**Gravity and potential**

**January 2002**

**Number 30**

Gravity acts on all bodies that have mass. The gravitational force between two bodies is:

### •

always attractive;### •

along the line joining the centers of mass of the two bodies;### •

of the same magnitude on both bodies, but acting in opposite directions;### •

*proportional to the product of the two masses F*∝

*m*

_{1 }m_{2};### •

*inversely proportional to the square of their seperation F*∝ .

** Newton’s Law of Gravitation states that the force between the**
attracting masses is :

*1*
*r2*

**Worked Example. Two heavyweight boxers stand 1 metre apart.**
**If they each have a mass of 80 kg, what is the gravitational force**
**of attraction between them?**

*m _{1} = m_{2} = 80 kg.*

*F =* * newtons.*

*This is about 4.3 *×* 10*−*7 _{ N, which is extremely small.}*

**Worked Example. Calculate the gravitational force of attraction**
**between one boxer (mass 80 kg ) and the Earth. Mass of Earth**
**m _{E} = 5.98 **×××××

**1024**

_{ kg. Radius of Earth r}* E = 6.36 *×××××

**10**

**6**_{ m.}This calculation is fairly straightforward but you should be careful with powers of ten, particularly when squaring distances in the denominator.

The calculation now looks like this:
*F =*
*F =* _{newtons}*F = 7.89 *× 102_{ N}*Gm _{1}m_{2}*

*r2*

**Gravitational field**

*A field is a region where we can detect a force. To find the gravitational*
field strength at a point, we place a mass m at the point and measure the
*(gravitational) force F.*

The gravitational field strength is defined to be the force per kg.
*This ratio is also called the acceleration due to gravity g. Since g = F/m,*
the field strength may be quoted in Nkg−1, or ms−2

.

**Field strength above the earth's surface**

*Consider a body of mass m at a point P a distance R from the centre of the*
Earth.

*Gm _{E }m*

*R2*

*Divide both sides by m gives the gravitation field strength g:*

**Gravitational field strength = = g = - an inverse square law.**_{m}F*GmE*
*R2*

**Factsheet**

**Factsheet**

**Physics**

**Physics**

**The gravitational force exerted by the earth on a mass is the weight of the**
*mass, which you will be familiar with from the formula W = mg, where g is*
the acceleration due to gravity.

*This can be used to derive a relationship between g and G:*
*Consider a mass m at the earth’s surface.*

*F =* *Gm1m2*
*r2*

*where G is the universal gravitational constant.*
*(G = 6.67 *× 10−11_{ Nm}2_{kg}−2_{).}
r_{E}
m_{E}
W = mg m
F F
r
m_{1} m_{2}

**Exam Hint: - Ensure you measure the seperation, r, between the****centres of mass of the two bodies, not between their surfaces.**

*6.67 *×* 10*−*11*×* _{ 80 }*×

_{ 80}*12*

*6.67 *× *10*−*11*× * _{80 }*×

*×*

_{ 5.98 }

_{10}24*( 6.36*×

*106*

_{)}2We can write two expressions for the gravitational force acting on this
*mass due to the earth by using W = mg and Newton's Law of Gravitation.*
Equating these gives:

*mg =*
*Cancelling m gives:*

*g =*

This approach can be used to calculate the acceleration due to gravity on any planet, by substituting the mass and radius of that planet for the mass and radius of the earth.

