A region of space in which an object experiences a force .

Define gravitational field .

A region of space in which any object with mass experiences a gravitational force .

The gravitational field around a point mass is .......... in shape, with field lines pointing towards the centre of mass.

The gravitational field around a point mass is radial in shape, with field lines pointing towards the centre of mass.

What does the direction of a gravitational field line represent?

The direction of the gravitational force that would act on a mass placed at that point, which is also the direction of the mass's acceleration .

True or False? Gravitational forces can be attractive or repulsive.

False. The gravitational force is always attractive — masses only ever attract each other, so field lines always point towards the centre of mass.

Define gravitational field strength .

The force per unit mass experienced by an object at a point in a gravitational field, measured in N kg -1 .

The gravitational force acting on an object due to a gravitational field — the 'force due to gravity'.

An object's .......... stays the same at all points in space, even though its weight can change.

An object's mass stays the same at all points in space, even though its weight can change.

What two factors determine the gravitational field strength at the surface of a planet?

The mass (or density) of the planet and its radius (or diameter).

True or False? An object's mass is greater on Jupiter than on Earth.

False. Mass is constant everywhere — it is the object's weight that is greater on Jupiter, because Jupiter's gravitational field strength g is larger.

What is the difference between g and G ?

g is the gravitational field strength , which varies with the mass producing the field and the distance from it. G is Newton's universal gravitational constant , fixed at 6.67 × 10 -11 N m 2 kg -2 .

State Newton's Law of Universal Gravitation .

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

The 1/ r 2 relationship in Newton's law of gravitation is called the .......... .

The 1/ r 2 relationship in Newton's law of gravitation is called the inverse square law .

If the distance between two masses doubles, what happens to the gravitational force between them?

It reduces by a factor of four , because doubling r changes 1/ r 2 to 1/(2 r ) 2 = 1/4 r 2 .

In Newton's law of gravitation, the separation r is measured between the .......... of the two masses.

In Newton's law of gravitation, the separation r is measured between the centres of the two masses.

True or False? If the separation triples, the gravitational force falls to one third.

False. The force follows the inverse square law , so tripling the separation reduces the force to one ninth (1/3 2 ).

For gravitational calculations, a spherical mass can be treated as a .......... located at the centre of the sphere.

For gravitational calculations, a spherical mass can be treated as a point mass located at the centre of the sphere.

The mass m that experiences the gravitational field produced by another mass M .

In the equation g = GM / r 2 , the mass M is the mass .......... the gravitational field.

In the equation g = GM / r 2 , the mass M is the mass producing the gravitational field.

True or False? The gravitational field strength at a point depends on the test mass placed there.

False. g = GM / r 2 depends only on the mass M producing the field and the distance r — not on the test mass placed in it.

Define gravitational potential .

The work done per unit mass in bringing a test mass from infinity to a defined point. It is measured in J kg -1 and given the symbol V .

Define gravitational potential energy in a radial field.

The energy an object possesses due to its position in a gravitational field.

Why is gravitational potential always negative ?

Gravity is always attractive , so work must be done on a mass to move it to infinity, where potential is defined as zero . Everywhere closer than infinity therefore has a value less than zero , so the potential is always negative.

Near the Earth's surface the gravitational field is approximated as .......... , but far from the surface it is .......... because the Earth is a sphere.

Near the Earth's surface the gravitational field is approximated as uniform , but far from the surface it is radial because the Earth is a sphere.

Why can the equation G.P.E = mg Δ h only be used for objects near the Earth's surface?

Near the surface the gravitational field is approximately uniform , so g is constant. Far from the surface the field becomes radial and g is no longer constant, so this equation no longer applies.

True or False? Gravitational potential and gravitational field strength both follow an inverse-square (1/ r 2 ) relationship with distance.

False. Gravitational potential follows V grav ∝ 1/ r , whereas gravitational field strength follows g ∝ 1/ r 2 .

State three key differences between gravitational and electric fields.

Gravitational force acts on particles with mass ; electrostatic force acts on particles with charge Gravitational force is always attractive ; electrostatic force can be attractive or repulsive Gravitational potential is always negative ; electric potential can be negative or positive

The gravitational force acts on particles with .......... , whilst the electrostatic force acts on particles with .......... .

The gravitational force acts on particles with mass , whilst the electrostatic force acts on particles with charge .

True or False? Gravitational potential can be either positive or negative.

False. Gravitational potential is always negative . It is electric potential that can be either positive or negative.

State two similarities between gravitational and electric fields (forces and field lines).

The force between two point masses/charges follows an inverse square law Field lines around spherical masses and charges are radial

In a radial field, how do field strength and potential depend on distance r for both gravitational and electric fields?

