5.4: Gravitational Fields, F Flashcards
Define: gravitational field
Region around a body in which other bodies around it experiences a non-contact force due to the mass of the body
Define: gravitational field strength
Any point in a gravitational field is the force acting per unit mass at that point, (g=F/m)
The force experienced by an object is always __________
The force experienced by an object is always attractive
Define: Newton’s Law of Gravitation?
- Gravitational force of attraction between two masses is directly proportional to the product of their masses and inversely proportional to the square of their separation.
- An inverse square law: F ∝1/r^2
Explain using Newton’s law of gravitation, what would happen if you manipulated one of the variables
-If the distance(r) increases then the force(F) will decrease
and
-If the distance(r) doubles then the force(F) will be one quarter(1/4) the strength of the original force
Define: Gravitational field lines
Lines which show the shape and direction of the force that masses would feel when placed at that point
The Earth’s gravitational field is radial. What does it mean by that
The lines of force meet at the centre of the Earth
In a ______ Field, g is _________ Proportional to r^2
In a radial field, g is inversely proportional to r^2
Define: Gravitational Potential
Work done in moving a Unit Mass from infinity to that point
Vg= - GM/r (Vg being gravitational potential in J/kg)
Why is Gravitational potential negative?
Work is done against the gravitational field to move an object out of the gravitational field.
Explain what Gravitational Potential difference is
Work done moving a Unit mass. 2 points at different distances will have different gravitational potentials ∴ the difference is the amount of energy you need to move the object
Define: Gravitational potential energy
Gravitational potential is work done per unit mass ∴ gravitational potential energy of an object at a point in a gravitational field is E =mVg
Define: Escape velocity
- Minimum lunch velocity required required to move an object from that point to infinity
- (minimum energy to escape a gravitational field)
How can you derive escape velocity?
By equating kinetic energy to gravitational potential energy
0.5mv^2 = GMm/r
rearrange and cancel m’s
What is a satellite?
Any small mass which orbits a larger mass
e.g. moon is a satellite of earth
What factors does the speed of an orbit rely on
- Radius
- Mass of the larger body
How can we derive the equation to find the speed of an orbit
By equating centripetal force with the force of gravity
F =-GMm/r^2 and F = mv^2/r
rearrange and cancel m’s to find v (orbit speed)
How can we derive the time period for one orbit
By equating 2 circular motion equations and subbing in the equation for orbit velocity
v=wr and w=2π/T
Define: Geostationary orbit
An orbit of the earth made by a satellite that has the same time period and orbital direction as the rotation of earth(24hrs) and is in the equatorial plane(above equator)
What are geostationary satellites used for
- Monitoring air traffic
- Monitoring weather and changes in environment
- TV signals and relying mobile telephone signals
What are the downsides of using geostationary satellites
- Expensive
- Small risk of something going wrong and it falling back to earth
State Kepler’s first law
Planets travel around the sun in elliptical orbits
State Kepler’s second law
A line joining the sun to a planets will sweep out equal areas in equal times
Define: Kepler’s third law
The time period squared of a planet orbiting the sun is proportional to the mean radius of its orbit cubed, or T^2 ∝ r^3
How can Newton’s law of gravitation be used to explain how thick a planet’s atmosphere is
- A planet exerts a force all the particles, otherwise they would float off into space.
- The larger a planet, the larger the force is further away from its surface ∴ the more atmosphere particles it can stop escaping, leading to a thicker atmosphere