*eg. Jupiter has mass 1.90 *×* 1027 kg and radius is 6.99 *×* 107* m.
*So the acceleration due to gravity on Jupiter is Gm _{J }/r_{J}2 _{= 26ms}-2*

*Gm m _{E}*

*r*

_{E}2*Gm _{E}*

*r*

_{E}2*where mE*= mass of the Earth
*r _{E}*

_{= radius of the Earth}

*The force F is given by F =*

m
F
P
F
m_{E}
R

**Field strength below the earth's surface**

For points below the earth's surface, the situation is slightly more complicated:

**Neutral point.**

For any two bodies there is a point P in between them where the pull of
one body to the right is balanced by the pull of the other body to the left.
*This is called a neutral point. A mass placed at P would be in equilibrium.*
Consider the planet Jupiter and its moon Ganymede:

**F**
*The graph below shows the variation in gravitational field strength (g)*

with distance from the centre of the Earth. Note that the value of 9.81 that we often use is only correct at or near to the Earth’s surface - but even at the top of mount Everest, the reduction is less than 0.4% The field strength at point P is due only to the shaded sphere, of radius R, rather than the whole mass of the earth.

If we assume the density of the earth is approximately constant, the mass
of the shaded sphere can be expressed as a fraction of the mass of the
Earth by using:
=
So: =
*So: m _{S} = m_{E }R3 _{/ r}*

*E*

*3*

The gravitational field strength will therefore be:
* g =*

=

*So below the earth's surface, g is directly proportional to distance from*
the centre.

**P**
R

mass of smaller sphere mass of Earth

volume of smaller sphere
volume of Earth
*m _{S}*

*m*4π

_{E}*R3*

_{/3}4π

*r*

_{E}3_{/3}

*Gm*

_{E }R3_{/ r}*E*

*3*

*R2*

*Gm*

_{E }R*r*

_{E}3**Worked Example**

**(a) Calculate the acceleration due to gravity at a height of 50 km**
**above the surface of the earth.**

* Mass of Earth m_{E} = 5.98 *×××××

**10****24**_{ kg.}*×××××*

**Radius of Earth r**_{E}**= 6.36**

**10****6**_{ m.}*×××××*

**G = 6.67***−−−−−*

**10***−−−−−*

**11**_{ Nm}**2**_{kg}

**2***Use g = Gm _{E}/R2*

*Note that R = radius of earth + 50km = 6.41 *×* 106 _{m}*

*So g = 6.67*×

*10*−

*11*×

*×*

_{ 5.98 }*×*

_{ 10}24_{ /(6.41 }

_{ 10}6_{)}2*r*

_{ g = 9.71 ms}-2_{E}2r

_{E}3r

_{E}4r

_{E}

### Earth

g ∝1 R2 9.81### 0

### 2

### 4

### 6

### 8

### 10

### g/Ng

-1**(b) Calculate the height above the earth at which g is a quarter of**
**its value on the surface of the earth.**

*We need to find the value of R when g = Gm _{E}/r_{E}2*

*Surface value of g = Gm*

_{E}/r_{E}2*At distance R from the center, we have g = Gm _{E}/R2*

*So we need to solve Gm*

_{E}/R2_{ = Gm}*E/4rE*

*2*

*Hence R2*

_{ = 4r}*E*

*2*

*So R = 2r*

_{E}*So the height above the earth's surface is r _{E} = 6.36 *×

*106*

_{m}**Weightlessness is a term often used. However, as can be seen from the**

*graphs and formula, there is always a force between the Earth and any*mass and so the mass always has a weight. When you see pictures of

*orbiting astronauts it is better to describe them as apparently weightless.*

**Field Lines**

**Field lines are used to show the presence of a gravitational field.**

### •

*The direction of the gravitational field lines shows the direction of the*

force acting on a mass in the field.

### •

*The spacing of the lines indicates the strength (or intensity) of the field.*Lines close together = strong field; lines wide apart = weak field. The diagram below shows the gravitational field of a spherical mass such as

*the Earth. A field like this is called a radial field.*

At increasing distance from the mass, the spacing increases showing that the field strength decreases - it follows the inverse square law detailed above.

