Unit 4: Gravitation

Question 1

What is a field and apply said definition to gravitation and the basic equation of it:

Answer 1

A field is a region in which other objects with certain properties experience a force, in the case of gravitational, objects with mass. g = F/m

Question 2

What are the units of a gravitational field:

Answer 2

NKg^-1 ( Newtons per kilogram )

Question 3

What other unit is popularly used to describe a gravitational field:

Answer 3

ms^-2

Question 4

How is a gravitational field drawn:

Answer 4

When drawing a gravitational field and field lines, one must draw 8 arrows with the directions of said arrows because a gravitational field is a vector quantity. in a radial shape.

Question 5

What type of field (radial or uniform) would one use when focussed on the surface of a large planet:

Answer 5

When focussed on the surface of the earth or planets, the field is treated as uniform

Question 6

What is Newton’s law of gravitation:

Answer 6

The force between two objects is proportional to the product of the masses and inversely proportional to their separations squared

Question 7

Why is Newton’s equation for his law of gravitation negative

Answer 7

The reason F = -GMm/r^2 is negative, is because gravity is always attractive

Question 8

Derive g = -GM/r^2 and state what it implies:

Answer 8

Combine g = F/m and F = -GMm/r^2 . It implies that field strength is proportional to the mass of the object creating the force and inversely proportional to the distance from the mass squared

Question 9

Describe a graph of g against r for the earth:

Answer 9

see graph

Question 10

Why is g = 0 at the centre of the earth:

Answer 10

Mass all around pulling outwards

Question 11

What provides centripetal force for satellites:

Answer 11

Gravity

Question 12

How does one derive velocity = sqrt(GM/r) and what does it imply:

Answer 12

Because gravity is the centripetal force mv^2/r = GMm/r^2, therefore v^2 = GM/r. The derived equation implies that satellites at certain heights require certain velocities to move.

Question 13

What types of orbit:

Answer 13

• Polar orbit -> low r value, move over both poles and perpendicular to the direction the earth is spinning
• Low earth orbit -> r is low therefore speed is high such that time period is low
• Geostationary -> rotate in the same direction as the earth and T=24 hours

Question 14

What are low earth orbit satellites used for:

Answer 14

• Communications
• Military -> because they cover a lot of the globe
• Scientific -> monitor climate and look out to space
• Weather monitor
• GPS

Question 15

What are the units of gravitational potential:

Answer 15

JKg^-1

Question 16

What is the symbol for gravitational potential:

Answer 16

Vg

Question 17

Define gravitational potential:

Answer 17

The work done in bringing a unit mass from infinity to that point.

Question 18

How can one calculate work done per unit mass to bring an object to a point in a gravitational field and what does the equation imply:

Answer 18

Vg = -GM/r. Implies that as M increases the magnitude of the potential increases and when r increases the magnitude of potential decreases proportionally.

Question 19

Define gravitational potential energy:

Answer 19

Work done to move a mass from infinity to a point in a gravitational field.

Question 20

How is gravitational potential energy calculated:

Answer 20

E = m*Vg but more useful to think about ΔE=mΔVg

Question 21

How does one change their gravitational potential energy in a uniform field:

Answer 21

Change their altitude

Question 22

How can one calculate and change their GEp in a radial field:

Answer 22

Combining equations ΔE=mΔVg and Vg = -GM/r to get E = -GMm/r one can see that to change GEp in a radial field, one must change radius.

Question 23

How can someone see changes in GEp from a force/distance diagram:

Answer 23

When looking at a force distance diagram where the force represents the force due to gravity on a body one will move, a change in GEp can be seen by the area beneath that curve.
See graph