-2)
In Physics code:
W = mg
On the Earth, the acceleration due to gravity is 10 m s-2 while on the Moon, it is 1.6 m s-2.
The Earth orbits the Sun, because the Sun is a really big object, and the force of gravity between the Earth and the Sun is much bigger than the Earth and other planets. The Moon orbits the Earth rather than the Sun because it's close to the Earth and the gravitational attraction is much bigger between the Moon and the Earth.
Anything that orbits the Earth is actually falling towards the Earth in free fall. Since the object is moving with a high forward velocity, the resulting path actually misses the Earth.
Weightlessness happens, NOT because there is no gravity, but because the spacecraft and its cosmonauts are falling at the same rate. At the height at which spacecraft commonly orbit, the acceleration due to gravity is about 9 m s-2.
The centripetal acceleration of the cosmonaut and his spacecraft is also 9 m/s2.
Grade A
The tides are caused by gravitational attraction. On the side nearest the moon, the gravity of the moon attracts the water, so it bulges by about 10 metres.
On the opposite side of the Earth, the water bulges too. This may seem odd, but it is because the Earth and moon have a common centre of mass around which they revolve. The water on the far side tries to fly off in a straight line, but is pulled in by the centripetal force.
The Sun is much further away, so its effect on the tides is much less. However, when it's lined up with the moon, high (spring) tides occur.