Chapter 7.1 + 7.2: Gravitational Fields

Question 1

The force felt in a force field is a non-contact force. True or False?

Answer 1

True

Question 2

Is force a vector or scalar quantity?

Answer 2

Vector

Question 3

What are some similarities between electrostatic and gravitational forces?

Answer 3

• inverse square force laws
• potential concept
• equipotential surfaces
• use of field lines

Question 4

What are some differences between electrostatic and gravitational forces?

Answer 4

The gravitational forces from masses always attract, while charges may repel or attract.

Question 5

What is gravity?

Answer 5

Gravity is the universal attractive force which acts between all matter.

Question 6

What is G?

Answer 6

The universal gravitational constant.
Approx. 6.67x10^-11 m^3kg^-1s^-2

Question 7

What can field lines tell you about a field?

Answer 7

The direction of the field and the strength of the field depending on the density of the field lines.

Question 8

What is g?

Answer 8

g is the force per unit area in a uniform field.
In radial field the magnitude of g is the proportionality constant at that point between force and mass
Let g = GM/r^2

Question 9

What is gravitational potential?

Answer 9

The potential energy per kilogram, at any point in the field. 0 potential is defined at infinity, hence at a point close to a mass the potential of an object would be negative.

Question 10

What is the work done by moving a mass in a field?

Answer 10

Mass x change in potential

Question 11

What is gravitational potential difference?

Answer 11

Gravitational potential difference is the difference in the gravitational potentials of 2 points in a gravitational field.

Question 12

What is an equipotential surface?

Answer 12

A surface in which every point on the surface has the same potential.

Question 13

How much work is done when you move 1km in any direction on an equipotential?

Answer 13

No work is done when moving across equipotentials, as the potential at each point is the same.

Question 14

Why is gravitational potential a negative value?

Answer 14

Work needs to be done to move an object from the inside the field to outside the field. Since outside the field’s potential is defined as 0 then the potential inside the field must be negative.

Question 15

How is the orbital period related to the radius of a circular orbit?

Answer 15

T^2 directly proportional to R^3

Question 16

What equations could one use to find the speed of an orbiting satellite?

Answer 16

The orbiting object (mass m) is in circular motion, so we would use F=ma with F=GMm/r^2 rearranged to a=v^2/r=ω^2xr. This can be sold to find the speed (v), angular speed (ω), the radius of the orbit or using T=2π/ω its period.

Question 17

Compare the PE and KE of a lower orbit to a higher one.

Answer 17

A lower orbit (smaller mass) has less potential energy and more kinetic energy than a higher orbit (bigger r).

Question 18

What is the period of a geosynchronous orbit?

Answer 18

Geosynchronous orbits have a period of one day.