Gravitational fields

What happens to the strength of magnetic fields as distance increases?

It decreases

To what point do all gravitational fields extend to?

Infinity

Where are gravitational fields attracted to?

The centre of mass of a spherical object

Where are gravitational fields approximately uniform?

Near the surfaces of large objects

What is newton’s law of gravitation in word form?

The gravitational force between two point masses is directly proportional to the product of the masses and inversely proportional to the square of the distance between their centres.

What is Newton’s law of gravitation in symbol form?

F = G(m₁m₂ / d²)

What is Kepler’s first law?

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

What is Kepler’s second law?

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

Where would a planet move fastest?

Closest to the sun

What is Kepler’s third law?

The square of the orbital period of a planet is directly proportional to the cube of its average distance from the sun

What is gravitational potential?

Work done per unit mass to bring that mass from infinity to a point in a gravitational field

Why is gravitational potential always negative?

Potential is always zero at infinity therefore since gravitational forces are attractive, gravitational potential is always negative

Is gravitational potential scalar or vector?

Scalar

What is the total gravitational potential at any point?

The sum of gravitational potentials from each of the masses involved

What is gravitational potential energy?

Work done to bring a mass from infinity to a point in a gravitational field

What is the gradient at any point on a r against V_g graph equal to?

-g

What is the area under a graph of g against r equal to?

Work done

What is escape velocity?

The velocity needed so an object has just enough KE to escape a gravitational field

At what point is escape velocity reached?

When gravitational force = centripetal force

What is the equation for escape velocity?

V = (2GM / r)^1/2

What happens to the energy of an object in orbit?

Energy is transferred between GPE and KE, but is overall constant

What do objects in binary systems orbit?

The centre of mass

What does the distance between gravitational field lines show?

The strength of the field

Where is a polar orbit?

Circling the poles

How are polar orbits useful?

They offer a complete view of Earth over a given period as the Earth rotates beneath the path of the satellite.

What are polar orbits useful for?

Mapping and reconnaissance

Where is a a Low-Earth orbit?

Close to the Earth

How are Low-Earth orbits useful?

Very short time for an orbit of the Earth (<2h) as T² = kr³

Where are geostationary satellites?

Above the equator

How are geostationary satellites useful?

Satellites remain in the same point above the Earth as it rotates

What are the three things required in order for a satellite to be in geostationary orbit?

• in orbit above the Earth’s equator

• rotating in the same direction as Earth

• have an orbital period of 24 hours