Gravitational Fields

Question 1

define gravitational field

Answer 1

region of space where a mass experiences a force due to the gravitational attraction of another mass

Question 2

direction of a gravitational field

Answer 2

towards the centre of mass

Question 3

define gravitational field strength

Answer 3

the gravitational field strength at a point is the force per unit mass of an object at that point

Question 4

differences between a gravitational field and electric field

Answer 4

• G is only between masses, E is between charges

- E can be both attractive and repulsive; G is only attractive

Question 5

what is a uniform sphere and why can it be considered a point mass

Answer 5

• evenly distributed mass

- its mass seems to act from the centre: a point

Question 6

when is an object regarded as a point mass

Answer 6

its gravitational field covers a very large distance as compared to its size, so, to study its size, motion or dimension can be neglected

Question 7

what are radial fields

Answer 7

• non uniform fields (not parallel, equally spaced lines)
• go circular around an object
• so the gravitational field strength varies with distance from the centre

Question 8

state newtons law of universal gravitation

Answer 8

the gravitational force between 2 point masses is proportional to the product of the masses and inversely proportional to the square of thier separation

Question 9

state the conditions required to use Fg=Gm1.m2/r^2

Answer 9

• objects must be point masses

- separation must be large

Question 10

what provides planets with the centripetal force to stay in their circular orbits.

Answer 10

gravitational force between sun and planet, which is perpendicular to the direction of travel
therefore there is centripetal acceleration as the direction is constantly changing but with a constant speed in its fixed orbit

Question 11

state Keplers 3rd law of planetary motion

Answer 11

for planets/satellites in circular orbit about the same central body, the square of the time period T is proportional to the cube of the radius of the orbit

Question 12

Formula for strength of gravitational field

Answer 12

g=Fg/m

Question 13

Formula of newton’s law of universal gravitation

Answer 13

FG= Gm1•m2 / r^2 hence G is the constant of proportionality

Question 14

What equation do you get after equating newton’s gravitation equation to centripetal force of a planet

Answer 14

v^2 = GM /r

Question 15

What is keplers 3rd law equation

Answer 15

T^2 = 4pi^2×r^3 /GM

Question 16

What does the gravitational field strengrh at a point describe

Answer 16

How strong or weak a gravitational field is at that point

Question 17

2nd formula for gravitational field strength

Answer 17

g= GM / r^2 where M is the one causing the field

Question 18

Is gravitational field strength vector or scalar

Answer 18

Vector

Question 19

What is the direction of g

Answer 19

Towards centre of the body creating the gravitational field

Question 20

Outside the earth’s surface g is not constant. How does it vary then?

Answer 20

As r increases g decreases by a factor or 1/r^2

Inverse Square law relationship

Question 21

Describe graph of g against r

Answer 21

-when rR, g is inversely proportional to r^2 ie a reciprocal graph: as x increases y rapidly decreases

Question 22

why is g approximately constant for small changes in height above earth’s surface

Answer 22

because compared to earths radius any heigh addition, according to the inverse sqaure law relationship g ~1/r^2, the value doesnt change by much

Question 23

define GPE on the earths surface

Answer 23

energy object has when lifted off ground because work is done to do this
ie change in GPE= mg*change in h

Question 24

define GPE outside the earths surface

Answer 24

energy object possesses due to its position in a gravitational field

Question 25

define gravitational potential

Answer 25

work done per unit mass in bringing a test mass from infinity to a defined point (aka gpe/unit mass)

Question 26

explain your understanding regarding gravitational potential

Answer 26

whe bringing an object from infinity back to earth energy is lost because work is done by gravity to attract the object back (positive work)
whereas in the opp case as an object moves away from earth, GPE is gained with increasing distance to infinity because work is done against gravity. and this GPE increases because with more distance more work must be done against gravity to move from there to infinity. At infinity GPE=0 so even though we say GPE is increasing with distance, it is denoted in the negative because this is the only correct mathematical way to reach infinity by obeying the thoery ie values becomes less negative and therefore both approach 0 and also increase

Question 27

what is the equation for gravitional potenial, fye

Answer 27

= -GM/r

Question 28

what is the significance of the - on the formula for gravitional potenial

Answer 28

since the gravitional potenial at infinity is 0, and the gravitional potenial of all points that are below infinity is less than 0, these all must be negative

Question 29

is kinetic energy related to gravitational force and centripetal force? (planets)

Answer 29

yes.

equate GF to CF then make v^2 subject and substitute in 0.5mv^2

Question 30

state what is meant by gravitational force

Answer 30

force exterted by an object in a gravitional field on another mass in that field