What provides the centripetal force needed for a planet or satellite to maintain a circular orbit?

The gravitational force between the orbited body and the orbiting body provides the centripetal force.

What is the direction of the gravitational force relative to a satellite's direction of travel in a circular orbit?

The gravitational force is perpendicular to the satellite's direction of travel.

Since the mass of the satellite cancels out when equating gravitational and centripetal force, all satellites at a given orbital radius travel at the same .......... , regardless of their mass.

Since the mass of the satellite cancels out when equating gravitational and centripetal force, all satellites at a given orbital radius travel at the same speed , regardless of their mass.

True or False? A more massive satellite orbiting at the same radius as a less massive satellite travels faster.

False. Since mass cancels out when equating gravitational and centripetal force, all satellites at a given orbital radius r travel at the same speed v , regardless of their mass.

A graph of log( T ) against log( r ) is plotted for planets orbiting the Sun, producing a straight line. Does this line pass through the origin?

No. The graph has a negative y -intercept, so it does not pass through the origin.

Define the total energy of an orbiting satellite.

The total energy of an orbiting satellite is the sum of its kinetic energy and gravitational potential energy , and it remains constant throughout the orbit.

Compare the orbital speed and time period of a satellite in a smaller-radius orbit with one in a larger-radius orbit.

Smaller radius: higher speed, shorter time period Larger radius: lower speed, longer time period

A satellite in a smaller-radius orbit experiences a .......... gravitational force than one in a larger-radius orbit.

A satellite in a smaller-radius orbit experiences a larger gravitational force than one in a larger-radius orbit.

True or False? A satellite in a larger orbital radius travels faster than one in a smaller orbital radius.

False. A satellite in a larger orbital radius travels slower , since the gravitational (centripetal) force is smaller at greater distances.

Define escape velocity .

The minimum speed that allows an object to escape a gravitational field with no further energy input.

Does escape velocity depend on the mass of the escaping object?

No. Escape velocity is the same for all masses in the same gravitational field (for example, a rocket and a tennis ball have the same escape velocity from Earth).

An object reaches escape velocity when all of its .......... has been transferred to gravitational potential energy.

An object reaches escape velocity when all of its kinetic energy has been transferred to gravitational potential energy.

True or False? A rocket launched from the Earth's surface must reach escape velocity to enter orbit around the Earth.

False. Rockets are continuously given energy through fuel and thrust, and less energy is needed to reach orbit than to escape Earth's gravitational field altogether.

What must an object actually escape to reach "escape velocity" — the planet itself, or something else?

The planet's gravitational field , not just the planet — this could mean travelling a large distance from the planet.

Define a synchronous orbit .

An orbit in which the orbital period of the satellite is equal to the rotational period (length of day) of the planet or body it is orbiting.

Define a geostationary orbit .

A geosynchronous orbit with a 24-hour period that lies in the plane of the equator, so the satellite stays above one fixed point on the Earth's surface.

What is a geosynchronous orbit?

An Earth-centred synchronous orbit with a 24-hour period; it may be inclined and slightly elliptical, and does not stay above one fixed point on the Earth's surface.

A synchronous orbit can be in any plane and at any .......... , and does not have to be above the equator.

A synchronous orbit can be in any plane and at any longitude , and does not have to be above the equator.

Why can the receiver dishes for geostationary satellite TV broadcasts be fixed in position?

Because a geostationary satellite's orbit is fixed relative to the ground, always above the same point, so a dish can point at a constant, fixed direction.

What are two uses of low-orbit satellites, such as those in a polar orbit?

Weather monitoring Military applications

True or False? A geosynchronous satellite always stays above the same fixed point on the Earth's surface.

False. Only a geostationary orbit (a special case of geosynchronous, in the plane of the equator) stays above a fixed point; other geosynchronous orbits may be inclined or elliptical and do not.

What is a polar orbit ?

A low orbit in which the satellite passes over the Earth's north and south poles.

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Orbits of Planets & Satellites (AQA A Level Physics): Flashcards

Exam code: 7408

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FrontCircular Orbits in Gravitational Fields
Define Kepler's third law.

