What causes gravitational fields?

Any object with mass

How do gravitational field lines represent gravitational fields? (3)

At right angles to object

Arrows show direction of force (always attractive)

Separation of lines shows strength of field

Define gravitational field strength

The gravitational force exerted per unit mass at that point in the gravitational field

How is gravitational field strength and distance related?

Gravitational field strength is inversely proportional to the distance from the centre of mass of the object

What is assumed about the Earth’s gravitational field strength?

It is constant close to the surface of the Earth as the field is considered to be uniform

What does Newton’s law of gravitation assume? (2)

Point masses

Mass is concentrated at centre of mass

Why is there a negative sign in Newton’s law of gravitation?

Gravity is always an attractive force

What is Kepler’s first law of planetary motion?

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

How is a gravitational field shown to be uniform?

Equidistant, parallel field lines

State Newton’s law of gravitation

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

What is eccentricity?

A measure of how elongated a circle is

What is Kepler’s second law of planetary motion?

A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time

What is Kepler’s third law of planetary motion?

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

How is Kepler’s third law derived? (3)

Model the planetary orbit as a circle

Centripetal force on planet = gravitational force on planet

Determine speed by dividing circumference by orbital period

What is a geostationary satellite?

A satellite that stays above the same area of Earth

What are the conditions for a geostationary satellite? (4)

36000km from Earth’s surface

Orbit above the equator

Time period of 24 hours

Orbit in the same direction as Earth’s rotation

What are polar orbits used for? (2)

Mapping

Reconnaissance

What are Low Earth Orbits used for? (2)

Photography

Weather

What are Medium Earth Orbits used for?

GPS

What are geostationary orbits used for? (2)

Communications

TV

What are High Earth Orbits used for?

Research

Define gravitational potential

The work done per unit mass to move an object to a point in a gravitational field from infinity

What does infinity refer to in the definition of gravitational potential?

A distance so far from the gravitational field that its field strength is zero

What is true about gravitational potential at infinity?

It is zero

Why are all values of gravitational potential negative? (2)

All masses attract each other

Work must be done to separate objects gravitational potential is zero at infinity

What does a graph of gravitational potential against distance look like? (3)

Most negative value at surface of object (graph starts here)

Inversely proportional relationship

All values of gravitational potential are negative

How does gravitational potential change when masses move? (2)

Moving towards a point mass decreases gravitational potential (more negative)

Moving away from a point mass increases gravitational potential (less negative)

What does the area under a force-distance graph for a point mass represent?

Work done

What is escape velocity? (2)

The minimum velocity required to escape the gravitational field of a mass

When loss in KE = gain in GPE