Chapter 11 - Out into Space

Question 1

Kepler’s Laws

Answer 1

1. Orbits are ellipses with the sun at one focus. 2. Line from the sun to a planet sweeps out equal areas in equal times. 3. Time period and radius of orbit related by: T^2 is proportional to r^3

Question 2

What is a point mass?

Answer 2

Objects that behave as if all their mass is concentrated in the centre.

Question 3

What is the gravitational field strength?

Answer 3

Force per unit mass given by the equation F = mg thus g = F/m

Question 4

Equation for gravitational field strength

Answer 4

g = - GM/r^2

Question 5

What is gravitational potential energy?

Answer 5

The work that would need to be done to move a mass against the force of gravity.

Question 6

Key points about GPE graph

Answer 6

1. Mass on earth’s surface has negative GPE. 2. As you move a mass away from the earth it gains GPE. 3. GPE is 0 at an infinite distance away from Earth. 4. Force is greatest at Earth’s surface where tangent to graph is steepest.

Question 7

What is gravitational potential?

Answer 7

Potential energy per unit mass.

Question 8

Centripetal force = gravitational force rearrangement

Answer 8

v = root(GM/r)

Question 9

What is a standard candle?

Answer 9

An object that you can measure the brightness of directly. E.g cepheid variable stars: their brightness changes in a certain pattern. thus finding such a star in a galaxy can allow you to determine the distance of the galaxy from us.

Question 10

What is 1 AU

Answer 10

Mean distance between the Earth and the Sun.

Question 11

Describe the field lines of the gravitational field close to the surface of the Earth

Answer 11

Gravitational field is almost uniform - field lines are almost parallel and may be assumed that field is perfectly uniform.

Question 12

velocity of a geostationary orbit

Answer 12

V = (2pi r)/T v = r w

Question 13

What must be done to put a satellite into a circular orbit around the earth?

Answer 13

Balance the centripetal and gravitational forces. mv^2/r = GMm/r^2 Therefore v = root(GM/r) or v = root(gr) since gr = GMr/r^2

Question 14

Inside a sphere of uniform mass, describe the relationship between the gravitational force and the distance of an object from the centre of a sphere.

Answer 14

Gravitational force is directly proportional to the distance of an object from the centre of the sphere.

Question 15

What must be done to completely leave the orbit of the earth?

Answer 15

Convert KE to PE.

1/2 mv² = GMm/r

Rearrange to find v = root(2GM/r)