Year 2 Chapter 3: Gravitation Flashcards
What is gravity?
A universal attractive force acting between all matter
What is a force field?
An area in which an object will experience a non-contact force
What is Newton’s Law of Gravitation?
F=GMm / r^2
What is G?
The universal constant of gravitation
6.67 x 10^-11 N m^2 kg^-2
What is a gravitational field?
A region surrounding a mass in which any other object with mass will experience an attractive force
What is gravitational field strength and what is it measured in?
The attractive gravitational force per unit mass exerted on a small test mass placed at a point in that field, measured in N / kg
What can gravitational fields be represented by and what do they show?
Field lines which represent the direction and strength of the field, showing the path that a test mass would take when placed within the gravitational field at that point
What are some features of gravitational field lines?
Point towards centre of mass producing the field
Show direction test mass moves when placed at that point in the field
Closeness represents field strength
Never cross over each other
What is a uniform field?
A field in which all of the field lines are parallel and equally spaced – field strength is equal in all areas of the field, eg gravity on earth’s surface
What is a radial field?
A field in which the field lines are all directed towards a single point, eg the centre of a planet
What is an equation for g for all fields?
g = F/m
What is an equation for g in radial fields?
g = GM/ r^2
What is the gravitational field strength on the surface of the earth?
9.81 N / kg
What is gravitational potential, V?
The work done per unit mass required to move a small test mass from infinity to a given point in a field, measured in J / kg
What is the equation for gravitational potential?
ΔV = gΔh
Why is gravitational potential negative?
Gravitational potential is defined as being zero at infinity and as energy is released as GPE is reduced, it decreases as it moves towards the centre of the field and so it is always a negative value
What is the equation for work done when moving a mass in a gravitational field?
ΔW = mΔV
What is gravitational potential energy?
The component of an object’s energy due to its position in a gravitational field
What is an equipotential surface?
A surface of constant potential. No work is done by the field when an object moves along an equipotential.
What is a feature of equipotential lines relating to field lines?
They are perpendicular to field lines
What is a potential gradient?
The change of potential per metre at a point in the field
What is the equation for gravitational potential gradient and why is it negative?
g = -ΔV / Δh or g = -ΔV / Δr
Potential gradient is positive direction upwards, field direction is downwards
What is the equation for V in a radial field?
V = -GM / r
How do you find ΔV from a graph of g against r?
Find the area under the curve
What is Kepler’s Third Law?
T^2 α R^3
What is escape velocity?
The minimum velocity required by an object to be able to escape a gravitational field of a mass when projected vertically from its surface
How can escape velocity be calculated?
Ek = mΔV = GMm/r
What is the centripetal force in orbits?
Gravitational force, resulting in circular motion
What is a synchronous orbit?
An orbit in which the period of the orbit is equal to the rotational period of the object that it is orbiting.
What is a geostationary satellite?
A satellite that orbits above the equator with a 24 hour period, so it will always remain above the same position on the Earth. They orbit approximately 36,000km above the surface of the Earth.
