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Xaverian Physics > Module 5 Gravitational Fields > Flashcards

Module 5 Gravitational Fields Flashcards

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1
Q

Define Newton’s law of gravitation?

A

the force of attraction between two masses is directly proportional to the product of masses and inversely proportional to the square of their separation

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2
Q

In the formula for newtons law of gravitation, why is the value given a negative?

A

attractive force

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3
Q

What value do you substitute in for r in newtons law of gravitation for a satellite orbiting a planet?

A

r is separation between centres
so take radius of planet plus height/distance from surface

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4
Q

What is m and M in equations of gravity?

A

M is central mass and m is the orbiting mass

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5
Q

How do you test if a gravitational field is present?

A

place a small mass m within the suspected field, it will accelerate towards M

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6
Q

How are gravitational fields shown in diagrams?

A

gravitational field lines are arrows in the direction of the field (usually towards centre)

For a radial field - check all field lines meet at the centre of M

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7
Q

What does the density of arrows in a gravitational field diagram represent?

A

strength of the field

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8
Q

How do you draw Earth’s local gravitational field?

A

arrows pointing down evenly spaced and at 90 degrees to surface

This represents a uniform field (equal magnitude and direction)

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9
Q

What can be said about earth’s local field in terms of variation?

A

virtually uniform with constant strength (magnitude) and direction

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10
Q

Define g?

A

gravitational force per unit mass

(g is gravitational field strength)

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11
Q

How can the inverse square law be applied to the relationship between g and R?

A

g=GM/R^2

G is a constant and M is constant for individual planets

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12
Q

What is Kepler’s first law?

A

the orbit of a planet is an ellipse with the sun at one of the foci

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13
Q

What is Kepler’s second law?

A

a line joining a planet and the sun sweeps out equal areas in equal time intervals

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14
Q

What does Kepler’s second law imply in terms of the speed of planets orbiting the sun in an elipse?

A

travel faster when it is closer to the sun.
Greater loss in PE=gain in KE

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15
Q

What is Kepler’s third law?

A

For planets orbiting the same star, time period squared is directly proportional to the orbital radius cubed

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16
Q

What is an approximate value for the mass of the earth?

A

6x10^24kg

17
Q

What is an approximate value for the radius of the earth?

A

6.4x10^6m

18
Q

When using Kepler’s third law in ratio form, what can be done concerning the use of radius or time period?

A

When using the ratio T^2 proportional to r^3

can use different units (days for example) if used consistently

19
Q

What is a geostationary orbit?

A

A satellite that remains in a fixed position above the earth’s surface

20
Q

What are the three properties of geostationary orbits?

A

Time period of 24 hours for the earth

Must orbit above the equator

Must rotate from west to east like the earths (same direction as planet)

21
Q

Explain how you find the height of a geostationary orbit for earth

A

Plug T=8.64x10^4s (24 hours) into Kepler’s third law equation
Find r
then subtract the radius of the earth to find the height

22
Q

What are polar satellites?

A

satellites passing over north and south poles in a short time period

23
Q

Give a use for polar satellites

A

weather monitoring

Military surveillance

Climate analysis

24
Q

If given a height of a satellite, what is the value of r?

A

height plus radius of planet

25
Q

Why for large values of h, can the formula for PE=mgh not be used?

A

g decreases with height so formula becomes inaccurate

26
Q

What is the area under a force against radius graph?

A

work done to move the mass to orbital radius r

27
Q

Why is PE defined as zero at a distance of infinity?

A

no gravitational field is present to cause a force so no associated energy

28
Q

Why is GPE considered negative?

A

GPE is defined as zero at infinity

Gravity is an attractive force

A mass must gain energy in order to escape gravitational field

29
Q

Define is gravitational potential Vg (simple summary)?

A

gravitational potential energy per unit mass

30
Q

Define gravitational potential?

A

Vg at a point is the work done per unit mass from infinity to that point, with the potential defined as zero at infinity

31
Q

How to convert from Vg to GPE?

A

multiply by m (SMALL MASS)

32
Q

How to calculate change in GPE from change in Vg

A

Change in GPE = mΔVg

33
Q

What is Vg (as words)

A

gravitational potential

34
Q

What is the unit for gravitational potential?

A

J kg^-1

35
Q

Define escape velocity?

A

minimum velocity needed to escape the gravitational force of a mass

be able to derive formula for escape velocity

36
Q

Explain why the earth has lost most of it’s hydrogen and helium from it’s atmosphere

A

All atoms/molecules have the same average KE in a gas at one temperature

Lighter atoms travel faster (larger root mean square velocity)

More likely to have a V>escape velocity

Remember - Spread of velocities in a gas even if one type of atom/molecule (Maxwell Boltzmann distribution)

37
Q

Explain why lighter planets lose their atmospheres?

A

Smaller mass, smaller escape velocities, particles in atmosphere escape more easier

38
Q

If asked to calculate the sum of energies for an object orbiting a planet, what must be done?

A

Find KE
Find PE
Add them (KE-PE as PE is negative)

39
Q

How can you use find KE from change in Vg?

A

mΔVg = ΔPE= ΔKE
m in both equations cancel out as you don’t need it
(remember Vg is energy per unit mass so to get energy multiply by mass)

Xaverian Physics (18 decks)

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