Field strength ∝ 1/ r 2 Potential ∝ 1/*r *

True or False? In a uniform field, gravitational and electric field lines look different.

False. In a uniform field the field lines are identical — parallel and equally spaced in both cases.

Define centripetal force (in the context of orbits).

The resultant force acting on an orbiting body, directed towards the centre of the orbit and perpendicular to its velocity. For an orbit, this force is provided by gravity .

What provides the centripetal force that keeps a planet or satellite in orbit?

The gravitational force between the orbiting body and the mass it orbits. It acts towards the centre, perpendicular to the velocity.

When gravitational force is equated to centripetal force, the satellite's .......... cancels out, so all satellites at the same orbital radius travel at the same .......... .

When gravitational force is equated to centripetal force, the satellite's mass cancels out, so all satellites at the same orbital radius travel at the same speed .

Explain why a satellite in a circular orbit is accelerating even when its speed is constant.

Its direction (and therefore velocity) is constantly changing. The gravitational force acts as a centripetal force towards the centre, producing centripetal acceleration .

True or False? At the same orbital radius, a heavier satellite must orbit faster than a lighter one.

False. Orbital speed depends only on the mass being orbited: v 2 = GM / r . It is independent of the satellite's mass , so all satellites at a given radius travel at the same speed.

A LevelPhysicsEdexcelFlashcards12. Gravitational FieldsGravitational Fields

Gravitational Fields (Edexcel A Level Physics): Flashcards

Exam code: 9PH0

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FrontGravitational Fields
Define force field.

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Cards in this collection (46)

