Gravitation

Question 1

What is a force field and what is an example of one?

Answer 1

1. A force filed is a region where an object will experience a non-contact force
2. Force fields cause interactions between objects or particles e.g. between static or moving charges, or in the case of gravity, between masses

Question 2

What is Newton’s law of attraction?

Answer 2

F = Gm1m2 / r2

Question 3

What can Newton’s law be used to calculate?

Answer 3

1. The force between two point masses
2. The force between a planet and a small mass
3. The force between two planets or stars

Question 4

What happens when a mass is placed in a gravitational field?

Answer 4

Any object with mass will experience an attractive force if you put it in the gravitational filled of another object

Question 5

How does gravity change?

Answer 5

1. On Earth gravity exerts a larger force on more massive objects
2. The moon has a smaller surface gravity than the Earth because it is a less massive body
3. A planet’s pull of gravity gets weaker further away from that planet’s surface

Question 6

What is G?

Answer 6

The universal constant of gravitation, 6.67x10^-11 Nm^2kg^-2

Question 7

What is the inverse square law?

Answer 7

1/r2

Question 8

What is gravitational field?

Answer 8

A region in space in which a massive object experience a gravitational force

Question 9

What is field strength?

Answer 9

The strength of the gravitational field measured in Nkg^-1

Question 10

What do field lines represent?

Answer 10

• The direction of the gravitational force on an object and strength of the field
• The spacing of the lines gives a measure of field strength

Question 11

What is the gravitational field strength defined as?

Answer 11

g = F/m (gravitational force, N), (mass, kg)

Question 12

What is the gravitational field like near the surface of a planet?

Answer 12

• The gravitational field is very early uniform

- This means that the field is one of the same strength and direction everywhere

Question 13

What is the shape of the field near to the Earth? What does this mean?

Answer 13

• Radial field

- Here the field lines all point to the centre of the planet

Question 14

Why can you still use Newton’s law of gravity to calculate the gravitational forces between two planets?

Answer 14

The field is exactly the same shape as it would have been if all the mass of the planet were concentrated at its centre

Question 15

How does gravitational field strength change?

Answer 15

The gravitation field strength DECREASES in strength with increasing distance from the centre of the planet

Question 16

What is another equation for gravitational field strength?

Answer 16

g=GM/r^2

Question 17

How do you calculate gravitational potential energy?

Answer 17

mgh

Question 18

What is gravitational potential difference?

Answer 18

• The gravitational energy difference per kilogram
• Gravitational potential and potential difference have unit Jkg-1
• Gravitational potential is given the symbol V, and the gravitational potential difference is given the symbol deltaV

Question 19

How do you calculate gravitational potential difference?

Answer 19

DeltaV = gDeltah

Question 20

What happens when an object moves along an equipotential line?

Answer 20

The potential energy (and therefore the potential) stays the same

Question 21

What are equipotential surfaces?

Answer 21

• They are always at right angles to the gravitational field
• When something moves at right angles to the field (and hence along the equipotential) NO WORK IS DONE by or against the filed, so there is NO potential energy change
• When something moves along a field line, there is a change of gravitational potential energy

Question 22

How is gravitational field linked to the gravitational potential gradient? Why is there a minus sign?

Answer 22

g = -DeltaV / Deltah
-The significance of the minus sign is that the potential gradient deltaV/deltah is in a positive direction upwards, because the potential increases as the height above the planet increases. The gravitational field direction is downwards

Question 23

How does the gravitational force on an object of mass m change near to planet of mass M?

Answer 23

• The graph shows that a force F, acts not he object at. distance r form the centre of the planet
• If the object is moved a small distance, or further away from the planet, we can calculate the increase, in the objects GPE
• DeltaEp = work done = fr

Question 24

Why is fdeltar more usually written as mgdeltah?

Answer 24

Because the force acting on the mass is equal to it weight, mg

Question 25

How do you calculate the change in potential energy?

Answer 25

DeltaEp = GMm [1/r1 - 1/r2]

Question 26

What is the increase in potential, deltaV?

Answer 26

DeltaEp/m | -This allows use to think about defining gravitational potential close to a planet

Question 27

What is the potential change moving r1 to infinity?

Answer 27

DeltaV = GM/r1

Question 28

What is the potential at a distance r from the centre of a planet of mass M?

Answer 28

V = -GM/r

Question 29

How does the potential gradient change?

Answer 29

1. The potential gradient is steep closer to the planet's surface 2. The filed lines are closer together near the surface, because the gravitational field strength, g, is stronger - These tow statements ar blinkend through g= -deltaV / deltah or g=-deltav/deltar - These are the same but deltah has been used for a change in height and delta has been used for a change in distance from the centre of a planet

Question 30

Why does the Earth has an atmosphere?

Answer 30

-The molecules of gas moving in our atmosphere do not have enough kinetic energy to escape the pull of gravity at the Earth's surface

Question 31

What happens when a fast moving object leaves the surface of a planet?

Answer 31

- The decrease in kinetic energy os equal to the increase in gravitational potential - This assume that the object is NOT affected by an atmosphere and is in free fall (this is not a spacecraft with a rocket)

Question 32

What happens when a n object is is about to just escape the pull of the planet?

Answer 32

Ke1 - Ke2 = mdelatV | -Its speed V2, will kist reach zero at an infinite distance from the planet

Question 33

What is escape velocity?

Answer 33

The minimum velocity an object mist have at the surface of a planet to escape the pull of gravity using its own kinetic energy

Question 34

What does the pull of gravity do in relation to the circular motion?

Answer 34

The pull of gravity provides the necessary centripetal force to keep the planet in orbit

Question 35

How do you work out the velocity of an orbit? What does it tell you?

Answer 35

V^2 = GM/r | -The speed of the orbit is faster for small orbits

Question 36

How do you link the time period of an orbit to the radius of the orbit?

Answer 36

-The speed of the orbit is linked to the circumference @pir and the time period, T of the orbit through the equation: V = wr = 2pir / T T^2 = (4pi^2 / GM) r^3

Question 37

What are the different orbits?

Answer 37

- Most orbits have an elliptical shape - Most planetary orbits are nearly circular - However comets, some minor planets have elongated (or eccentric) elliptical orbits - The Sun's solar wind always blows the comets tail away from the sun

Question 38

Describe the motion of an elliptical orbit A to D, starting at A nearest the Sun

Answer 38

- When a planet or comet moves in an elliptical orbit, its speed changes, but the total energy of the body stays the same - A: comest moves at fastest orbital speed, but smallest GPE because ti is closest to the Sun - B: component of the Sun's gravitational pull on the comet which slows it down - C: It is at its furtherest point form the sun and it has its lowest KE at this point but highest GPE - D: The comet is falling back towards the Sun, there is a component of the Sun's gravitational pull, which speeds it ip. The comets potential energy is being transferred into KE, and the comet reaches it maximum speed again at A

Question 39

How are satellite orbits use?

Answer 39

- Satellites are in orbit around the Earth (for communications and observations) - Low orbit: able to take photos of world below - High orbit: useful for communication because messages may be sent form one part of the world to another via the satellite

Question 40

Whta is a geosynchronous orbit?

Answer 40

- In this case the satellite is placed in orbit above the Earth;s equator, at such a height that it takes exactly one day to complete an orbit - The orbit is synchronised with the Earth's rotation, so that it remains in the same place above the Earth's surface - This means that our satellite dishes e.g. can be aligned with a satellite which always lies in the same position relative to Earth

Question 41

State Newton's law of gravitation

Answer 41

- Forces of attraction between two point masses - Is proportional to the product of the masses - Inversely proportional to the square of the distance between them