Gravitational fields

Question 1

Define a gravitational field

Answer 1

A field is a region in which a force may be exerted by an object.

Question 2

What are the rules for drawing gravitational field lines?

Answer 2

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).

Question 3

Define the strength of a gravitational field (g)

Answer 3

The force per unit mass on a small test mass placed in the field.

Question 4

Describe a radial field

Answer 4

The field lines are like the spokes of a wheel, towards the centre.

Question 5

What happens to g in a radial field?

Answer 5

Decreases with increasing distance from the massive body.

Question 6

Describe a uniform field

Answer 6

Field lines are parallel and equally spaced.

Question 7

What happens to g in a uniform field?

Answer 7

Magnitude and direction of g is constant.

Question 8

Define gravitational potential energy

Answer 8

The gravitational potential energy (W) at a point in a field, is the work done to move an object from infinity to that point.

Question 9

Define gravitational potential

Answer 9

Gravitational potential at a point is the work done per unit mass to move a small object from infinity to that point.

Question 10

What are equipotentials?

Answer 10

surfaces that join up points of equal potential.

Question 11

Define potential gradient (gravitational)

Answer 11

Potential gradient at a point in a gravitational field is the change in potential per metre at that point.

Question 12

What is Kepler’s Third Law?

Answer 12

The period of planet’s orbit squared is proportional to the cube of the average radius of its orbit.

Question 13

What are the assumptions in Newton’s law of gravitation?

Answer 13

Always attractive
Proportional to the mass of each object
Proportional to 1/r^2 (r is distance between objects)

Question 14

State Kepler’s first law

Answer 14

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

Question 15

Define escape velocity

Answer 15

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

Question 16

What is the formula to remember for escape veloctiy?

Answer 16

v(esc)=sqrt(2gR)
g is gravitational field strength
R is the radius of the planet

Question 17

State Kepler’s second law

Answer 17

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

Question 18

State the orbital time period for a geostationary satellite

Answer 18

24 hours

Question 19

State the criteria for a geostationary orbit

Answer 19

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)

Question 20

State some benefits of a polar (low) orbit

Answer 20

A cheaper launch cost

Eventually cover the whole earth as the satellite doesn’t stay above the same point

Question 21

Why is gravitational potential always negative?

Answer 21

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.

Question 22

If the radius of a orbit decreases what happens to the speed, kinetic energy, potential energy and total energy?

Answer 22

Total energy - constant
Total energy = kinetic + potential
Potential energy - decreases
Kinetic energy increases

Question 23

How are gravitational potential and gravitational potential energy related?

Answer 23

V = W/m
V - Gravitational potential (J/kg)
W - GPE (J)
m - mass (kg)

Question 24

Is gravitational potential a vector or scalar?

Answer 24

Scalar

Question 25

Why is the total energy supplied by a fuel to launch a satellite greater than the change in potential energy of the satellite?

Answer 25

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

Question 26

What can be found from a graph of force against separation

Answer 26

Area = work done = GPE

Question 27

What can be found from a graph of gravitational field strength against separation

Answer 27

Area = Gravitational potential

Question 28

What can be found from a graph of GPE against separation

Answer 28

Gradient = force

Question 29

What can be found from a graph of gravitational potential against separation

Answer 29

Gradient = g

Question 30

State two reasons why rockets launched from Earth do not need to achieve escape velocity to reach their orbit?

Answer 30

Energy continually added in flight by thrust provided by fuel
Less energy is need to achieve orbit rather than escaping Earth’s gravitational field

Question 31

Explain why mass of an object is constant but it’s weight may change

Answer 31

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