P

Jupiter Ganymede

x

Since the gravitational force due to Jupiter is equal to that due to Ganymede at point P, we have

=

Substituting in and rearranging gives:
*y2 _{ ( 1.90 }*×

_{ 10}27

_{ ) = x}2_{(1.46 }×

_{ 10}23

_{)}

Square-rooting gives:

4.36 × 1013 * _{y = 3.82 }*×

_{ 10}11

_{x}So *114y = x*

*Since x + y = Ganymede's orbital radius, we have 115y = 1.06 *× 109_{,}

giving y = 9.2 × 106_{ m}
*Gm _{J}*

*x2*

*Gm*

_{G}*y2*y

Ganymede's orbital radius:1.06 × 109_{ m}

*m*_{G}: 1.46 × 1023_{ kg}

**Circular motion**

Circular motion is covered fully in Factsheet 19, but to understand this section you just need to know that for a mass moving in a circle with constant speed:

### •

Angular speed ω (*rad s-1*

_{), linear speed v (ms}-1_{) and radius of circular}*path r (m) are related by the equation v =*ω

*r*

### •

The period of the motion (the time taken for one complete rotation) is given by 2π/ω### •

There must be a force directed towards the centre of the circle (the*centripetal force)*

### •

*The magnitude of the centripetal force is m*ω

*2*

_{r}*Consider the moon orbiting the Earth:*

*The force F which is necessary for circular motion of moons*
*around planets, or planets around the sun, is provided by gravity.*

Since the centripetal force is supplied by gravity, we have:

Hence: *Gm _{E} =*ω2

_{R}

3
_{R}

**Planetary system - Kepler's Laws**

Applying the same principle as above to any of the planets in the solar system orbiting the sun, we obtain:

*Gm _{S} = *ω2

*3*

_{R}*where m _{S} is the mass of the sun, and R is the orbital radius of the planet.*

*But since the orbital period, T = 2*π/ω, we have ω = 2π/

*T:*

*Gm _{S} = *

*R*3

Rearranging: *Gm _{S}T2_{ = 4}*π

*2*

_{R}3*This gives us: T2*

### ∝

_{R}3This result is Kepler's Third Law of Planetary Motion.

*NB: This derivation relies upon approximating the planets' orbits by*
*circles, although they actually move in ellipses around the sun. However*
*the result still holds true despite this.*

**Gravitational Potential Energy**

Consider a mass m in the Earth’s gravitational field.

*ii) Again a = r _{E} , but b = 3r_{E} . *∆

*(pe) = G m*

_{E}m∆* (pe) = 6.67 *×* 10*−*11*× *6 *×* 1024*×* 1 *×
∆ *(pe) = 4.19* × *107 J*
*1*
*r _{E}*

*[ ]*

*.*

_{3r}1*E*−

### [ ]

*3*×

*6.36 2*×

*106*

*iii) Again a = r*∞

_{E}, but now, b =*.*∆

*(pe) = Gm*

_{E}m*Note that = 0 and so,*

∆* (pe) = 6.67 *×* 10*−*11*×* _{ 6 }*×

*×*

_{ 10}24*× ∆ (pe) = 6.3 × 10−7*

_{ 1 }_{J or 63 MJ}

*1*

*r*

_{E}### [ ]

−_{ ∞}

*1*.

*1*∞

### [ ]

*1*

*6.36*×

*106*

There is an important idea contained in this last part. The 1 kg mass has
*been taken to ‘infinity’ but the work done or gain in pe has increased by a*
*finite amount ( 6.3 *× 107_{ J).}
*Gm _{M}m_{E}*

*R2*

*= mM*ω

*2*m

_{R}_{E}F mM R 2π

*T*

### (

### )

2 b ∆h a F mg m*To raise the mass from level a to b, the force F will do work F *× ∆*h on the*
mass, which increases the potential energy of the mass by this amount
*Since F = mg , the increase in potential energy is given by *∆*(p.e.) = mg*∆*h.*

There are two important things to note here:

### •

*We are only finding the increase in potential energy, we do not know*

*the potential energy at a or b.*

### •

*We have assumed that g is constant between the levels a and b.*

*The assumption that g is constant is valid close to the Earth's surface, but*
for positions further away from the earth, another approach is needed.

