3. Gravitational fields Flashcards
Define weight:
Weight is the force due to gravity acting on a mass
Define gravitational field:
It is an example of a field force and is defined as the force per unit mass
In what direction does a gravitational field act?
It always directed towards the centre of the mass it is acting on
How do you calculate gravitational field strength?
g = F or W / m
Gravitational field strength = force due to gravity or weight / mass
What do gravitational field lines represent?
They show the direction and magnitude of a gravitational field
What are the two types of gravitational fields?
Uniform and non-uniform
What are radial gravitational fields?
Radial or non-uniform g-fields are those where the g-field-strength decreases with distance from the centre of the object; the g-field-lines are directed towards the centre of the object
What are uniform gravitational fields?
Uniform g-fields are those where the g-strength is the same throughout the field; it is represented by equally spaced parallel lines
What type of gravitational field is near the Earth’s surface?
The Earth’s gravitational field near the surface is almost uniform, therefore, it is approximated as uniform
What is a point mass?
An object which has an even mass distribution and the distance being considered is larger than its size
What is Newton’s law of gravitation?
The gravitational force between two bodies outside a uniform field is defined by Newton’s law of gravitation, it states that:
The gravitational force between two point masses is proportional to the product of the masses and inversely proportional to the square of their separation
What is the equation for Newton’s law of gravitation?
F = Gm1m2 / r^2
What is the value of G?
G or Newton’s gravitational constant is equal to 6.67x10^-11 Nm^2kg^-2
What is the inverse square law?
This means that when a mass is twice as far away from another, the gravitational force reduces by a quarter:
Inverse square law = 1 / r^2
What is a geostationary satellite?
- The satellite remains directly above the equator, therefore, it always orbits at the same point above the Earth’s surface
- Moves from East to West
- Has an orbital period equal to the Earth’s rotational period of 24 hours
What are geostationary satellites used for?
They are used to telecommunication transmissions (radios) and television broadcasts
How do you calculate gravitational field strength at a single point?
The gravitational field strength at a point describes how strong or weak a gravitational field is at that point; it is given by:
g = F / m
How do you calculate gravitational field strength due to a point mass placed in a gravitational field of a bigger object?
g = GM / r^2
M = mass of the body producing the gravitational field
r = distance between the point source and position in field
Define gravitational potential (φ):
The work done per unit mass in bringing a small test mass from infinity to the point
Why does gravitational potential always have a negative value?
- It is defined as having a value of zero at infinity
- Since the gravitational force is attractive, work must be done on a mass to reach infinity
How do you calculate gravitational potential?
φ = - GM / r
r = distance from the centre of the mass to the point mass
How do calculate change in gravitational potential?
Δφ = (- GM / r1) - (-1GM / r2)
Define gravitational potential energy:
The work done to assemble the system from the infinite separation of the components of the system
How do you calculate the EP between two point masses?
EP = - Gm1m2 / r
r = the distance between the centres of m1 and m2
How do you calculate change in EP?
ΔEP = Gm1m2 * (1 / r2 - 1 / r1)
Define work done in a gravitational field:
Work done against a gravitational field is equal to the change in gravitational potential energy of an object in a gravitational field
What is the difference between EP and GPE?
GPE is only relevant for an object lifted in a uniform gravitational field (close to the surface of the Earth)
