Define gravitational field .

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

A field line (or line of force) is an arrow representing the direction of the gravitational force, and therefore the acceleration , that would act on a mass placed at that point.

What shape is the gravitational field around a point mass, and in which direction do the field lines point?

The field is radial , with field lines directed toward the centre of mass .

Why are gravitational field lines always directed toward the centre of mass of a body?

Because the gravitational force is always attractive .

Gravitational field lines are directed toward the .......... of a body because the gravitational force is .......... .

Gravitational field lines are directed toward the centre of mass of a body because the gravitational force is attractive .

True or False? Gravitational field lines show the direction of the velocity of a mass placed in the field.

False. Field lines show the direction of the gravitational force (and hence acceleration) on a mass, not necessarily its velocity.

State two factors that affect the gravitational field strength at the surface of a planet.

The radius (or diameter) of the planet The mass (or density) of the planet

What is the key difference between g and G ?

g is the gravitational field strength , which varies with the mass and radius of the body producing the field; G is Newton's Universal Gravitational Constant , a fixed value of 6.67 × 10 -11 N m 2 kg -2 .

An object's .......... remains the same at all points in space, but its .......... changes with the local gravitational field strength.

An object's mass remains the same at all points in space, but its weight changes with the local gravitational field strength.

True or False? On a planet with a larger value of g , an object has a greater mass than it does on a planet with a smaller value of g .

False. An object's mass stays the same everywhere; only its weight (the gravitational force on it) changes with g .

When calculating r in Newton's Law of Gravitation for a satellite orbiting a planet, what must you remember to include?

The planet's radius must be added to the satellite's altitude, since r is measured from centre to centre of the two masses.

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

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

True or False? Doubling the separation between two masses halves the gravitational force between them.

False. Gravitational force follows an inverse square law , so doubling the separation reduces the force to a quarter , not a half.

A test mass ( m ) is a mass that experiences the gravitational field of another mass M , without itself being the mass causing the field.

Define gravitational field strength at a point due to a mass M .

The force per unit test mass experienced by an object at that point in the field.

For calculations involving gravitational fields, how can a spherical mass be treated?

As a point mass located at the centre of the sphere.

The equation g = GM / r 2 is derived by substituting Newton's Law of Universal Gravitation into g = .......... , then cancelling the test mass.

The equation g = GM / r 2 is derived by substituting Newton's Law of Universal Gravitation into g = F/m , then cancelling the test mass.

True or False? In the equation g = GM / r 2 , the mass M refers to the test mass placed in the field.

False. M is the mass causing the gravitational field; the test mass that experiences it is m .

Define gravitational potential ( V ).

Gravitational potential is the work done per unit mass in bringing a test mass from infinity to a defined point in the field, measured in J kg –1 .

Why is gravitational potential always negative?

Gravitational potential is defined as zero at infinity , and work must be done against gravity to move a mass away from another mass, so the potential at every finite point is negative .

Why is the equation G.P.E = mg Δ h only valid near the Earth's surface?

Because the gravitational field is only approximately uniform close to the surface; further away, the field becomes radial .

Gravitational potential is defined to be .......... at a point infinitely far from the mass causing the field.

Gravitational potential is defined to be zero at a point infinitely far from the mass causing the field.

Define radial field .

A radial field is the field around a spherical mass or charge, with field lines directed toward or away from the centre. Both gravitational and electric fields are radial around spherical sources.

State three differences between gravitational and electric fields.

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

Gravitational potential is always .......... , whereas electric potential can be either negative or .......... .

Gravitational potential is always negative , whereas electric potential can be either negative or positive .

True or False? Like the electric force, the gravitational force can be either attractive or repulsive.

False. The gravitational force is always attractive ; only the electric force can be repulsive, since charge (unlike mass) can be positive or negative.

Define centripetal force in the context of orbital motion.

In orbital motion, the centripetal force is the resultant force acting on a satellite or planet, directed towards the centre of the mass it orbits and perpendicular to its velocity. This force is provided by the gravitational force between the two masses.

Define centripetal acceleration .

The acceleration of a body moving in a circular orbit, caused by the continuous change in the direction of its velocity, directed towards the centre of the orbit.

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

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

True or False? A satellite in a circular orbit has zero acceleration because its speed is constant.

False. Although the speed is constant, the direction of the velocity is constantly changing, so the satellite has a centripetal acceleration directed towards the centre of the orbit.

International A Level (IAL)PhysicsEdexcelFlashcards5. Thermodynamics, Radiation, Oscillations & CosmologyGravitational Fields

Gravitational Fields (Edexcel International A Level (IAL) Physics): Flashcards

Exam code: YPH11

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Cards in this collection (42)

