2. Gravity and motion

Question 1

In the Star Wars Episode IV movie, Han Solo states about his spaceship, the Millennium Falcon: It’s the ship that made the Kessel Run in less than twelve parsecs. Explain if this statement makes sense or not.

Answer 1

Parsec is a measure of distance, not speed.

Question 2

Why do objects move at a constant velocity if no force acts on them?

Answer 2

Newtons first law

Question 3

What are Kepler’s laws of motion?

Answer 3

1. The orbit of a planet is an ellipse with the Sun at one of the two foci
2. A line segment joining a planet and the sun sweeps out equal areas during equal intervals of time
3. The square of a planets orbital period is proportional to the cube of the length of the semi major axis of its orbit

Question 4

What is velocity?

Answer 4

Velocity is the rate and direction of an objects movement

Example: if you move 5kmph in a circle and finish where you started, your speed is 5kmph but your velocity is 0

Question 5

Explain how we can use Newton’s version of Kepler’s third law to measure the total mass of two stars in a binary system.

Answer 5

We can measure how far they move apart from each other (their orbital distance) and how long it takes them to move around each other (their orbital period). Then you use the formula to calculate M1+M2.

Question 6

What is the centre of mass?

Answer 6

The unique point

Weighted Relative Position

Distributed mass

Sums to zero

Question 7

What is mass?

Answer 7

Mass is the amount of matter in an object

Question 8

What determines the strength of gravity?

Answer 8

The masses of objects and the distance between them

Question 9

Standard time is defined…

Answer 9

With respect to the time zones

Question 10

What is weight?

Answer 10

Weight is the the force acting on an object due to gravity

Question 11

What are Newtons 3 laws?

Answer 11

1. Every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force
2. The change of motion of a body is proportional to and in the direction of the force acting on it
3. For every action there is an equal and opposite reaction

Question 12

What is latitude an longitude?

Answer 12

A system of lines used to describe the location of any place on Earth. Lines of latitude run in an east-west direction across Earth. Lines of longitude run in a north-south direction

Question 13

What is force?

Answer 13

Force is a physical influence, which when applied to an object causes it to accelerate in the direction from which it was applied

Question 14

Newtons version of Kepler’s 3rd law

Answer 14

Newtons formulation introduces the additional factor of the masses of the Sun (M1) and the planet (M2), both expressed in units of mass.

a^3=(M1+M2)xP^2

Question 15

How does the tilt of the Earth cause the seasons?

Answer 15

The Earths spin axis is tilted with respect to its orbital plane. When the Earths axis points towards the Sun it is summer for the hemisphere

Question 16

Mean solar time

Answer 16

Defined locally based on the average solar day over the course of the year. Contains exactly 24 hours and is what we use in our every daytime keeping

Question 17

Describe the shape of planetary orbits in our solar system

Answer 17

Elliptical

Question 18

What is angular momentum?

Answer 18

Angular momentum is the rotational analogue of linear momentum. It is a product of mass, velocity and radius.

Question 19

What is acceleration?

Answer 19

Acceleration is the rate of change of velocity

Question 20

Explain why orbital period is independent of eccentricity

Answer 20

Eccentricity is the ratio of the distance between the foci to the length of the semi major axis of the ellipse.

Orbital period is the time it take a planet to travel once around the Sun.

Orbital period depends on the distance to the centre of the object being orbited and the mass of that object.

Question 21

What keeps a planet rotating and orbiting the Sun?

Answer 21

Gravity

Question 22

Formula for Newtons law of gravity

Answer 22

Question 23

If you could somehow compress the Sun so that it still had all its mass but had a radius of only three kilometres, it would become a black hole!
What effect would this have on the Earth’s orbit?

Answer 23

While 8.3 minutes after this occurred the Sun would disappear from the Earth’s view, its gravitational force would remain. Gravity simply depends on masses of objects and their separation. Neither the size nor density matter, and so indeed there would be no effect on the orbit.

Question 24

Explain how planets are under constant acceleration from the force of gravity from the Sun

Answer 24

The gravitational attraction between two bodies is proportional to their masses. The more mass an object has, the stronger the pull of its gravitational force

Question 25

Suppose a solar system has a star that is four times more massive than our Sun. If that solar system has a planet the same size as Earth, orbiting at a distance of 1 AU, what is the orbital period of the planet?

Answer 25

From Kepler’s law, we see that the period depends on the inverse square root of the object masses. Thus, if we have a star four times as massive as the Sun, the period of a planet orbiting at 1 AU will be half that of the Earth, or 6 months

Question 26

What causes eclipses?

Answer 26

An eclipse happens when one astronomical body blocks light from or to another

Question 27

What is gravitational acceleration?

Answer 27

Gravitational acceleration is the acceleration of an object in free fall within a vacuum. This is the steady gain in speed caused exclusively by the force of gravitational attraction

Question 28

What is an ellipse? How is it different from a circle?

Answer 28

An ellipse is a plane curve surrounding two focal points, such that for all points of the curve the sum of the two distances to the focal point is a constant

Question 29

Sidereal day is...

Answer 29

4 minutes shorter than a solar day

Question 30

How long does the cycle of lunar phases take?

Answer 30

29.5 days

Question 31

How do we mark the progression of the seasons?

Answer 31

We define 4 special points: summer and winter solstice, spring and fall equinox

Question 32

Describe the components of an ellipse

Answer 32

Centre Two foci Major and semi major axis Minor and semi minor axis

Question 33

How do we locate objects on the celestial sphere?

Answer 33

Right ascension and declination

Question 34

What is momentum?

Answer 34

Momentum is the product of the mass and velocity of an object

Question 35

Why do we see phases of the Moon?

Answer 35

Its orbit around Earth and Earths orbit around the Sun

Question 36

What is speed?

Answer 36

Speed is the time rate at which an object is moving along a path

Question 37

What is a sidereal day?

Answer 37

The time it takes for the Earth to complete one rotation about its axis with respect to "fixed" stars 4 minutes faster Particular star will rise 4 minutes earlier every night Reason why different constellations are only visible at specific times of the year

Question 38

P^2=a^3

Answer 38

``` P = orbital period in years a = distance in AU ``` Kepler’s third law, says that a planet’s orbital period squared is proportional to the semimajor axis of its orbit cubed.