5.3 Gravitational Fields

Question 1

What are gravitational fields due to?

Answer 1

Objects having mass

Question 2

What are gravitational field lines?

Answer 2

Show direction of the force that masses would feel in a gravitational field

Question 3

What is a gravitational field?

Answer 3

Region where objects experience a non contact force

Question 4

What are some properties of gravitational fields?

Answer 4

Force felt in field is Always attractive

It’s a vector

Question 5

What is Newton’s law of graviton?

Answer 5

F=-GMm/r^2

Question 6

What is gravitational field strength?

Answer 6

Force per unit mass

Question 7

Where is gravitational field strength uniform?

Answer 7

Surface of earth

Question 8

What is Kepler’s first law?

Answer 8

Each planet moves in an ellipse around the sun, with the sun at one focus

Question 9

What is Kepler’s second law?

Answer 9

A line joining the sun to a planet will sweep out equal areas in equal times

Question 10

What is Kepler’s third law?

Answer 10

The period of the orbit and the mean distance between the sun and the planet are related by Kepler’s third law, T^2 proportional to r^3

Question 11

What provides the centripetal force on a planet?

Answer 11

The centripetal force on a planet is provided by

the gravitational force between it and the Sun

Question 12

How do you derive the equation

T^2=(4pi^2/GM)r^3

Answer 12

Use F=-GMm/r^2

F=mv^2/r and v=2pir/T

Question 13

What are geostationary satellites?

Answer 13

Orbit directly over the equator and are always above the same point on Earth

Question 14

What is the angular speed of a geostationary satellite the same as?

Answer 14

Same angular speed as the Earth turn below it

Question 15

How long does a geostationary satellites orbit take?

Answer 15

24 hours

Question 16

What are the uses of geostationary satellites?

Answer 16

Sending TV and telephone signals improving communication in the world

Question 17

Why don’t you have to alter the angle of receivers or transmitters on earth?

Answer 17

The satellite is stationery relative to a certain point on the Earth

Question 18

What are the downsides to satellites?

Answer 18

They are expensive and pose a small risk of falling back to Earth

Question 19

How can Newton’s law of graviton help to explain the thickness of a planets atmosphere?

Answer 19

Planets gravitational field exerts a force on everything around it, including the particles which make up its atmosphere (otherwise particles would float up to space)

The bigger the planet, the larger the force is further from the planet’s surface, so the more atmosphere particles it can stop escaping into space

So thicker atmosphere

Question 20

Define gravitational potential

Answer 20

the work done in bringing unit mass from infinity to the point; gravitational potential is zero at infinity

Question 21

Describe a force distance graph due to the gravitational field of a point mass, what is the area underneath?

Answer 21

Decreasing negative gradient, (starting from r1)

Area underneath is work done

Question 22

Define escape velocity

Answer 22

Velocity needed so an object has just enough kinetic energy to escape a gravitational field.

Question 23

When does an object have a high enough escape velocity?

Answer 23

When an object kinetic energy is equal and opposite to its gravitational potential energy, so total energy is Zero

Question 24

How do you derive equation for escape velocity?

Answer 24

1/2mv^2 + (-GMm/r) = 0

Cancel cancel

V=root(2GM/r)