Gravitational Fields/ Orbits Flashcards
Gravitational field def
A region of space where a mass experiences a force due to the gravitational attraction of another mass
Can gravitational fields be repulsive?
No
What is the relationship between the size of a gravitational field and mass
- There is a greater gravitational force around objects with a large mass
- There is a smaller gravitational force around objects with a small mass
What do the components of the gravitational field strength equation mean
What does the gravitational field strength equation show
- The larger the mass of an object, the greater its pull on another object
- On planets with a large value of g, the gravitational force per unit mass is greater than on planets with a smaller value of g
What factors affect the gravitational field strength at the surface of a planet
- The radius (or diameter) of the planet
- The mass (or density) of the planet
Draw the gravitational field lines for a point mass and a uniform gravitational field
Why are gravitational fields non-uniform
The gravitational field strength g is different depending on how far you are from the centre
Why are parallel field lines on the Earth’s surface considered uniform
The gravitational field strength g is the same throughout
When is an object regarded as a point mass
When a body covers a very large distance as compared to its size, so, to study its motion, its size or dimensions can be neglected
What is Newton’s law of gravitation
The gravitational force between two point masses is proportional to the product of the masses and inversely proportional to the square of their separation
What do the components of Newton’s law of gravitation equation mean
Describe the inverse square law within Newton’s law of gravitation equation
Describe the gravitational field strength (in a radial field) equation
Is gravitational field strength vector or scalar
vector
Describe the direction of gravitational field strength
- The direction of g is always towards the centre of the body creating the gravitational field
- This is the same direction as the gravitational field lines
Draw and describe the graph of a the distance away from a planet, against its gravitational field strength
What is the G.P.E at the surface of the earth assumed to be (when objects are near the earth’s surface)
0
Gravitational potential definition
The work done per unit mass in bringing a test mass from infinity to a defined point
What is the symbol and unit of gravitational potential
The symbol is V and is measured in J kg-1
Why is gravitational potential always a negative value
Because:
- It is defined as zero at infinity
- Since the gravitational force is attractive, work must be done on a mass to reach infinity
- This means that the gravitational potential is negative on the surface of a mass (such as a planet), and increases with distance from that mass (becomes less negative)
- Work has to be done against the gravitational pull of the planet to take a unit mass away from the planet
- The gravitational potential at a point depends on the mass of the object producing the gravitational field and the distance the point is from that mass
Why do two points with different distances from a mass have different gravitational potentials
because the gravitational potential increases with distance from a mass
What do the components of the gravitational potential equation mean
Is gravitational potential vector or scalar
Vector
What do the components of the g, V, r equation mean
Draw and describe the key features of the graph of V against r
Draw and describe the key features of the graph of g against r
What is the equation for gravitational field strength that is not on the formula sheet and what does it mean
What do the components of the work done in moving a mass equation mean
What is the work done in moving a mass equal to
the change in gravitational potential energy
How can ‘the change in G.P.E, or work done, for an object of mass m at a distance r1 from the centre of a larger mass M, to a distance of r2 further away’ be written in an equation
What is this equation only used for
- for an object lifted in a uniform gravitational field (close to the Earth’s surface)
- The new equation for G.P.E will not include g, because this varies for different planets and is no longer a constant (decreases by 1/r2) outside the surface of a planet.
What do equipotential lines/ surfaces join
points that have the same gravitational potential
What are the rules of equipotential lines/surfaces
They are always:
- Perpendicular to the gravitational field lines in both radial and uniform fields
- Represented by dotted lines (unlike field lines, which are solid lines with arrows)
What are equipotential lines like in radial/ uniform fields
In a radial field (eg. a planet), the equipotential lines:
- Are concentric circles around the planet
-Become further apart further away from the planet
In a uniform field (eg. near the Earth’s surface), the equipotential lines are:
- Horizontal straight lines
- Parallel
- Equally spaced
Is work done as an object moves along an equipotential line
No
Draw the equipotential lines in a radial field
Draw the equipotential lines in a uniform field
What are the components of the linear speed in orbit equation, which is not on the formula sheet
What is the equation relating the time period T and orbital radius r, which is not on the formula sheet
What is Kepler’s third law
For planets or satellites in a circular orbit about the same central body, the square of the time period is proportional to the cube of the radius of the orbit
How can Kepler’s third law be summarised
How can an orbiting satellite’s total energy be calculated
Total energy = Kinetic energy + Gravitational potential energy
What happens to the KE and GPE of a satellite if there is a change in orbital radius
- If the orbital radius of a satellite decreases its KE increases and its GPE decreases
- If the orbital radius of a satellite increases its KE decreases and its GPE increases
Escape velocity def
The minimum speed that will allow an object to escape a gravitational field with no further energy input
(It is the same for all masses in the same gravitational field ie. the escape velocity of a rocket is the same as a tennis ball on Earth)
When does an object reach escape velocity
when all its kinetic energy has been transferred to gravitational potential energy
What is the equation used for calculating an object’s escape velocity
What is the derive version of the escape velocity equation
Why do rockets launched from the Earth’s surface not need to achieve escape velocity
- They are continuously given energy through fuel and thrust to help them move
- Less energy is needed to achieve orbit than to escape from Earth’s gravitational field
What is a synchronous orbit
When an orbiting body has a time period equal to that of the body being orbited and in the same direction of rotation as that body
What do synchronous orbits usually refer to
Satellites (the orbiting body) around planets (the body being orbited)
What is a geosynchronous orbit
When the plane of a synchronous orbit is directly above the equator
Describe the key features of a geosynchronous orbit
- Remains directly above the equator
- Is in the plane of the equator
- Always orbits at the same point above the Earth’s surface
- Moves from west to east (same direction as the Earth spins)
- Has an orbital time period equal to Earth’s rotational period of 24 hours
What are geosynchronous orbits used for and how does this work
Geostationary satellites are used for telecommunication transmissions (e.g. radio) and television broadcast:
- A base station on Earth sends the TV signal up to the satellite where it is amplified and broadcasted back to the ground to the desired locations
- The satellite receiver dishes on the surface must point towards the same point in the sky
- Since the geostationary orbits of the satellites are fixed, the receiver dishes can be fixed too
What are low orbits
Some satellites are in low orbits, which means their altitude is closer to the Earth’s surface
What are low orbits useful for
Low orbits are useful for taking high-quality photographs of the Earth’s surface. This could be used for:
- Weather
- Military applications
