Gravitational fields Flashcards
Gravitational fields
Define a gravitational field
A field is a region in which a force may be exerted by an object.
What are the rules for drawing gravitational field lines?
Gravitational Field lines represent the direction of force on a mass at a point.
The separation between fields lines represents their strength
Field lines drawn for a single mass (gravitational or otherwise) never cross each other.
Gravitational field lines always point towards the centre of mass. (Gravitational force is always attractive).
Define the strength of a gravitational field (g)
The force per unit mass on a small test mass placed in the field.
Describe a radial field
The field lines are like the spokes of a wheel, towards the centre.
What happens to g in a radial field?
Decreases with increasing distance from the massive body.
Describe a uniform field
Field lines are parallel and equally spaced.
What happens to g in a uniform field?
Magnitude and direction of g is constant.
Define gravitational potential energy
The gravitational potential energy (W) at a point in a field, is the work done to move an object from infinity to that point.
Define gravitational potential
Gravitational potential at a point is the work done per unit mass to move a small object from infinity to that point.
What are equipotentials?
surfaces that join up points of equal potential.
Define potential gradient (gravitational)
Potential gradient at a point in a gravitational field is the change in potential per metre at that point.
What is Kepler’s Third Law?
The period of planet’s orbit squared is proportional to the cube of the average radius of its orbit.
What are the assumptions in Newton’s law of gravitation?
Always attractive
Proportional to the mass of each object
Proportional to 1/r^2 (r is distance between objects)
State Kepler’s first law
The orbital path of a planet around the Sun is an ellipse, not a perfect circle. The Sun lies at one of the foci of the ellipse
Define escape velocity
The minimum velocity an object must be given to escape from the planet when projected vertically from the surface.
For this to happen, the kinetic energy of the mass must be equal to the GPE gained
What is the formula to remember for escape veloctiy?
v(esc)=sqrt(2gR)
g is gravitational field strength
R is the radius of the planet
State Kepler’s second law
An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse over equal intervals of time. KE + PE = a constant
State the orbital time period for a geostationary satellite
24 hours
State the criteria for a geostationary orbit
HAVE A PERIOD OF 24 HOURS
BE OF HEIGHT ABOUT 36 000 KM ABOVE THE EARTH’S SURFACE
BE CIRCULAR
BE EQUATORIAL (in the plane of the equator)
BE IN THE SAME DIRECTION AS THE EARTH’S ROTATION (west to east)
State some benefits of a polar (low) orbit
A cheaper launch cost
Eventually cover the whole earth as the satellite doesn’t stay above the same point
Why is gravitational potential always negative?
GPE is zero at infinity.
The further you raise the object, the more work you do and the greater the change in gpe. GPE increase with distance but can only increase to 0 at infinity.
If the radius of a orbit decreases what happens to the speed, kinetic energy, potential energy and total energy?
Total energy - constant
Total energy = kinetic + potential
Potential energy - decreases
Kinetic energy increases
How are gravitational potential and gravitational potential energy related?
V = W/m
V - Gravitational potential (J/kg)
W - GPE (J)
m - mass (kg)
Is gravitational potential a vector or scalar?
Scalar
Why is the total energy supplied by a fuel to launch a satellite greater than the change in potential energy of the satellite?
Energy lost to friction (heat) and sound
Combustion of fuel is not 100% efficient (light wasted)
Satellite given KE and PE as it moves upwards
Fuel given some KE as ejected downwards
Fuel is needed to raise the height of the fuel, rocket and satellite combined
What can be found from a graph of force against separation
Area = work done = GPE
What can be found from a graph of gravitational field strength against separation
Area = Gravitational potential
What can be found from a graph of GPE against separation
Gradient = force
What can be found from a graph of gravitational potential against separation
Gradient = g
State two reasons why rockets launched from Earth do not need to achieve escape velocity to reach their orbit?
Energy continually added in flight by thrust provided by fuel
Less energy is need to achieve orbit rather than escaping Earth’s gravitational field
Explain why mass of an object is constant but it’s weight may change
Mass only depends on the number of particles present.
Weight is a force which depends on gravitational field strength
Gravitational fields strength varies on every planet
