Gravitational Fields

Question 1

what is a gravitational field?

Answer 1

a gravitational field is the region around a body in which other bodies will feel a force due to the mass of the body

Question 2

what do gravitational field lines show?

Answer 2

the shape and direction of the field line at a point is the direction in which a small mass would move when placed at that point

Question 3

what is gravitational field strength?

Answer 3

the gravitational field strength at any point in a gravitational field is the force acting per unit mass at that point, g = F / m (units are NKg^-1)

Question 4

how do gravitational field lines act around a spherical mass?

Answer 4

they act radially inwards, towards the centre because spherical objects can be modelled as a point mass at its centre

Question 5

what are gravitational field caused by?

Answer 5

they occur because objects have mass, the more mass that body has, the more that it will attract the smaller masses around it

Question 6

what happens to the field lines of Earth as you get closer and closer to the surface of the Earth?

Answer 6

the gravitational field lines become uniform and straight, the lines converge to become almost parallel, the radial field becomes apparent the further we move away from the Earth and observe it from space

Question 7

state Newton’s law of gravitation?

Answer 7

Newton’s law of gravitation states that the gravitational force of attraction between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation

Question 8

what is the equation for Newton’s law of gravitation?

Answer 8

F = - GMm / r^2
where M and m are the masses of the two bodies and r is the separation of their centres
(the minus shows it is an attractive force as bound)

Question 9

what is G?

Answer 9

the universal gravitational constant, 6.67 x 10^-11

Question 10

what law does Newton’s law of gravitation follow in terms of force? (think of intensity)

Answer 10

follows an inverse square law, forces gets smaller with distance because the strength changes

Question 11

a spherical mass is often assumed to be what?

Answer 11

a point mass with all its mass concentrated at the centre

Question 12

what is the other equation for gravitational field strength for a point mass and how can you get to it?

Answer 12

first g = F / m
F = - GMm / r^2, equate Fs
leading to g = - GMm / mr^2, cancel little m
leading to g = - GM / r^2

Question 13

what is gravitational field strength equal to on earth?

Answer 13

g = 9.81 ms^-2 (acceleration due to freefall)

Question 14

what are Kepler’s free laws of planetary motion?

Answer 14

1. planets travel around the sun in elliptical orbits
2. a line joining the Sun to a planet will sweep out equal areas in equal times
3. the time period of the orbit squared is proportional to the mean radius of the orbit cubed, or T^2 ∝ r^3

Question 15

what are Kepler’s free laws of planetary motion?

Answer 15

1. planets travel around the sun in elliptical orbits
2. a line joining the Sun to a planet will sweep out equal areas in equal times
3. the time period of the orbit squared is proportional to the mean radius of the orbit cubed, or T^2 ∝ r^3

Question 16

what is Kepler’s third law?

Answer 16

kepler’s third law states that the square of the period of a planet orbiting the Sun is proportional to the mean radius of its orbit cubed (his law also applies to other planetary stems, to the orbits of moons around planets and to binary stars)

Question 17

what is a geostationary orbit?

Answer 17

a geostationary orbit is an orbit of the Earth made by a satellite that has the same time period and orbital direction as the rotation of the Earth (i.e 24 hours) and is in the equatorial plane

Question 18

what are geostationary satellites used for?

Answer 18

geostationary satellites are used for communication. eg. TV and mobile phones

Question 19

what are polar orbiting satellites

Answer 19

polar orbiting satellites orbit over the poles from North to South, such that the Earth rotates West to East under them to provide a complete image of the Earth

Question 20

what are polar orbiting satellites used for

Answer 20

polar orbiting satelites are used for mapping and reconnaissance

Question 21

what are low earth orbits

Answer 21

low earth orbits take less than 2 hours to orbit the earth. eg. ISS

Question 22

what is the equation that relates time period to mean radius (kepler’s third law)?

Answer 22

T^2 = (4π^2 / GM ) x r^3. derived from F=(mv^2)/r=GMm/(r^2)

Question 23

what is gravitational potential?

Answer 23

work done per unit mass to move an object from infinity to the given point./ always negative as bound in field

Question 24

what is gravitational potential energy?

Answer 24

energy needed to get from distance r to infinity. always negative as bound

Question 25

what is escape velocity?

Answer 25

the velocity needed for an object to escape the gravitational field of a body

Question 26

what is the radius of a black hole?

Answer 26

the Schwarzschild radius - the minimum radius that allows a proton (velocity=c) to escape a body of mass M