Textbook: Energy | Technical Science for Grade 10 PDF Download

 Page 1


 technical science GRADE 10 179
c hapter 7 energy 
Part of this course is based on what you learnt about energy in Natural Sciences, including 
the following:
•	 Forms of energy: potential energy and kinetic energy 
Potential energy can take various forms, such as gravitational potential energy, strain energy 
and chemical energy.
•	 Energy in transfer: electrical energy, heat energy, light energy and sound energy
You can summarise what you learnt about energy in Natural Sciences with a simple statement: 
Energy is needed for life and to make things work.
We will study forms of energy in this chapter, and we will study temperature, heat and energy 
in transfer in Chapter 16 on Heat and Thermodynamics.
unit 7.1 Gravitational potential energy
Because of the force of gravity, everything on Earth has weight – a boat on the sea, 
the international space station orbiting the Earth, a particle of gas – everything that you can see 
or cannot see has weight!
Because an object has weight (and mass), it takes energy to lift it up. As an object is lifted up 
from one position to a higher position, the energy that is used to lift it up is transferred to the 
object. The object gains energy as it is lifted up.
The energy that an object gains as it is physically lifted up from one position to a higher 
position is gravitational potential energy.
definition:  The gravitational potential energy of an object is the energy it has 
because of its position in the gravitational field.
The description “gravitational potential” is full of meaning.
•	 The word “gravitational” shows that the energy is the result of the gravitational 
attraction of the Earth for the object. 
•	 The word “potential” shows that the energy can be used later to do work.
The box is
gaining energy
The man is using energy
The box has gained gravitational
potential energy
The man has
less chemical
potential energy  
The man has a lot of chemical potential
energy available in his muscles
The box has a
little gravitational
potential energy 
Figure 7 .1
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   179 2015/12/17   10:02 AM
Page 2


 technical science GRADE 10 179
c hapter 7 energy 
Part of this course is based on what you learnt about energy in Natural Sciences, including 
the following:
•	 Forms of energy: potential energy and kinetic energy 
Potential energy can take various forms, such as gravitational potential energy, strain energy 
and chemical energy.
•	 Energy in transfer: electrical energy, heat energy, light energy and sound energy
You can summarise what you learnt about energy in Natural Sciences with a simple statement: 
Energy is needed for life and to make things work.
We will study forms of energy in this chapter, and we will study temperature, heat and energy 
in transfer in Chapter 16 on Heat and Thermodynamics.
unit 7.1 Gravitational potential energy
Because of the force of gravity, everything on Earth has weight – a boat on the sea, 
the international space station orbiting the Earth, a particle of gas – everything that you can see 
or cannot see has weight!
Because an object has weight (and mass), it takes energy to lift it up. As an object is lifted up 
from one position to a higher position, the energy that is used to lift it up is transferred to the 
object. The object gains energy as it is lifted up.
The energy that an object gains as it is physically lifted up from one position to a higher 
position is gravitational potential energy.
definition:  The gravitational potential energy of an object is the energy it has 
because of its position in the gravitational field.
The description “gravitational potential” is full of meaning.
•	 The word “gravitational” shows that the energy is the result of the gravitational 
attraction of the Earth for the object. 
•	 The word “potential” shows that the energy can be used later to do work.
The box is
gaining energy
The man is using energy
The box has gained gravitational
potential energy
The man has
less chemical
potential energy  
The man has a lot of chemical potential
energy available in his muscles
The box has a
little gravitational
potential energy 
Figure 7 .1
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   179 2015/12/17   10:02 AM
180 c hapter 7 ENERGY
For example, the heavy head of the pole-driver in  
Figure 7.2 gains potential energy when it is lifted up.  
When the heavy head is dropped down, the energy of  
the head drives the pole into the ground.
activity 1  investigate the effect of height and 
mass when a ball rolls down a ramp
In this investigation, balls will roll down a ramp and across a 
smooth, level floor. You will investigate the effect of (a) the 
mass of the ball or (b) the height from which it is released, on 
the distance that the ball rolls.
Figure 7 .3 Investigate the effect of height and mass.
A. Divide the class into two groups, one to investigate the 
effect of mass and the other to investigate the effect 
of height. Your teacher will lead you through the 
investigations.
B. Do your investigation using the format given in the 
section on How to do an investigation in the 
Resource Pages.
C. Record all your information and results in a table.
D. Use the results of your experiment to draw a graph of one 
of the following: 
•	 mass versus distance
•	 height versus distance
E. Based on your results and the information that your 
graph gives you, draw a conclusion that relates the distance that a ball rolls to the mass of 
the ball or the height from which it was released.
On the basis of the investigation, you will have concluded that the distance travelled by the 
ball depends on both of the following:
•	 the height from which the ball was released
•	 the mass of the ball
In this activity you followed a scientific process:
•	 You thought about the problem.
•	 You developed a hypothesis.
Figure 7 .2 The gravitational potential 
energy of the driver head when it is 
lifted up is used to drive the pole into 
the ground.
apparatus
 ? three steel balls (from recycled 
ball bearings), of different sizes 
(bigger than 8 mm in diameter) 
and of known mass
 ? a metre rule
 ? a plank and strips of card, 
or recycled curtain rail about  
1 m long
 ? a few books
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   180 2015/12/17   10:02 AM
Page 3


