Chapter 21 - Gravitational fields Flashcards
What is a force field?
Region in which a body experiences a non-contact force
What is a gravitational field?
Force field around a mass
What is a (gravitational) field line?
Path a smaller mass would follow if pulled towards a larger mass.
Define the strength of a gravitational field(g)
Force per unit mass on a smaller test mass placed in the field
Give the equation for gravitational field strength (g)
g = F/m (NKg^-1)
Give the equation for acceleration of free fall in a gravitational field.
a = force/ mass = mg/m = g
What’s a radial field?
Where field lines are like spokes of a wheel. Towards the centre

What happens to g in a radial field?
Decreases with increasing distance from the massive body.
What is a uniform field?
Field lines are parallel and equally spaced.
What happens to g in a uniform field?
Magnitude + direction of g is constant throughout the field
Is earth’s gravitational field uniform or radial?
- Radial (Uniform over a very small distance)
- g decreases with distance from the earth increasing
Define gravitational potential energy
Energy of an object due to its position in a gravitational field
Define gravitational potential (V)? Give the equation
The gravitational potential at a point is the work done per unit mass to move a small object from infinity to that point. (As gravitational potential is zero at infinity)
V = W/m (unit: JKg^-1)
What’s the equation for change in gpe (Ep) if a small object of mass m is moved from gravitational potential V1 to gravitational potential V2?
Change in Ep = m(V2-V1)
What are equipotentials?
- Surfaces of constant potential. So no work needs to be done to move along an equipotential surface.
Define potential gradient (gravitational)
Potential gradient at a point in a gravitational field is the change of potential per metre at that point
Draw the equipotentials on a diagram of a planet

Why do the equipotential get further apart as the distance increases from the planet?
gravitational fields become weaker so gain of gravitational potential energy per metre of height gain becomes less.
Give the equation for potential gradient for change in V and change in r (small distance
potential gradient = change in V/ change in r
Give the equation for g (gravitational field strength) using potential gradient
g = - change in V/ change in r = - potential gradient
What is Kepler’s third law?
- The value of r^3/T^2 was the same for all planets.
where r = average radius of it’s orbit and T = time period
What are the assumptions in Newton’s laws of gravitation?
Gravitational force between any 2 point objects (planets were taken to be point masses) is:
- always an attractive force
- Proportional to the product of the masses of each object
- Proportional to 1/r^2 where r is their distance apart
What is G and give a value and units
Universal constant of gravitation, G
6.67x10^-11 Nm^2Kg^-2
Give the equation for gravitational force, F, using Newton’s laws of gravitation.
Gravitational force F = Gm1m2/r^2