Suppose we wish to move the mass between the two points from
*where R = a to R = b and find the work done for this displacement.*
* Because the force F is varying we cannot use the equation work = F*× d.
Fortunately there is a simple way round the problem. First we find the

*work done in taking the mass from the point where R = a to infinity. For*this displacement, the work done is

NB: This equation only works in a radial field.

*To take the mass from the point where R = b out to infinity would require*
work of to be done.

So the change in potential energy between the two points is
−* = Gm _{E}m *−
m

_{E}F = G mE m R2

*m*

_{R = a}*R = b*R

### m

*Gm*

_{E}m*a*

*Gm*

_{E}m*b*

*Gm*

_{E}m*a*

*Gm*

_{E}m*b*

*1*

*a*

*1*

*b*

**Worked Example**

**The Earth has a mass m _{E} = 6.0 **×××××

**1024**

_{ kg and radius r}**E = 6.36 **×××××** 10**
**6 _{ m.}**

**Find the gain in p.e. of a 1 kg mass when it is lifted from the Earth’s**
**surface to a height of i)** **h = r**_{E} and

**ii) h = 2r**_{E.}

* iii) h = *∞∞∞∞∞

*×××××*

**(infinity) Take G = 6.67***−−−−−*

**10***−−−−−*

**11**_{ Nm}**2**_{kg}

**2**∆* (pe) = G m _{E}m*
∆

*(pe) = 6.67*×

*10*−

*11*×

*6*×

*1024*×

*1*× joules ∆

*(pe) = 3.15*×

*107 J*

*1*

*r*

_{E}### [ ]

*.*

_{2r}1*E*−

### [ ]

*1*

*2*×

*6.36*×

*106*

*i) Using the above, we have a =r*

_{E}and b = 2r_{E}Lines of
equipotential
b
a
a
q
p
**Worked Example**

**A piece of space junk of mass 10 kg detaches itself from**
**a spacecraft at p and falls freely to point q. Find the kinetic**
**energy gained by the 10 kg mass in falling to Earth.**

**( r**_{p}** = 30 **×××××** 106**_{m, r}

**q**** = 12 **×××××** 10**

**6**_{m m}

**E**** = 6 **×××××** 10**

**24 _{kg)}**

*Since the mass falls freely from p to q, the gain in kinetic energy*
*is equal to the loss in potential energy. The potential energies at points*
*p and q are m *×* V _{p} and m *×

*V*

_{q}.*where.*
*Gain in ke = pe ( p ) – pe ( q )*
*= ( mV _{p} ) – ( mV_{q} )*

*= m (V*−

_{p}*V*

_{q})*= m*

*= mGm*

_{E}*= 10*×

*6.67*×

*10*−

*11*×

*6*×

*1024*

*= 2.00*×

*108J*

*V*= −

_{R }*GmE*

*R*−

*GmE*−

*r*

_{p}*Gm*

_{E}*r*−

_{q}### [ ]

*.*

_{r}1*q*

*1*

*r*−

_{p}### [

### −

*30*×

*1 106*

### ]

.*1*

*12*×

*106*

### ( )

### ( )

**Defining Gravitational Potential Energy**

To define the actual potential energy, rather than working out differences
*in it, it is first necessary to decide at which point a body will have zero*
*potential energy. At first sight it may seem strange but it has been agreed*
that this point shall be at ‘infinity’.

*Since moving a point from the earth's surface to infinity increases its*
potential energy (as seen in the previous example), if the potential energy
is zero at infinity, it must be negative elsewhere.

*Gravitational potential energy of a mass at a point in a*
*gravitational field is the work done in bringing the mass from*
*infinity to that point.*

**Gravitational Potential (V)**

*Gravitational potential (V) is the gravitational potential energy*
*per unit mass.*

*(Jkg*−*1*_{)}
*Gm _{E}*

*R*
*V = *−

The diagram below shows how the potential varies with the distance from the earth’s centre.

