Gravitational field def
2 points
Force per unit mass
A region of space where mass experiences a force
Field lines show what two things?
(make ref to arrows and spacing)
- arrows show the direction of the gravitational force
- spacing indicates strength of the gravitational field
Point mass
when mass is concentrated at the centre of the body
Centre of mass
the point at which the total mass of the object is considered to be concentrated
Uniform gravitational field has what feature
equally spaced field lines
Newton’s law of gravitation
The force between two point masses is proportional to the product of their masses and inversely proportional to the square of seperation
F∝Mm/ r²
Four points on geostationary satellites
- It remains at the same point above the earth’s surface, has the same angular speed
- It has an orbital period of 24 hrs
- It travels from west to east
- Is remains directly above the equator
Formula for gravitational field of a point mass
g= F/m
therfore
g= GM/ r²
Definition of gravitational field strength
Gravitational force at that point per unit mass
Why is g approximately constant for small changes in height near the Earth’s surface
2 marks
Value of Earth’s radius is so large that small changes in height near the Earth’s surface make little difference in the value of g
so field lines are effectively parallel
Gravitational potential definition
Work done per unit mass in bringing a small test mass from infinity to that point
Gravitational potential formula
-GM/ r
Why is gravitational potential negative?
3 points
- gravitational potential at infinity is zero
- gravitational force is attractive
- as mass moves from infinity, work is done and potential decreases
Gravitational force definition
Force acting between two masses
