Gravitational fields

Question 1

Newton’s universal law of gravitation

Answer 1

All masses attract each other.
The strength of Fg of attraction between any 2 masses is proportional to the magnitude of each of the masses.

Question 2

Inverse square law

Answer 2

Fg between 2 masses gets weaker as distance between them increases.
It’s inversely proportional to the square of the distance between the centres of the 2 masses.

Question 3

Value of gravitational force

Answer 3

The value of the gravitational constant is very small, and over short distances, g-forces are generally weaker than electrical and magnetic forces.
Radially inward net force on a satellite.

Question 4

Gravitational field

Answer 4

A vector field describing the property of space that causes an object with mass to experience a force in a particular direction.
The larger the primary mass, the greater the magnitude of g-field.

Question 5

Fg of a body on Earth

Answer 5

“F on body by Earth”

Question 6

Field

Answer 6

A physical quantity that has a value at each point in space.

Question 7

Gravitational field strength

Answer 7

g in Nkg^-1

Question 8

g-Field arrows

Answer 8

Show the direction of F that a mass at a point would experience.

Question 9

Spacing of field lines

Answer 9

Indicate field strength - closer lines indicate a stronger field.

Question 10

Why do field lines never touch/intersect?

Answer 10

As the force on an object cannot have multiple magnitudes or directions at the same time.

Question 11

Why are g-fields non-uniform?

Answer 11

The field lines become further apart as the distance from a primary increases, showing that the field is weakening.

Question 12

Primary

Answer 12

The body orbited by a smaller satellite or companion.

Question 13

Earth’s g-field

Answer 13

Is static, it doesn’t change with time.

Question 14

g-fields from 2 masses

Answer 14

The vector (direction) sum of the 2 fields

Question 15

Acceleration from g-fields

Answer 15

When Fg is the only force acting on a mass, the mass is in ‘free fall’.
In free fall, acceleration experienced by the mass is equal to the g-field at that point in space.

Question 16

Satellite

Answer 16

An object that’s orbiting a larger central mass. Satellites can be natural or man-made.
Circle a primary at constant speeds.

Question 17

Centripetal acceleration

Answer 17

The acceleration towards the centre of a circle experienced by an object moving in a circular motion.
Causes velocity to constantly change direction.

Question 18

Velocity

Answer 18

= 2 pi r / T

Question 19

Orbital period

Answer 19

The time taken for a satellite to complete one orbit around a central object.

Question 20

Newton’s gravitational relationship

Answer 20

G M / 4 pi ^2 = r ^3 / T ^2

Question 21

Kepler’s 3rd Law

Answer 21

Applies to all satellites going around the same central mass.
(r1 / r2)^3 = (T1 / T2)^2
You must be consistent with units used.

Question 22

Geostationary

Answer 22

Stationary relative to a point directly below it on Earth’s surface. A geostationary orbit has the same period as the rotation of Earth.

Question 23

Geostationary satellites

Answer 23

Required to stay constantly above one place on Earth’s surface.
So, they must have an orbital period that’s the same as the time for the primary to complete one rotation about its axis.

Question 24

Astronaut in a spacecraft

Answer 24

The astronaut’s acceleration is independent of the spacecraft.
There’s no normal force unless the astronaut is strapped to a surface.

Question 25

Weightlessness

Answer 25

When there is no forces acting on a mass, no gravitational field, and therefore, the mass experience’s freefall.

Question 26

Apparent weightlessness

Answer 26

When there is no normal force acting on a mass. As gravity is a non-contact force, it cannot be felt without any opposing force.

Question 27

Kinetic energy

Answer 27

The energy associated with the movement of an object. A scalar quantity.

Question 28

Gravitational potential energy

Answer 28

Energy stored in an object as a result of its position relative to another object (primary) to which it’s attracted by Fg.

Question 29

Force-distance graph

Answer 29

The change in GPE = area under graph

Question 30

Field-distance graph

Answer 30

The change in GPE = area under graph x mass as area = (newtons per kilogram) x metre, so we must multiply by mass to calculate energy.

Question 31

G-field close to Earth’s surface

Answer 31

Is relatively constant as the distance from the primary doesn’t significantly vary.