Define gravitational field.
A gravitational field is a region of space in which any object with mass experiences a gravitational force.
Define field line.
A field line (or line of force) is an arrow representing the direction of the gravitational force, and therefore the acceleration, that would act on a mass placed at that point.
What shape is the gravitational field around a point mass, and in which direction do the field lines point?
The field is radial, with field lines directed toward the centre of mass.
Why are gravitational field lines always directed toward the centre of mass of a body?
Because the gravitational force is always attractive.
Gravitational field lines are directed toward the .......... of a body because the gravitational force is ...........
Gravitational field lines are directed toward the centre of mass of a body because the gravitational force is attractive.
True or False?
Gravitational field lines show the direction of the velocity of a mass placed in the field.
False.
Field lines show the direction of the gravitational force (and hence acceleration) on a mass, not necessarily its velocity.
Define gravitational field strength.
Gravitational field strength ( g ) is the force per unit mass acting on an object at a point in a gravitational field:
$g = \frac{F}{m}$
Define weight.
Weight is the force due to gravity acting on an object's mass, equal to $F = m g$
State two factors that affect the gravitational field strength at the surface of a planet.
- The radius (or diameter) of the planet
- The mass (or density) of the planet
What is the key difference between g and G?
g is the gravitational field strength, which varies with the mass and radius of the body producing the field; G is Newton's Universal Gravitational Constant, a fixed value of 6.67 × 10^-11 N m² kg^-2.
An object's .......... remains the same at all points in space, but its .......... changes with the local gravitational field strength.
An object's mass remains the same at all points in space, but its weight changes with the local gravitational field strength.
True or False?
On a planet with a larger value of g, an object has a greater mass than it does on a planet with a smaller value of g.
False.
An object's mass stays the same everywhere; only its weight (the gravitational force on it) changes with g.
Define Newton's Law of Universal Gravitation.
The gravitational force F_G between two masses m₁ and m₂ is proportional to the product of their masses and inversely proportional to the square of their separation r:
$F_{G} = \frac{G m_{1} m_{2}}{r^{2}}$
Define the inverse square law as applied to gravitational force.
The inverse square law describes a $\frac{1}{r^{2}}$ relationship between the gravitational force and the separation r of two masses.
If the distance between two masses doubles, what happens to the gravitational force between them?
It reduces by a factor of four, since $F_{G} \propto \frac{1}{r^{2}}$
When calculating r in Newton's Law of Gravitation for a satellite orbiting a planet, what must you remember to include?
The planet's radius must be added to the satellite's altitude, since r is measured from centre to centre of the two masses.
The gravitational force between two masses is proportional to the product of their masses and inversely proportional to the .......... of their separation.
The gravitational force between two masses is proportional to the product of their masses and inversely proportional to the square of their separation.
True or False?
Doubling the separation between two masses halves the gravitational force between them.
False.
Gravitational force follows an inverse square law, so doubling the separation reduces the force to a quarter, not a half.
Define test mass.
A test mass ( m ) is a mass that experiences the gravitational field of another mass M, without itself being the mass causing the field.
Define gravitational field strength at a point due to a mass M.
The force per unit test mass experienced by an object at that point in the field.
State the equation for gravitational field strength g at a distance r from a point mass M.
$g = \frac{G M}{r^{2}}$
For calculations involving gravitational fields, how can a spherical mass be treated?
As a point mass located at the centre of the sphere.
The equation g = GM/r² is derived by substituting Newton's Law of Universal Gravitation into g = .........., then cancelling the test mass.
The equation g = GM/r² is derived by substituting Newton's Law of Universal Gravitation into g = F/m, then cancelling the test mass.
True or False?
In the equation g = GM/r², the mass M refers to the test mass placed in the field.
False.
M is the mass causing the gravitational field; the test mass that experiences it is m.
Define gravitational potential ( V ).
Gravitational potential is the work done per unit mass in bringing a test mass from infinity to a defined point in the field, measured in J kg^–1.
Why is gravitational potential always negative?
Gravitational potential is defined as zero at infinity, and work must be done against gravity to move a mass away from another mass, so the potential at every finite point is negative.
Why is the equation G.P.E = mgΔh only valid near the Earth's surface?
Because the gravitational field is only approximately uniform close to the surface; further away, the field becomes radial.
State the equation for gravitational potential V at a distance r from a point mass M.
$V = - \frac{G M}{r}$
Gravitational potential is defined to be .......... at a point infinitely far from the mass causing the field.
Gravitational potential is defined to be zero at a point infinitely far from the mass causing the field.
True or False?
Gravitational potential becomes more negative as you move further from a mass.
False.
Gravitational potential increases (becomes less negative) with distance from the mass, since $V \propto \frac{1}{r}$
Define radial field.
A radial field is the field around a spherical mass or charge, with field lines directed toward or away from the centre. Both gravitational and electric fields are radial around spherical sources.
How do gravitational field strength and electric field strength both vary with distance r in a radial field?
Both have a $\frac{1}{r^{2}}$ relationship with r.
How do gravitational potential and electric potential both vary with distance r from a point mass or charge?
Both have a $\frac{1}{r}$ relationship with r.
State three differences between gravitational and electric fields.
- Gravitational force acts on mass; electric force acts on charge
- Gravitational force is always attractive; electric force can be attractive or repulsive
- Gravitational potential is always negative; electric potential can be positive or negative
Gravitational potential is always .........., whereas electric potential can be either negative or ...........
Gravitational potential is always negative, whereas electric potential can be either negative or positive.
True or False?
Like the electric force, the gravitational force can be either attractive or repulsive.
False.
The gravitational force is always attractive; only the electric force can be repulsive, since charge (unlike mass) can be positive or negative.
Define centripetal force in the context of orbital motion.
In orbital motion, the centripetal force is the resultant force acting on a satellite or planet, directed towards the centre of the mass it orbits and perpendicular to its velocity. This force is provided by the gravitational force between the two masses.
Define centripetal acceleration.
The acceleration of a body moving in a circular orbit, caused by the continuous change in the direction of its velocity, directed towards the centre of the orbit.
State the equation for the orbital (linear) speed v of a satellite in terms of G, M and r.
$v = \sqrt{\frac{G M}{r}}$
State the equation relating the orbital time period T and orbital radius r.
$T^{2} = \frac{4 π^{2} r^{3}}{G M}$
Since the mass of the satellite cancels when equating the gravitational force to the centripetal force, all satellites at a given orbital radius travel at the speed, regardless of their mass.
Since the mass of the satellite cancels when equating the gravitational force to the centripetal force, all satellites at a given orbital radius travel at the same speed, regardless of their mass.
True or False?
A satellite in a circular orbit has zero acceleration because its speed is constant.
False.
Although the speed is constant, the direction of the velocity is constantly changing, so the satellite has a centripetal acceleration directed towards the centre of the orbit.

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