**Typical Exam Question.**

**(i)** **Use Newton’s law of gravitational to find the units for G**
**(ii)** **Hence show that the expression**

**does have the unit joule.**

*Answer:*

(i) *Newton’s law:* * re-arrange*

*Substituting the known units, we obtain the units for G :*
*[G] =* * or Nm2 _{kg}*−

*2*

(ii) *pe = . Substitute the units:*
*[pe] =*− *kg *× *(Nm*

*2 _{kg}*−

*2*×

_{) }

_{ kg}*m*

= (−)*Nm.*

* Since work (or energy) is force *×* displacement it follows that Nm*
* is the same as joules.*

* F = Gm1m2*
*r _{2}*

*G =*

*Fr2*

*m*

_{1}m_{2}*Nm2*

*kg2*

*Gm*

_{E}*R*−

*m*×

*R = distance from centre of earth (R >r _{E})*

*To find the potential energy of a body at a given point, multiply*
*the potential at the point by the mass of the body.*

V/107_{ Jkg}−1
0
−2
−4
−6
Earth
1
R
V ∝
5r_{E} 10r_{E} 15r_{E} **R**
**Gm**_{E}* R*
−−−−−

*×××××*

**m****Lines of equipotential**

These are lines which surround a body, all points on the line being at the same potential. They are shown as dotted lines in the diagram below.

Note that the lines of equipotential are perpendicular to the field lines. If
you move along the curved path from a to b then the displacement is
*always perpendicular to the direction of the force. This means no work is*
done and there is no gain (or loss) in potential energy; hence the curve
through ab is a line of equipotential.

**Escape Velocity**

The escape velocity is the velocity a mass must have to escape from the
Earth ( or any celestial body) and not return. The mass must not be driven
by motors of any kind, but have a velocity imparted to it that will carry it
completely away from the Earth ie to infinity. As the mass gets further
from the Earth, its potential energy increases. At infinity, the potential
energy has increased to zero. When the mass has its escape velocity u, it
must have sufficient kinetic energy to match the necessary gain in potential
*energy in taking it to infinity. Let u = the escape velocity for the Earth.*
*We use conservation of energy for a mass m.*

Loss in kinetic energy = Gain in potential energy.
*½ mu*2_{ = pe(}∞_{) }− _{pe(r}
E)
*½ mu*2_{ = 0 }−

### [

*2GmEm*

### ]

*r*−

_{E}*2Gm*

_{E}m*r*= 2 × 6.67 × 10−11 ×

_{ 6 }×

_{ 10}24 6.36 × 106 = 11 × 10 3

_{ ms}−1

*u =*Lines of Field

**a) Write down an expression for the gravitational potential at a**
**distance R from the centre of a planet of mass M. Hence write**
**down an expression for the potential energy of a mass m at a**
**height h above the Earth’s surface ; radius of Earth = R**_{E} , mass

**Earth = M _{E} .**

**[3]**

*Potential *

* Potential energy = mgh. ! 1/3*

**b) An amount of energy E **E E **E is transferred to a satellite of mass m**E
**when launched by a certain rocket from the surface of the**
**Earth. If the vertical height reached by the satellite is h, obtain**
**an equation relating h to E **E E E **E and the other terms. [2]**
*E = mgh = m h. ! 0/2*

**c) Show that if = R _{E}, the energy E **E E

**E is given by EEEEE = [2]**E

*Putting R _{E} = h, E = m ! 0/2*

**d) The graph below shows how the force acting on the satellite**
**varies with distance from Earth’s centre.**

**Exam Workshop**

*V =* *GM*
*R*

Minus sign omitted. Has not understood how to find potential energy
from potential . Formula quoted only valid when h « R_{E} . In this case
h = R_{E} .

*GM _{E}*

*R*

_{E}2A fatal error has been carried forward and gains no credit. Substituting for g = is meaningless as this expression is only true at the Earth's surface. This candidate has relied too much on quoting formula without understanding the physics.

