Gravitational fields Flashcards
Gravitational fields
Define a gravitational field
A field is a region in which a force may be exerted by an object.
What are the rules for drawing gravitational field lines?
Gravitational Field lines represent the direction of force on a mass at a point.
The separation between fields lines represents their strength Field lines drawn for a single mass (gravitational or otherwise) never cross each other.
Gravitational field lines always point towards the centre of mass. (Gravitational force is always attractive).
Define the strength of a gravitational field (g)
The force per unit mass on a small test mass placed in the field.
Describe a radial field
The field lines are like the spokes of a wheel, towards the centre.

What happens to g in a radial field?
Decreases with increasing distance from the massive body.
Describe a uniform field
Field lines are parallel and equally spaced.

What happens to g in a uniform field?
Magnitude and direction of g is constant.
Sketch a graph of gravitational field strength against separation.
State what can be calculated from this graph
See diagram
Area = Gravitational potential

Explain why mass of an object is constant but weight may change
- Mass only depends on the number of particles present.
- Weight is a force which depends on gravitational field strength
- Gravitational fields strength varies on every planet
Define Newton’s law of gravitation?
Attractive force
Proportional to the mass of each object
Inversley Proportional to r2 (r is distance between objects)
When using Newtons law of atteaction equation why can planets be assumed to be point masses?
The diamater of a planet is much smaller than the distance between the two planets.
Sketch a graph of force against separation.
State what can be found from this graph
See diagram
Area = work done = GPE

State Kepler’s first law
The orbital path of a planet around the Sun is an ellipse, not a perfect circle. The Sun lies at one of the foci of the ellipse

State Kepler’s second law
An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse over equal intervals of time.
KE + PE = a constant

If the radius of a orbit decreases what happens to the speed, kinetic energy, potential energy and total energy?
Total energy is constant
Total energy = kinetic + potential
Potential energy - decreases so
Kinetic energy and speed increases
If the radius of a orbit increases what happens to the speed, kinetic energy, potential energy and total energy?
Total energy is constant
Total energy = kinetic + potential
Potential energy - increases so
Kinetic energy and speed decreases
What is Kepler’s Third Law?
The period of planet’s orbit squared is proportional to the cube of the average radius of its orbit.
T2 proportional r3
State the equations needed to derive the orbital velocity equation
F = mv2/r and
F = GM1M2/r2

State the equations needed to derive the orbital time period equation equation (Keplers 3rd Law)
F = mv2/r and
F = GM1M2/r2 and
v = 2πr/T
State the orbital time period for a geostationary satellite
24 hours
State the criteria for a geostationary orbit
- Have a time period of 24 hours
- Be at a height of 36 000 km above the Earths surface
- Be circular
- Be equatorial (in the plane of the equator)
- Rotate in the same direction of the earth (west to east)
State some benefits of a polar (low) orbit
- A cheaper launch cost
- Eventually cover the whole earth as the satellite doesn’t stay above the same point
Define gravitational potential energy
The gravitational potential energy (W) at a point in a field, is the work done to move an object from infinity to that point.
It is measured in joules
State the equation for gravitational potential energy
Ep = - GMm / r






