5.4: Gravitational Fields, F

Question 1

Define: gravitational field

Answer 1

Region around a body in which other bodies around it experiences a non-contact force due to the mass of the body

Question 2

Define: gravitational field strength

Answer 2

Any point in a gravitational field is the force acting per unit mass at that point, (g=F/m)

Question 3

The force experienced by an object is always __________

Answer 3

The force experienced by an object is always attractive

Question 4

Define: Newton’s Law of Gravitation?

Answer 4

• Gravitational force of attraction between two masses is directly proportional to the product of their masses and inversely proportional to the square of their separation.
• An inverse square law: F ∝1/r^2

Question 5

Explain using Newton’s law of gravitation, what would happen if you manipulated one of the variables

Answer 5

-If the distance(r) increases then the force(F) will decrease
and
-If the distance(r) doubles then the force(F) will be one quarter(1/4) the strength of the original force

Question 6

Define: Gravitational field lines

Answer 6

Lines which show the shape and direction of the force that masses would feel when placed at that point

Question 7

The Earth’s gravitational field is radial. What does it mean by that

Answer 7

The lines of force meet at the centre of the Earth

Question 8

In a ______ Field, g is _________ Proportional to r^2

Answer 8

In a radial field, g is inversely proportional to r^2

Question 9

Define: Gravitational Potential

Answer 9

Work done in moving a Unit Mass from infinity to that point

Question 10

Vg= - GM/r (Vg being gravitational potential in J/kg)

Why is Gravitational potential negative?

Answer 10

Work is done against the gravitational field to move an object out of the gravitational field.

Question 11

Explain what Gravitational Potential difference is

Answer 11

Work done moving a Unit mass. 2 points at different distances will have different gravitational potentials ∴ the difference is the amount of energy you need to move the object

Question 12

Define: Gravitational potential energy

Answer 12

Gravitational potential is work done per unit mass ∴ gravitational potential energy of an object at a point in a gravitational field is E =mVg

Question 13

Define: Escape velocity

Answer 13

• Minimum lunch velocity required required to move an object from that point to infinity
• (minimum energy to escape a gravitational field)

Question 14

How can you derive escape velocity?

Answer 14

By equating kinetic energy to gravitational potential energy
0.5mv^2 = GMm/r
rearrange and cancel m’s

Question 15

What is a satellite?

Answer 15

Any small mass which orbits a larger mass

e.g. moon is a satellite of earth

Question 16

What factors does the speed of an orbit rely on

Answer 16

• Radius

- Mass of the larger body

Question 17

How can we derive the equation to find the speed of an orbit

Answer 17

By equating centripetal force with the force of gravity
F =-GMm/r^2 and F = mv^2/r

rearrange and cancel m’s to find v (orbit speed)

Question 18

How can we derive the time period for one orbit

Answer 18

By equating 2 circular motion equations and subbing in the equation for orbit velocity
v=wr and w=2π/T

Question 19

Define: Geostationary orbit

Answer 19

An orbit of the earth made by a satellite that has the same time period and orbital direction as the rotation of earth(24hrs) and is in the equatorial plane(above equator)

Question 20

What are geostationary satellites used for

Answer 20

• Monitoring air traffic
• Monitoring weather and changes in environment
• TV signals and relying mobile telephone signals

Question 21

What are the downsides of using geostationary satellites

Answer 21

• Expensive

- Small risk of something going wrong and it falling back to earth

Question 22

State Kepler’s first law

Answer 22

Planets travel around the sun in elliptical orbits

Question 23

State Kepler’s second law

Answer 23

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

Question 24

Define: Kepler’s third law

Answer 24

The time period squared of a planet orbiting the sun is proportional to the mean radius of its orbit cubed, or T^2 ∝ r^3

Question 25

How can Newton’s law of gravitation be used to explain how thick a planet’s atmosphere is

Answer 25

• A planet exerts a force all the particles, otherwise they would float off into space.
• The larger a planet, the larger the force is further away from its surface ∴ the more atmosphere particles it can stop escaping, leading to a thicker atmosphere