*GM _{E}*

*R*

_{E}2

**GM**_{E}**m**

**2R**_{E}*GM*

_{E}*R*

_{E}**Write the equation that shows how this force can be calculated**
**and given that the force at the Earth’s surface is 10 kN,****calculate the force on the satellite at a height h = R**_{E}**. [3]**

Force on satellite /kN
R_{E} 2R_{E} 3R_{E} 4R_{E}
0
2
4
6
8
10

Distance from Earth centre

*The force follows Newton's Law of Gravity.*

*2*

*R*
*Mm*
*G*
*F*=

*When h = R, the distance is doubled so *

### ( )

*2*

*2*

*R*

*Mm*

*G*

*4*

*1*

*2R*

*Mm*

*G*

*F*= =

*The force is reduced to a quarter, so force at height R is:*
*2.5kN*
*4*
*10kN* _{=}
!
!
!
!
!
!
!!
!
!
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!
!
!
!

This answer is correct and gains full credit. It is testing the inverse square law.

**e) Indicate on the graph the quantity that represents the**
**energy **E.E.E.E.E. ** [2]**
Force on satellite /kN
R_{E} 2R_{E} 3R_{E} 4R_{E}
0
2
4
6
8
10

Distance from Earth centre ! !! !! ! ! ! ! !

This answer is correct and gains full credit. It is testing how to find the work done by a force which varies with distance.

This is a general result and is used elsewhere in physics.

**Examiner’s answers**

*a)*

*R*

*GM*

*V*

*potential*=−

*E*

*potential energy of mass m = m *×* V, at height h, R = R*
*E*
* + h*
*and so p.e. = *

_{(}

_{)}

*h*

*R*

*GM*

*m*

*E*

*E*+ × −

*b) Energy E is transferred so by conservation,*

E + _{}− _{}=_{}−

_{(}

_{+}

_{)}

_{}

*h*

*R*

*GM*

*m*

*R*

*GM*

*m*

*E*

*E*

*E*

*E*

*c) When h = R*

*E*

*we get*E + ⇒ − = −

*E*

*E*

*E*

*E*

*2R*

*GM*

*m*

*R*

*GM*

*m*E =

*E*

*E*

*2R*

*GM*

*m*

*d) As above, force follows inverse square law:- *

*2*

*R*
*mM*
*G*
*F*=

*Doubling distance gives *

*4*
*1*
*2*
*1* *2*
=
* of force.*
*Force when h = R _{E} *

*2.5kN*

*4*

*10*=

*e) Shade area under graph from R _{E} to 2R_{E}*

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!! _{!}_{!}_{!}_{!}_{!}

**Typical Exam Question**

**1) i)** **Explain the term ‘gravitational potential’ as applied to a**
**point in the Earth’s gravitational field.**

**ii) The gravitational potential at the Earth’s surface is**

−−−−−**63 MJ kg****-1**_{ ; taking the Earth to be a sphere of radius R}

**what are the potentials at points A and B at heights R and****3R respectively above the Earth’s surface?**

**iii) A 10 kg mass is moved from A to B. Does its potential****energy increase or decrease and by how much ?**

**iv) A 1 kg mass at the Earth’s surface is taken to infinity.****What is the minimum amount of energy to achieve this ?**

*i)* *The gravitational potential at a point is the work done per*
*kilogram by an external force in moving a mass from the*
*point to infinity (or zero potential) Or, alternatively, the work*
*done per kilogram by the field as the mass ‘falls’ from infinity*

*to the point.* **[2]**

*ii) At the Earth’s surface radius R, the potential is given by*

*and the potentials at A and B are given by*

*V _{A} = *−

*31.5MJ kg-1*

_{ and V}*B =* −*15.75 MJ kg*

*-1* _{[4]}

* iii) The potential energy is m *×* V joules. So, for the 10 kgmass*
*at points A and B, we have: p.e. (A) = 10 *×* (31.5) MJ = *−*315*
*MJ p.e.(B) = 10 *× * (*−*15.75 ) MJ = *−*157.5 MJ The potential*
*energy at B is less negative than at A, hence in moving from*
*A to B, the potential energy increase* **[3]**

