Chapter 4 - Gravity

Question 1

What direction does gravity act?

Answer 1

It acts towards the centre of mass.

Question 2

3 rules for drawing a field?

Answer 2

• use lines
• arrows for direction
• lines never touch or cross

Question 3

3 rules for gravitational field lines on sphere?

Answer 3

• Enter at right angle to surface of Earth
• fade out towards centre of Earth
• Start at infinity and end on matter

Question 4

Gravitational force - distance law?

Answer 4

The gravitational force between two objects is inversely proportional to the square of the distance between them.

F = k(1/d^2)

Question 5

What is gravitational field strength (GFS)?

Answer 5

The force per unit mass at any point (N/kg).

Question 6

GFS equation?

Answer 6

GFS = force/mass (N/kg)

Question 7

What is the difference between GFS and g?

Answer 7

GFS - tells you about the gravitational pull of one body on another

g - tell you about the acceleration of a mass under a force

Question 8

When are point masses appropriate?

Answer 8

When the distance between objects is much more than the size of the objects.

Question 9

What is Newtons law of gravity?

Answer 9

The attractive force between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them.

Question 10

Newtons equation for gravity?

Answer 10

F = -GMm/r^2

Question 11

What is a sidereal day?

Answer 11

The time taken for the Earth to rotate on its axis 360degrees (23hrs 56mins).

Question 12

What is a solar day?

Answer 12

The time taken for the sun to be in the same point in the sky (24hours).

Question 13

Equation for g used to find GFS of a point mass?

Answer 13

g= -GM/r^2

Question 14

Equation joining CPF and F?

Answer 14

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

mv^2/r = GMm/r^2

v^2r/G = M

Question 15

Equation joining both Fs?

Answer 15

F = -GMm/r^2
F = mg

g =GM/r^2

Question 16

Why do astronauts appear weightless?

Answer 16

Because their gravitational force (weight) is being used to keep them in orbit (it provides CF and acceleration).

Question 17

Keplers 3rd law?

Answer 17

T^2 is proportional to r^3

Question 18

Keplers equation and how to get there?

Answer 18

T^2= (4π^2/GM)r^3

v=2πr/T

v^2 = 4π^2/T^2

Also v^2 = GM/r

Equating: 4π^2/T^2 = GM/r

Rearrange to get final equation

Question 19

Define satellite?

Answer 19

A body that orbits something else.

Question 20

4 rules for geostationary satellites?

Answer 20

• Orbital period = 24hours
• Spins same way as Earth (W to E)
• Centred on centre of Earth
• In line with equator

Question 21

Define geostationary satellite?

Answer 21

.

Question 22

Uses of geostationary satellites?

Answer 22

Weather forecasting
Radio
Tv

Question 23

Problems with geostationary satellites?

Answer 23

-Very expensive - costs lots to get it into orbit since large gains in KE and GPE must be made.

Question 24

Define a field?

Answer 24

A region in which an object may experience a force.