Define point mass approximation .

For a point outside a uniform sphere, the sphere's mass can be treated as concentrated at a single point at its centre of mass.

Define uniform sphere .

A uniform sphere is one whose mass is distributed evenly throughout.

What two conditions allow an object to be treated as a point mass?

It has an even mass distribution The distance being considered is larger than its size

True or False? The gravitational field lines around a uniform sphere differ from those around a point mass.

False. The field lines around a uniform sphere are identical to those around a point mass, since a point outside the sphere cannot distinguish it from a point mass at its centre.

To what kind of astronomical bodies does the point mass approximation commonly apply?

Planets and stars , since they are approximately uniform spheres and the distances considered (e.g. orbital separations) are much larger than their radii.

State Newton's law of gravitation .

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

What two conditions allow stars and planets to be treated as point masses in Newton's law of gravitation?

They are approximately uniform spheres Their separation is much larger than their radii

True or False? If the distance between two point masses doubles, the gravitational force between them halves.

False. By the inverse square law, doubling the separation reduces the gravitational force to one quarter , not one half.

When calculating the distance r between an orbiting satellite and a planet, what must be added to the satellite's altitude above the surface?

The radius of the planet , since r is measured from the centre of the planet, not its surface.

What provides the centripetal force for a planet or satellite moving in a circular orbit?

The gravitational force between the orbiting body and the mass it orbits provides the centripetal force.

True or False? A satellite in a circular orbit of fixed radius has a constant velocity.

False. The satellite's speed is constant, but its velocity is not, since the direction is continuously changing as it orbits.

In what direction does the centripetal acceleration of an orbiting satellite act?

Towards the centre of the body it is orbiting, caused by the gravitational force acting as the centripetal force.

Define a geostationary orbit .

A geostationary orbit is one in which a satellite remains directly above the equator , moving west to east , with an orbital period equal to Earth's rotational period of 24 hours .

What are geostationary satellites commonly used for?

Telecommunication transmissions (e.g. radio) and television broadcast.

Describe how a geostationary satellite is used to broadcast a television signal.

A base station on Earth sends the TV signal up to the satellite, where it is amplified and broadcast back down to the desired locations.

A geostationary satellite moves from west to .......... , the same direction as the Earth spins.

A geostationary satellite moves from west to east , the same direction as the Earth spins.

True or False? Satellite receiver dishes on Earth must continuously rotate to track a geostationary satellite.

False. Because geostationary orbits are fixed, the satellite always remains at the same point in the sky, so receiver dishes can be fixed too.

A LevelPhysicsCambridge (CIE)Flashcards13. Gravitational FieldsGravitational Force Between Point Masses

Gravitational Force Between Point Masses (Cambridge (CIE) A Level Physics): Flashcards

Exam code: 9702

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FrontPoint Mass Approximation
Define point mass approximation.
FrontPoint Mass Approximation
What two conditions allow an object to be treated as a point mass?
BackPoint Mass Approximation
It has an even mass distribution
The distance being considered is larger than its size

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Cards in this collection (21)

Define point mass approximation.
For a point outside a uniform sphere, the sphere's mass can be treated as concentrated at a single point at its centre of mass.
Define uniform sphere.
A uniform sphere is one whose mass is distributed evenly throughout.
What two conditions allow an object to be treated as a point mass?
- It has an even mass distribution
- The distance being considered is larger than its size
True or False?
The gravitational field lines around a uniform sphere differ from those around a point mass.
False.
The field lines around a uniform sphere are identical to those around a point mass, since a point outside the sphere cannot distinguish it from a point mass at its centre.
To what kind of astronomical bodies does the point mass approximation commonly apply?
Planets and stars, since they are approximately uniform spheres and the distances considered (e.g. orbital separations) are much larger than their radii.
State Newton's law of gravitation.
The gravitational force between two point masses is proportional to the product of the masses and inversely proportional to the square of their separation.
Define the inverse square law as it applies to gravitational force.
The inverse square law describes how gravitational force is proportional to $\frac{1}{r^{2}}$ , where r is the separation between the two masses.
In $F_{G} = \frac{G m_{1} m_{2}}{r^{2}}$ , the distance r is measured between the .......... of the two masses, not their surfaces.
In $F_{G} = \frac{G m_{1} m_{2}}{r^{2}}$ , the distance r is measured between the centres of the two masses, not their surfaces.
What two conditions allow stars and planets to be treated as point masses in Newton's law of gravitation?
- They are approximately uniform spheres
- Their separation is much larger than their radii
True or False?
If the distance between two point masses doubles, the gravitational force between them halves.
False.
By the inverse square law, doubling the separation reduces the gravitational force to one quarter, not one half.
When calculating the distance r between an orbiting satellite and a planet, what must be added to the satellite's altitude above the surface?
The radius of the planet, since r is measured from the centre of the planet, not its surface.
What provides the centripetal force for a planet or satellite moving in a circular orbit?
The gravitational force between the orbiting body and the mass it orbits provides the centripetal force.
Equating gravitational and centripetal force gives $v = \sqrt{\frac{G M}{r}}$ , where .......... is the mass of the larger body being orbited.
Equating gravitational and centripetal force gives $v = \sqrt{\frac{G M}{r}}$ , where M is the mass of the larger body being orbited.
Why does the mass of the orbiting satellite cancel out when deriving the orbital speed equation $v = \sqrt{\frac{G M}{r}}$ ?
Both the gravitational force and the centripetal force are proportional to the satellite's mass m, so m cancels from both sides, meaning orbital speed is independent of the satellite's mass.
True or False?
A satellite in a circular orbit of fixed radius has a constant velocity.
False.
The satellite's speed is constant, but its velocity is not, since the direction is continuously changing as it orbits.
In what direction does the centripetal acceleration of an orbiting satellite act?
Towards the centre of the body it is orbiting, caused by the gravitational force acting as the centripetal force.
Define a geostationary orbit.
A geostationary orbit is one in which a satellite remains directly above the equator, moving west to east, with an orbital period equal to Earth's rotational period of 24 hours.
What are geostationary satellites commonly used for?
Telecommunication transmissions (e.g. radio) and television broadcast.
Describe how a geostationary satellite is used to broadcast a television signal.
A base station on Earth sends the TV signal up to the satellite, where it is amplified and broadcast back down to the desired locations.
A geostationary satellite moves from west to .........., the same direction as the Earth spins.
A geostationary satellite moves from west to east, the same direction as the Earth spins.
True or False?
Satellite receiver dishes on Earth must continuously rotate to track a geostationary satellite.
False.
Because geostationary orbits are fixed, the satellite always remains at the same point in the sky, so receiver dishes can be fixed too.

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