Gravity

Question 1

what is gravity?

Answer 1

• gravity acts on any object which has a mass and is always attractive

Question 2

what is Newton’s Law of gravitation?

Answer 2

• the magnitude of the gravitational force between two masses is directly proportional to the product of the masses.
• inversely proportional to the square of the distances between them

Question 3

what is the equation for Newton’s Law of Gravitation?

Answer 3

F = Gm1m2/r^2

Question 4

what is the value of G

Answer 4

6.67x10^-11

Question 5

what is gravitational field strength?

Answer 5

• force per unit mass exerted by a gravitational field on an object

g = F / m

Question 6

how can we calculate gravitational field strength?

Answer 6

g = F / m

or

g = GM/r^2 ( radial fields only )

Question 7

what is gravitational potential?

Answer 7

• work done per unit mass against gravitational force to move an object from infinity to a given point

Question 8

why is gravitational potential negative?

Answer 8

• gravitational potential at infinity is zero
• as an object moves from infinity to a point, energy is released as the GPE is reduced

Question 9

what is the equation for gravitational potential?

Answer 9

V = - GM / r

Question 10

what is gravitational potential difference?

Answer 10

• energy needed to move a unit mass between two points in a g field

Question 11

how do you calculate the work done to move a mass

Answer 11

mass x change in potential

Question 12

what are equipotential surfaces

Answer 12

• surfaces which are created by joining points of equal potential together
• potential on an equipotential surface is constant everywhere

Question 13

how can you measure the gravitational field strength graphically

Answer 13

• drawing a tangent to the curve at that distance and calculating its gradient
• multiply the answer by -1

( for a V-r graph )

Question 14

what is keplers third law

Answer 14

• square of the orbital period is directly proportional to the cube of the radius

Question 15

how can we derive keplers third law

Answer 15

mv^2 / r = GMm / r^2

v^2 = GM / r

v = 2rpi / T so v^2 = 4r^2pi^2 / T^2

4r^2pi^2 / T^2 = GM / r

T^2 = 4pi^2r^3 / GM

Question 16

what is the total energy of an orbiting satellite

Answer 16

• total energy is made up of its kinetic and potential energy
• if the height is decreased its GPE will decrease, however it will travel at a higher speed meaning its KE increases
• total energy ALWAYS kept constant

Question 17

what is meant by escape velocity?

Answer 17

• minimum velocity it must travel at, in order to escape the gravitational field at the surface of a mass

Question 18

what is the equation for escape velocity

Answer 18

v = root 2GM / r

Question 19

what is a synchronous orbit?

Answer 19

• period of the satellite is equal to the rotational period of the object its orbiting

Question 20

what is a geostationary satellite

Answer 20

• orbital period of 24 hours
• orbit directly above the equator
• useful for sending TV and telephone signals, so no need to alter position of the aerial or transmitter

Question 21

what does the area under a g-r graph show?

Answer 21

the gravitational potential difference