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OCR Physics A2 > Gravitational Field (DONE) > Flashcards

Gravitational Field (DONE) Flashcards

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1
Q

Gravitational field equation

A

g=f/m

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2
Q

Gravitational field units

A

Nkg^-1

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3
Q

What is a field

A

A region where an object experiences a force

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4
Q

What other units can a gravitational field have?

A

Ms^-2 (acceleration due to gravity)

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5
Q

What kind of quantity is a gravitational field?

A
  • vector quantity
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6
Q

How to draw radial gravitational field diagram

A
  • straight lines towards centre of mass

- with arrows (towards centre of mass)

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7
Q

Types of gravitational field

A
  • radial (drawn from centre point, the field is stronger nearer the centre of mass)
  • uniform (field lines are parallel, even field strength throughout field)
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8
Q

Where are gravitational fields found

A
  • around any object with a mass
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9
Q

How do gravitational fields of different objects interact in space

A
  • attractive forces would eventually pull objects together
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10
Q

What is the relationship between the size of attractive force and mass

A

Force is directly proportional to mass

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11
Q

What does Newton’s third law show when talking about the attractive forces of 2 objects

A
  • the attractive force of object A on object B, is equal to the force of object B on object A.
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12
Q

What is the relationship between attractive force and distance between 2 centre of masses?

A

F is directly proportional to 1/(r^2)

- meaning if r (distance between centre of masses) doubles the force will quarter.

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13
Q

By bringing all 3 gravitational field equations together, what is newtons law of gravitation?

A

F is directly proportional to Mm/(r^2)
Therefore:
F = -GMm/(r^2)

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14
Q

What s the value of constant G?

A

6.67 x 10^-11

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15
Q

What are the units of the constant G?

A

Nm^2kg^-2

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16
Q

Why is the force of a gravitational field negative?

A
  • because the force is attractive towards centre of mass
17
Q

What is newtons law of gravitation

A

F=-GMm/(r^2)

18
Q

Summarise newtons law of gravitation

A
  • the force between 2 objects is proportional to the product of the masses and inversely proportional to their separation squared.
19
Q

What is the equation for gravitational field strength

A

g=-GM/(r^2)

20
Q

How do you derive the equation for gravitational field strength

A 
1. Sub F=-GMm/(r^2) into g=f/m
Therefore:
g=-GMm/(r^2)m
2. Cancel terms
So:
g= -Gm/(r^2)
21
Q

gravitational field strengths relationship with mass is…

A

Directly proportional to mass

22
Q

What is gravitational field strengths relationship with radius

A

g is inversely proportional to r

G is proportional to 1/(r^2)

23
Q

How can gravitational field strength and radius be represented on a graph

A

r on x axis
g on y axis

  • from r0 1/(r^2) relationship, line never touches x axis but gets closer and closer
  • linear relationship from surface of mass towards centre of mass, gravitational field strength is zero at centre of mass
24
Q

What is the gravitational field strength at the centre of mass

A

Zero

25
Q

Summary of escape velocity

A
  • the speed needed to escape the gravitational pull of a mass
26
Q

When talking about the escape velocity, what is the equation for the potential energy

A

Ep= GMm/r

27
Q

How do you derive the equation for escape velocity

A

*provided the kinetic energy exceeds potential energy
Ek (kinetic energy) = Ep (potential energy)
(1/2)mv^2 = GMm/r
1. Cancel m’s from equation
2. V= root(2GM/r)

28
Q

What is keplers first law

A
  • the orbit of a planet is an ellipse with the sun at one of two foci
29
Q

What is a foci

A
  • the 2 centre points that form an ellipse
30
Q

When talking about keplers first law what does the distance from 2 foci to the same point on an ellipse equal?

A

The distance from a to point x and b to point x equals a constant for any point on an ellipse
a + b = constant

31
Q

What is keplers 2nd law?

A
  • a line segment joining a planet and the sun sweeps out an equal area during equal time intervals.
32
Q

What is keplers 3rd law?

A
  • the square of the orbital period T, is directly proportional to the cube of the average distance from the sun r.
    Therefore:
    T^2 proportional to r^3
33
Q

What is the force causing a planet to move in a circular path?

A
  • centripetal force
34
Q

What is the equation for the centripetal force?

A

F=mv^2/r

35
Q

Prove that T squared is proportional to r cubed

A
  1. Use equations F=-GMm/(r^2) and F=mv^2/r
  2. Substitute
  3. Simplify to GM/r = v^2
  4. Use S=D/T to find v=2(pie)r/T for a planet
  5. Substitute v into equation (3)
  6. Rearrange to find T^2= [4(pie^2)/GM]r^3
  7. Therefore T squared is proportional to r cubed
36
Q

How is the centripetal force provided

A
  • centripetal force is provided by the gravitational pull
37
Q

What does it mean when a satellite has a low earth orbit?

A
  • the value of r is very small (close to earths surface)
  • some rotate at 90 degrees to earths rotation (from north to south poles)
  • used for:
    Communications
    Military
    Weather
    Gps
38
Q

What effect does a low value of r have on a satellite?

A
  • low value of r leads to high value of v

v=root(GM/r)

39
Q

What is a geostationary satellite?

A
  • satellites which orbit above equator with a time period of 24 hours equal to the speed of the earths rotation
  • occupy fixed portion of land
  • good for communications

OCR Physics A2 (15 decks)

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