Topic 12 - Gravitational Fields

Question 1

Gravitational field

Answer 1

a region of space where a mass experiences an attractive force towards a second force.

Question 2

What is newtons law of gravitation?

Answer 2

an inverse square law

Question 3

Newtons law of gravitation

Answer 3

F = GMm/r^2

Question 4

Gravitational field strength equation

Answer 4

g = F/m = FM/r^2

Question 5

differences between electric fields and gravitational fields

Answer 5

• electric fields are attractive and repulsive whereas gravitational is just attractice
• objects can be shielded from electric fields but not gravitational
• size of an electric field depends on the medium between the objects whereas gravitational does not
• The force between (unit) charges at a given separation is much stronger than the force between (unit) masses at the same separation

Question 6

Similarities between electric fields and gravitational fields

Answer 6

• both are unit forces (gravitational - force per unit mass, electric - force per unit positive charge)
• both are inverse square laws
• both have infinite range

Question 7

When is energy transferred?

Answer 7

when a mass or charge moves in a field

Question 8

Why do satellites stay in orbit?

Answer 8

Because they’re constantly in free fall along a curve path around the earth

Question 9

What are the different types of orbit?

Answer 9

• eccentric (altitude varies)
• polar orbit (passes over noth/south poles)
• low earth orbit (orbit close to earths surface)
• geostationary orbit ( takes a constant time to orbit earth)

Question 10

What can orbits be presumed as?

Answer 10

circular

Question 11

How is presuming orbits circular helpful?

Answer 11

F = GMm/r^2 = mv^2/r

Question 12

What do field lines show?

Answer 12

The direction of the force a mass would feel

Question 13

Properties of gravitational fields

Answer 13

• always attractive
• can’t be shielded
• always the same no matter whats placed in between
• objects have to be large for the field to have any effects i.e. planet size

Question 14

Gravitational field strength, g

Answer 14

force per unit mass

Question 15

What happens to gravitational field strength below the surface of the earth.

Answer 15

g is proportional to the radius at that point. g increases the further you are from the centre until you reach the surface.

Question 16

Gravitational potential at a point alternative definition

Answer 16

the work done in moving a unit mass from infinity to that point

Question 17

Why is gravitational potential negative?

Answer 17

you have to do work against the gravitational field to move an object out of it.

Question 18

The further you are from the centre of a radial field…

Answer 18

the small the gravitational field strength/ the gravitational potential

Question 19

At an infinite distance from a point mass the gravitational potential will be…

Answer 19

0

Question 20

is a mass in moved along equipotentials…

Answer 20

no work is done against gravity

Question 21

g =

Answer 21

= F/m

= GM/r^2

Question 22

gravitational force of attraction =

Answer 22

= mass * centripetal acceleration

GMm/r^2 = m*v^2/r
g = v^2/r

Question 23

gravitational potential, V =

Answer 23

-GM/r

Question 24

Why is gravitational potential negative

Answer 24

because gravity pulls masses together

Question 25

gravitational potential at a point

Answer 25

the work done agaisnt gravity when a 1kg mass is brought from infinity to that point

Question 26

Energy change when an object moves in a field =

Answer 26

change in gravitational potential * mass

Question 27

Keplers third law

Answer 27

T^2 is proportional r^3

Question 28

potential gradient

Answer 28

rate of increase of gravitational potential with distance r

g= dV/dr

Question 29

escape velocity

Answer 29

the speed needed to escape a gravitational orbit

Question 30

escape velocity =

Answer 30

v = √(2GM/R)