Flashcards in Gravitational fields Deck (31)

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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