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Theory and vocab for AQA A-level gravitational fields.
What is a test mass?
A mass that is tiny compared to that of the source. Its own field is negligible compared to that of the field and can be ignored.
What is the source?
A mass that is massive enough to generate a (gravitational) field
What experiences gravitational fields?
An object with mass (particles like neutrinos and electrons have negligible mass, so do not feel gravity)
What experiences electric fields?
An object with charge.
What experiences magentic fields
An object with charge that is moving.
Describe two masses that are attracted to each other.
*system in equilibrium?
Both masses have a gravitational field.
One mass attracts the other.
The other mass attracts the first mass.
The force felt by each mass is equal and opposite, creating an action-reaction pair
The masses do not have to be equal. F = ma
What is a radial field?
What is a uniform field?
A field that has the same strength throughout, it does not change. The field lines are parallel.
Why are the field lines parallel in a uniform field?
The density of the field lines is proportional to the strength of the field. Stronger the part of the field, the more dense the lines. A uniform field has a constant strength, so the density of field lines must remain constant. Therefore the lines must be parallel (so they do not converge/diverge).
How would an otherwise uniform field be distorted by a dense collection of matter?
What happens to the work done when a test mass moves along the field lines? (radial field)
The change in potential decreases (moves from high to low). The work done by gravity is positive.
What happens to the gravitational potential energy when a test mass moves along the field lines? (radial field)
The GPE (of the test mass) decreases as energy from GPE is transferred to the kinetic energy of the test mass by gravity.
What happens to the kinetic energy when a test mass moves along the field lines? (radial field)
The kinetic energy increases. Energy from the GPE is transferred to Ek store. GPE becomes more negative.
What happens to the work done when a test mass moves against the field lines? (radial field)
The work done by gravity is negative, and the change in potential increases.
What happens to the gravitational potential energy when a test mass moves against the field lines? (radial field)
The GPE (of the test mass) increases as energy from the kinetic store is transferred to the GPE store of the object by gravity.
What happens to the kinetic energy when a test mass moves against the field lines? (radial field)
The kinetic energy of the test mass decreases as the energy is transferred to the GPE store by gravity.
Explain why values of the potential energy are negative.
The potential at infinity is 0
Force of gravity needs to do work to bring a test mass from infinity to a point.
The value must decrease and become negative.
What is the gravitational field strength vector?
The field lines point towards the centre of mass of the source. The magnitude is N/kg or the amount of force per unit of mass.
Explain why astronauts in an orbiting space vehicle experience weightlessness.
Both the astronaut and the vehicle are travelling at the same orbital speed, they have the same centripetal acceleration. They are in freefall.
There is no normal reaction between the astronaut and the vehicle.
What is gravitational potential energy?
The energy of an object due to its position in a gravitational field. Equal to the work done to move a test mass from infinity to a point in space.
What is the gravitational potential (of a point)?
The work done per unit mass to move a test mass from infinity to that point in space.
What is the (gravitational) potential difference?
The change in work done per unit mass.
Describe how you would measure the gravitational field strength close to the surface of the earth.
Note 
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