* iv)* *The potential at infinity is zero, hence the potential energy of*
*any mass at infinity is zero. The potential energy of 1 kg mass*
*at Earth’s surface is –63 MJ. To move it to infinity the potential*
*energy must increase to zero. Hence, the minimum energy*

*required is 63 MJ.* **[3]**
*V = *− *GM*= −*63MJ kg-1*
*R*
*V _{A} = *−

*GMand V*= −

_{B}*and so*

*2R*

*GNM*

*4R*

*V*−

_{A}=*MJ kg-1*

_{ and V}*B =*−

*MJ kg*

*-1*

*62*

*2*

*63*

*4*! ! ! ! !

_{!}! ! ! ! ! !

(Where necessary, take G = 6.67 × 10-11_{ Nm}2_{kg}-2_{ )}

1 a) Explain the term ‘gravitational field’ [2] b) Define the term ‘gravitational field strength’ [2] c) A communications satellite orbits the Earth in a circular path of

radius 4.2 ×107_{ m. Calculate the gravitational field strength at a}

point in this orbit (Mass of earth =6×1024_{kg) [3 ]}

d) The figure below shows a spherical planet with some equipotentials.

**Questions**

a
d
b
c
-1500 Jkg-1
-1000 Jkg-1
-700 Jkg-1
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0 -10 -20 2 4 6 8 R/I V/kJ kg-1 X Yiii) Find the work done in moving the 10 kg mass from point X to
point Y (6×104_{m from planet’s centre).} _{[2]}

iv) Write down the gravitational potential at point X and hence deduce the gravitational field strength (intensity) at point X. What force acts on the 10kg mass? [2 + 1] 2 a) Define the term ‘gravitational potential’ at a point in a radial

gravitational field.

b) Use this definition to calculate the ‘escape velocity’ for the Earth
with mass M_{E} = 6×1024_{ kg and radius R}

E = 6.36×10
6_{ m.}

c) A 1kg mass is to be catapulted from the Earth’s surface so that it will not return. How much energy must be given to the mass? What is this energy in kilowatt hours and how much would it cost (at the domestic rate of 5.5pence / kWh) to expel this mass? d) The planet Mercury is smaller than Earth with a mass

*M _{M} = 3.24 *×

*1023*

_{ kg and radius R}*M = 2.34 *×* 10*
*6 _{ m.}*

Calculate the escape velocity for Mercury and, giving reasons,
comment on the likelihood of Mercury retaining any atmosphere.
*Assume G = 6.67 *×* 10-11 _{ Nm}2_{kg}-2*

e) i) *Write down an expression for the gravitational potential at a*
distance R from the centre of a planet of mass M. [2]
* ii) Using the graph below find the potential energy of a 10 kg mass*

at 4×104_{m from the planet’s centre (point X).} _{[2]}

Calculate the gain in potential energy when a 10 kg mass is moved i) from a to b

ii) from b to c

3. a) Write down Newton’s law of gravity for two attracting masses. b) Assuming the Earth to be a uniform sphere of mass M and radius

R show that the acceleration of free fall at the Earth’s surface may be deduced from Newton’s law and is given by:

c) Given that the mean density of the Earth is ρ, show that

d) Show that the units on both sides of this equation are consistent.
e) A spaceship hovering above the surface of Mars releases a mass at a
height of 190m . This mass hits the surface of the planet at 38ms-1_{.}

Calculate a value for the acceleration of free fall at the planet’s surface.
f) Given that Mars has a radius of 3.4 × 106 _{m, estimate a value for the}

*density of the planet. Assume G = 6.67 *×* 10-11 _{ Nm}2_{kg}-2*

*g =*

*GM*

*R2*

*3g*

*G =*

*4*πρ

*R*

1. a) Space around a mass where gravitational effect (force on second mass ) can be detected.

