Gravitational fields Flashcards Preview

A Level Physics - LVI B > Gravitational fields > Flashcards

Flashcards in Gravitational fields Deck (31)
Loading flashcards...
1

Define a gravitational field

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

2

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).

3

Define the strength of a gravitational field (g)

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

4

Describe a radial field

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

5

What happens to g in a radial field?

Decreases with increasing distance from the massive body.

6

Describe a uniform field

Field lines are parallel and equally spaced.

7

What happens to g in a uniform field?

Magnitude and direction of g is constant.

8

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.

9

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.

10

What are equipotentials?

surfaces that join up points of equal potential.

11

Define potential gradient (gravitational)

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

12

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.

13

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)

14

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

15

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

16

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

17

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

18

State the orbital time period for a geostationary satellite

24 hours

19

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)

20

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

21

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.

22

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

23

How are gravitational potential and gravitational potential energy related?

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

24

Is gravitational potential a vector or scalar?

Scalar

25

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

26

What can be found from a graph of force against separation

Area = work done = GPE

27

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

Area = Gravitational potential

28

What can be found from a graph of GPE against separation

Gradient = force

29

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

Gradient = g

30

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