Rotational Motion and Astrophysics

Question 1

Define gravitational potential.

Answer 1

The work done by an external force in moving a unit mass from infinity to a point in a gravitational field.

Question 2

Define the conservation of angular momentum

Answer 2

Angular momentum is conserved in the absence of external torques.
Total angular momentum before = total angular momentum after, in the absence of external torques.

Question 3

Define Gravitational field strength

Answer 3

The gravitational force acting on a unit mass.

g=F/M

Question 4

Define ‘Escape velocity’

Answer 4

Escape velocity is the minimum velocity required to just escape a planet’s gravitational field and reach infinity with zero kinetic energy or zero velocity.

Question 5

Will a high moment of inertia mean an object spins faster or slower?

Answer 5

High moment of inetria= spins slower
(Rotating bodies veritasium video)

Question 6

What is Centripetal Force?

Answer 6

It is the unbalanced force which keeps an object moving in a circle.

Question 7

Define Schwarzschild Radius

Answer 7

The distance from the centre of the black hole to it’s event horizon.

Question 8

State what it is meant by the ‘Equivalence Principle’

Answer 8

It is impossible to tell the difference between the effects of a uniform gravitational field and of a constant acceleration.

Question 9

What is it meant by an inertial frame of reference?

Answer 9

An inertial frame of reference is one that is non-accelerating/ has a constant velocity. (Theory of Special Relativity)

Question 10

What is it meant by a non-inertial frame of reference?

Answer 10

A non-inertial frame of reference is one that is accelerating. (Theory of General Relativity)

Question 11

World Lines

Question 12

Will clocks run slower or faster in a stronger gravitational field?

Answer 12

Clocks run slower in a stronger gravitational field (compared to clocks in a weaker gravitational field).

Question 13

What is the value of the Universal constant of gravitation? (G)

Answer 13

G = 6.67x10-11 m3 kg-1 s-2

Question 14

Define Velocity

Answer 14

Velocity is the rate of change of displacement with time.

Question 15

Define Acceleration

Answer 15

Acceleration is the rate of change of velocity with time.

Question 16

How are displacement, velocity and acceleration linked?

Answer 16

a= dv/dt
v= ds/dt
s

Question 17

What tool can you use to find the area under a line on a graph?

Answer 17

Integration.

Question 18

What is the area under the velocity-time graph equal to?

Answer 18

Displacement

Question 19

What is it meant by the ‘moment of inertia of an object’?

Answer 19

It is a measure of the object’s reluctance to a change in its rotational motion.

Question 20

What is moment of inertia dependant upon?

Answer 20

The mass and the distribution of mass about a given axis of rotation.

Question 21

Does the path taken to move a mass between two points in a gravitational field affect the energy required to move that mass?

Answer 21

No.
The energy required to move a mass between two points in a gravitational field is independent of the path taken.

Question 22

What is spacetime considered to consist of?

Answer 22

3 dimensions of space and 1 dimension of time.

Question 23

What is a geodesic?

Answer 23

A path with the shortest distance between two points.

Question 24

What is the escape velocity from the event horizon of a black hole equal to?

Answer 24

The speed of light.

Question 25

If you looked at the event horizon of a black hole, what would you describe time to appear as?

Answer 25

Time would appear to be frozen to a distant observer at a black hole’s event horizon.

Question 26

How are stars born?

Answer 26

Stars are formed in interstellar clouds.
Gravitational forces overcome thermal pressure and cause a molecular cloud to contract until the core becomes hot enough to sustain nuclear fusion.
This provides a thermal pressure that balances the gravitational force.

Question 27

What does the mass of a star determine?

Answer 27

Its lifetime.

Question 28

What does light or a freely moving object follow in spacetime?

Answer 28

A geodesic

Question 29

What is meant by a ‘black body’?

Answer 29

A black body absorbs radiation and emits radiation at the same rate.

Question 30

What is one assumption we make about stars?

Answer 30

That all stars behave as black bodies

A black body absorbs radiation and emits radiation at the same rate

CFE Specimen Q4, https://lco.global/spacebook/light/black-body-radiation/#:~:text=Stars%20behave%20approximately%20like%20blackbodies,green%20part%20of%20the%20spectrum.