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Cards in this collection (27)

Define Kepler's third law.
For planets or satellites in a circular orbit about the same central body, the square of the time period is proportional to the cube of the orbital radius:
$T^{2} \propto r^{3}$
What provides the centripetal force needed for a planet or satellite to maintain a circular orbit?
The gravitational force between the orbited body and the orbiting body provides the centripetal force.
What is the direction of the gravitational force relative to a satellite's direction of travel in a circular orbit?
The gravitational force is perpendicular to the satellite's direction of travel.
Since the mass of the satellite cancels out when equating gravitational and centripetal force, all satellites at a given orbital radius travel at the same .........., regardless of their mass.
Since the mass of the satellite cancels out when equating gravitational and centripetal force, all satellites at a given orbital radius travel at the same speed, regardless of their mass.
True or False?
A more massive satellite orbiting at the same radius as a less massive satellite travels faster.
False.
Since mass cancels out when equating gravitational and centripetal force, all satellites at a given orbital radius r travel at the same speed v, regardless of their mass.
A graph of log(T) against log(r) is plotted for planets orbiting the Sun, producing a straight line. Does this line pass through the origin?
No. The graph has a negative y-intercept, so it does not pass through the origin.
State the equation relating orbital time period T and orbital radius r for a satellite, in terms of G and M.
$T^{2} = \frac{4 π^{2} r^{3}}{G M}$
Define the total energy of an orbiting satellite.
The total energy of an orbiting satellite is the sum of its kinetic energy and gravitational potential energy, and it remains constant throughout the orbit.
How do a satellite's kinetic energy (KE) and gravitational potential energy (GPE) change as its orbital radius decreases?
- KE increases
- GPE decreases
Compare the orbital speed and time period of a satellite in a smaller-radius orbit with one in a larger-radius orbit.
- Smaller radius: higher speed, shorter time period
- Larger radius: lower speed, longer time period
A satellite in a smaller-radius orbit experiences a .......... gravitational force than one in a larger-radius orbit.
A satellite in a smaller-radius orbit experiences a larger gravitational force than one in a larger-radius orbit.
True or False?
A satellite in a larger orbital radius travels faster than one in a smaller orbital radius.
False.
A satellite in a larger orbital radius travels slower, since the gravitational (centripetal) force is smaller at greater distances.
Define escape velocity.
The minimum speed that allows an object to escape a gravitational field with no further energy input.
What two properties of the object creating a gravitational field determine the escape velocity from its surface?
Its mass and its radius.
Does escape velocity depend on the mass of the escaping object?
No. Escape velocity is the same for all masses in the same gravitational field (for example, a rocket and a tennis ball have the same escape velocity from Earth).
An object reaches escape velocity when all of its .......... has been transferred to gravitational potential energy.
An object reaches escape velocity when all of its kinetic energy has been transferred to gravitational potential energy.
Derive the equation for escape velocity v by equating kinetic energy and gravitational potential energy.
$\frac{1}{2} m v^{2} = \frac{G M m}{r}$
$v = \sqrt{\frac{2 G M}{r}}$
True or False?
A rocket launched from the Earth's surface must reach escape velocity to enter orbit around the Earth.
False.
Rockets are continuously given energy through fuel and thrust, and less energy is needed to reach orbit than to escape Earth's gravitational field altogether.
What must an object actually escape to reach "escape velocity" — the planet itself, or something else?
The planet's gravitational field, not just the planet — this could mean travelling a large distance from the planet.
Define a synchronous orbit.
An orbit in which the orbital period of the satellite is equal to the rotational period (length of day) of the planet or body it is orbiting.
Define a geostationary orbit.
A geosynchronous orbit with a 24-hour period that lies in the plane of the equator, so the satellite stays above one fixed point on the Earth's surface.
What is a geosynchronous orbit?
An Earth-centred synchronous orbit with a 24-hour period; it may be inclined and slightly elliptical, and does not stay above one fixed point on the Earth's surface.
A synchronous orbit can be in any plane and at any .........., and does not have to be above the equator.
A synchronous orbit can be in any plane and at any longitude, and does not have to be above the equator.
Why can the receiver dishes for geostationary satellite TV broadcasts be fixed in position?
Because a geostationary satellite's orbit is fixed relative to the ground, always above the same point, so a dish can point at a constant, fixed direction.
What are two uses of low-orbit satellites, such as those in a polar orbit?
- Weather monitoring
- Military applications
True or False?
A geosynchronous satellite always stays above the same fixed point on the Earth's surface.
False.
Only a geostationary orbit (a special case of geosynchronous, in the plane of the equator) stays above a fixed point; other geosynchronous orbits may be inclined or elliptical and do not.
What is a polar orbit?
A low orbit in which the satellite passes over the Earth's north and south poles.

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