AstroPhysics Revision

Which statement best describes the nature of a gravitational field?

• It is the region where a mass experiences a force due to other charges.

• It is the region where a mass experiences a force due to other masses.

• It is a region where energy is stored as electric potential.

• It is the region around a magnet where magnetic forces act.

In a gravitational field, the direction of the field lines around a planet is:

• Radially outward from the planet.

• Parallel to the planet’s surface.

• Radially inward toward the planet’s centre.

• Tangential to the planet’s orbit.

The gravitational potential at a point is negative. This means:

• A positive amount of work must be done by gravity to move a mass there from infinity.

• Work must be done against gravity to move a mass there from infinity.

• The point is infinitely far from any mass.

• The gravitational field strength is zero at that point.

Why is the gravitational potential energy of a system taken as zero at infinite separation?

• Because gravitational potential energy is undefined there.

• Because at infinite separation, there is no interaction between the masses.

• Because gravitational potential energy is maximum at that point.

• Because field strength is strongest at infinity.

The orbits of planets around the Sun are stable because:

• Gravitational and centripetal forces cancel exactly.

• The gravitational force continually changes the planet’s speed.

• The gravitational force acts as the centripetal force, constantly changing direction of motion.

• No net force acts on the planet.

A geostationary satellite must orbit directly above the equator. Why?

• So it can remain stationary relative to the Earth’s poles.

• So its orbital period equals the Earth’s rotational period.

• So its orbital velocity matches the Earth’s escape velocity.

• So it experiences zero gravitational force.

Gravitational potential and electric potential share which feature?

• Both are vector quantities.

• Both are positive for attractive forces.

• Both are scalar quantities describing energy per unit mass or charge.

• Both increase toward the attracting body.

Which of the following best explains why astronauts in orbit appear weightless?

• They are too far from Earth to experience gravity.

• Gravity is exactly zero at orbital altitude.

• They are in continuous free fall around Earth.

• The rocket’s thrust cancels the gravitational force.

If the Sun were replaced by a black hole of the same mass, what would happen to Earth’s orbit?

• Earth would spiral into the black hole immediately.

• Earth’s orbit would remain almost unchanged.

• Earth would be ejected from orbit due to high gravity.

• Earth would stop moving and fall straight in.

What determines whether a star becomes a white dwarf, neutron star, or black hole?

• Its luminosity during main sequence

• Its mass at the start of its life

• Its age when fusion stops

• Its distance from the Milky Way centre