*b) Gravitational field strength /intensity at a point defined as force*
*per unit mass at that point =*

c)
*= 0.227 Nkg-1 _{ (ms}-2_{)}*
d) pe = m ×V
gain pe(ab) = (-10×1000) − (-10×1500) = 5000J
gain pe(bc) = (-10×700) − (-10×1000) = 3000J
gain pe (dc) = zero.
e) i) potential

*ii) from graph, at X, potential = -11.5kJkg*-1

⇒ pe(x) = -11.5×*10 kJ = -115 kJ*

(allow for sensible error in graph e.g. 11<V<12 ) iii) pe (Y) = -7.5×10 kJ. Work done = increase in pe

= (-75kJ) –(-115kJ) =40kJ. iv) For sphere,

*at X, = 2.875 ms*-2_{ }≈* _{ 2.9 ms}*-2

force = 10 ×* 2.9 = 29 N*

2. a) work done per kg in bringing mass from infinity to the point.
b) Escape = = 11.2 × 103 _{ ms}-1

c) = 6.3 × 107 * _{ joule 1kWh = 3.6 }*×

_{ 10}6

_{ J k.e. = 17.5 kWh}Cost = 96.25 pence !
*d) u*_{escape}= 4.3 × 103_{ ms}-1

smaller speed, easier escape, atmosphere less likely. (See also kinetic theory; molecular k.e. ∝ Temperature)

3. a) & b) see text

*c) put M = *ρ × volume = ρ×4π*R*3_{ /3}

*d) sub in units, change N to kg *×* ms*-1

*e) g = 3.8ms*-2

f) ρ = 4.0×103* _{ kgm}*-3

**Answers**

**Acknowledgements:**

*This Factsheet was researched and written by Keith Cooper*

**Curriculum Press, Unit 305B, The Big Peg, 120 Vyse Street, Birmingham, B18 6NF****Physics Factsheets may be copied free of charge by teaching staff or students,**

*provided that their school is a registered subscriber.*

*No part of these Factsheets may be reproduced, stored in a retrieval system, or*
*transmitted, in any other form or by any other means, without the prior*
**permission of the publisher. ISSN 1351-5136**

*F _{1} and F_{2} are mutual gravitational attraction. F_{1} = F_{2}*

*ii) centripetal force = F*

_{2}*iii) Kepler’s third law : T2*∝_{ R}3

*iv) GM = R3*ω2

*i.e. Kepler’s third law : T2*∝* _{ R}3*a

*GMm*

*R2*

*= m*×

*R*ω

*2*ω

*= = GMT2*π

*2*π

_{ = 4}*2*

_{R}3*T*

*i)*! ! ! ! ! ! !

**2) A moon of mass m is orbiting its planet of mass M in a circular**
**orbit of radius R.**

**i)** **Draw a diagram showing the forces acting on both bodies.**
**How do these forces arise and are they equal in magnitude**

**or not ?** **[2]**

**ii) Which of the two forces you have drawn provides the**
**centripetal force necessary for circular motion ? The moon**
**orbits the planet in a time T.** **[1]**
**iii) State Kepler’s third law of motion as applied to the moon**

**[2]**
**iv) Show how Kepler’s third law may be deduced from**

**Newton’s law of universal gravitation.** **[3]**

M

F_{1} _{F}

2

m

**Typical Exam Question**

*F*
*m*
*GM*
*R*
*V = *−
6.67× 10-11×_{ 6}×_{ 10}24
*F = GmM*
*R2* ⇒
*F*
*m =*
*GM*
*R =* (4.2× 107_{)}2
*V= G* *mM*
*R2* ⇒
*V*
*R =*
*GM*
*R2* *= g*
-11.5× 103
*=* _{4}_{× }_{ 10}4
*g*
*R*
*2GM*
*k.e.=mu*
*2*
*2*