Define force field.
A region of space in which an object experiences a force.
Define gravitational field.
A region of space in which any object with mass experiences a gravitational force.
The gravitational field around a point mass is .......... in shape, with field lines pointing towards the centre of mass.
The gravitational field around a point mass is radial in shape, with field lines pointing towards the centre of mass.
What does the direction of a gravitational field line represent?
The direction of the gravitational force that would act on a mass placed at that point, which is also the direction of the mass's acceleration.
True or False?
Gravitational forces can be attractive or repulsive.
False.
The gravitational force is always attractive — masses only ever attract each other, so field lines always point towards the centre of mass.
Name an everyday effect on Earth caused by the Moon and Sun's gravitational fields.
The tides.
Define gravitational field strength.
The force per unit mass experienced by an object at a point in a gravitational field, measured in N kg^-1.
State the equation for gravitational field strength g.
$g = \frac{F}{m}$
Where F = force due to gravity, or weight (N) and m = mass (kg).
Define weight.
The gravitational force acting on an object due to a gravitational field — the 'force due to gravity'.
An object's .......... stays the same at all points in space, even though its weight can change.
An object's mass stays the same at all points in space, even though its weight can change.
What two factors determine the gravitational field strength at the surface of a planet?
The mass (or density) of the planet and its radius (or diameter).
True or False?
An object's mass is greater on Jupiter than on Earth.
False.
Mass is constant everywhere — it is the object's weight that is greater on Jupiter, because Jupiter's gravitational field strength g is larger.
What is the difference between g and G?
g is the gravitational field strength, which varies with the mass producing the field and the distance from it.
G is Newton's universal gravitational constant, fixed at 6.67 × 10^-11 N m² kg^-2.
State Newton's Law of Universal Gravitation.
The gravitational force between two masses is proportional to the product of their masses and inversely proportional to the square of their separation.
State the equation for Newton's Law of Universal Gravitation.
$F_{G} = \frac{G m_{1} m_{2}}{r^{2}}$
Where F_G = gravitational force (N), G = gravitational constant, m₁ and m₂ = the two masses (kg), r = distance between their centres (m).
The 1/r² relationship in Newton's law of gravitation is called the ...........
The 1/r² relationship in Newton's law of gravitation is called the inverse square law.
If the distance between two masses doubles, what happens to the gravitational force between them?
It reduces by a factor of four, because doubling r changes 1/r² to 1/(2r)² = 1/4r².
In Newton's law of gravitation, the separation r is measured between the .......... of the two masses.
In Newton's law of gravitation, the separation r is measured between the centres of the two masses.
True or False?
If the separation triples, the gravitational force falls to one third.
False.
The force follows the inverse square law, so tripling the separation reduces the force to one ninth (1/3²).
State the equation for the gravitational field strength due to a point mass.
$g = \frac{G M}{r^{2}}$
Where M = mass producing the field (kg) and r = distance from the centre of M to the point (m).
How is the equation $g = \frac{G M}{r^{2}}$ derived?
Combine Newton's law $F_{G} = \frac{G M m}{r^{2}}$ with the definition $g = \frac{F}{m}$ .
The test mass m cancels, leaving $g = \frac{G M}{r^{2}}$ .
For gravitational calculations, a spherical mass can be treated as a .......... located at the centre of the sphere.
For gravitational calculations, a spherical mass can be treated as a point mass located at the centre of the sphere.
Define test mass.
The mass m that experiences the gravitational field produced by another mass M.
In the equation g = GM/r², the mass M is the mass .......... the gravitational field.
In the equation g = GM/r², the mass M is the mass producing the gravitational field.
True or False?
The gravitational field strength at a point depends on the test mass placed there.
False.
g = GM/r² depends only on the mass M producing the field and the distance r — not on the test mass placed in it.
Define gravitational potential.
The work done per unit mass in bringing a test mass from infinity to a defined point.
It is measured in J kg^-1 and given the symbol V.
Define gravitational potential energy in a radial field.
The energy an object possesses due to its position in a gravitational field.
Why is gravitational potential always negative?
Gravity is always attractive, so work must be done on a mass to move it to infinity, where potential is defined as zero.
Everywhere closer than infinity therefore has a value less than zero, so the potential is always negative.
State the equation for gravitational potential V_grav a distance r from a mass M.
$V_{g r a v} = - \frac{G M}{r}$
where M = mass producing the field (kg) and r = distance from the centre of the mass (m).
Near the Earth's surface the gravitational field is approximated as .........., but far from the surface it is .......... because the Earth is a sphere.
Near the Earth's surface the gravitational field is approximated as uniform, but far from the surface it is radial because the Earth is a sphere.
Why can the equation G.P.E = mgΔh only be used for objects near the Earth's surface?
Near the surface the gravitational field is approximately uniform, so g is constant.
Far from the surface the field becomes radial and g is no longer constant, so this equation no longer applies.
True or False?
Gravitational potential and gravitational field strength both follow an inverse-square (1/r²) relationship with distance.
False.
Gravitational potential follows V_grav ∝ 1/r, whereas gravitational field strength follows g ∝ 1/r².
State three key differences between gravitational and electric fields.
- Gravitational force acts on particles with mass; electrostatic force acts on particles with charge
- Gravitational force is always attractive; electrostatic force can be attractive or repulsive
- Gravitational potential is always negative; electric potential can be negative or positive
The gravitational force acts on particles with .........., whilst the electrostatic force acts on particles with ...........
The gravitational force acts on particles with mass, whilst the electrostatic force acts on particles with charge.
True or False?
Gravitational potential can be either positive or negative.
False.
Gravitational potential is always negative. It is electric potential that can be either positive or negative.
State two similarities between gravitational and electric fields (forces and field lines).
- The force between two point masses/charges follows an inverse square law
- Field lines around spherical masses and charges are radial
In a radial field, how do field strength and potential depend on distance r for both gravitational and electric fields?
- Field strength ∝ 1/r²
- Potential ∝ 1/*r*
True or False?
In a uniform field, gravitational and electric field lines look different.
False.
In a uniform field the field lines are identical — parallel and equally spaced in both cases.
Define centripetal force (in the context of orbits).
The resultant force acting on an orbiting body, directed towards the centre of the orbit and perpendicular to its velocity.
For an orbit, this force is provided by gravity.
What provides the centripetal force that keeps a planet or satellite in orbit?
The gravitational force between the orbiting body and the mass it orbits.
It acts towards the centre, perpendicular to the velocity.
By equating gravitational force to centripetal force, state the equation for the orbital speed v of a satellite.
$\frac{G M m}{r^{2}} = \frac{m v^{2}}{r} \Rightarrow v^{2} = \frac{G M}{r}$
The satellite mass m cancels, so all satellites at a given orbital radius r travel at the same speed.
When gravitational force is equated to centripetal force, the satellite's .......... cancels out, so all satellites at the same orbital radius travel at the same ...........
When gravitational force is equated to centripetal force, the satellite's mass cancels out, so all satellites at the same orbital radius travel at the same speed.
State the relationship between orbital period T and orbital radius r.
$T^{2} = \frac{4 π^{2} r^{3}}{G M}$
so $T^{2} \propto r^{3}$
Explain why a satellite in a circular orbit is accelerating even when its speed is constant.
Its direction (and therefore velocity) is constantly changing.
The gravitational force acts as a centripetal force towards the centre, producing centripetal acceleration.
True or False?
At the same orbital radius, a heavier satellite must orbit faster than a lighter one.
False.
Orbital speed depends only on the mass being orbited: v² = GM/r. It is independent of the satellite's mass, so all satellites at a given radius travel at the same speed.
How is a satellite's orbital speed v related to its orbital radius r and period T?
$v = \frac{2 π r}{T}$
since it travels the circumference 2πr in one period (speed = distance / time).

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