 technical science GRADE 10 179
c hapter 7 energy 
Part of this course is based on what you learnt about energy in Natural Sciences, including 
the following:
•	 Forms of energy: potential energy and kinetic energy 
Potential energy can take various forms, such as gravitational potential energy, strain energy 
and chemical energy.
•	 Energy in transfer: electrical energy, heat energy, light energy and sound energy
You can summarise what you learnt about energy in Natural Sciences with a simple statement: 
Energy is needed for life and to make things work.
We will study forms of energy in this chapter, and we will study temperature, heat and energy 
in transfer in Chapter 16 on Heat and Thermodynamics.
unit 7.1 Gravitational potential energy
Because of the force of gravity, everything on Earth has weight – a boat on the sea, 
the international space station orbiting the Earth, a particle of gas – everything that you can see 
or cannot see has weight!
Because an object has weight (and mass), it takes energy to lift it up. As an object is lifted up 
from one position to a higher position, the energy that is used to lift it up is transferred to the 
object. The object gains energy as it is lifted up.
The energy that an object gains as it is physically lifted up from one position to a higher 
position is gravitational potential energy.
definition:  The gravitational potential energy of an object is the energy it has 
because of its position in the gravitational field.
The description “gravitational potential” is full of meaning.
•	 The word “gravitational” shows that the energy is the result of the gravitational 
attraction of the Earth for the object. 
•	 The word “potential” shows that the energy can be used later to do work.
The box is
gaining energy
The man is using energy
The box has gained gravitational
potential energy
The man has
less chemical
potential energy  
The man has a lot of chemical potential
energy available in his muscles
The box has a
little gravitational
potential energy 
Figure 7 .1
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   179 2015/12/17   10:02 AM
180 c hapter 7 ENERGY
For example, the heavy head of the pole-driver in  
Figure 7.2 gains potential energy when it is lifted up.  
When the heavy head is dropped down, the energy of  
the head drives the pole into the ground.
activity 1  investigate the effect of height and 
mass when a ball rolls down a ramp
In this investigation, balls will roll down a ramp and across a 
smooth, level floor. You will investigate the effect of (a) the 
mass of the ball or (b) the height from which it is released, on 
the distance that the ball rolls.
Figure 7 .3 Investigate the effect of height and mass.
A. Divide the class into two groups, one to investigate the 
effect of mass and the other to investigate the effect 
of height. Your teacher will lead you through the 
investigations.
B. Do your investigation using the format given in the 
section on How to do an investigation in the 
Resource Pages.
C. Record all your information and results in a table.
D. Use the results of your experiment to draw a graph of one 
of the following: 
•	 mass versus distance
•	 height versus distance
E. Based on your results and the information that your 
graph gives you, draw a conclusion that relates the distance that a ball rolls to the mass of 
the ball or the height from which it was released.
On the basis of the investigation, you will have concluded that the distance travelled by the 
ball depends on both of the following:
•	 the height from which the ball was released
•	 the mass of the ball
In this activity you followed a scientific process:
•	 You thought about the problem.
•	 You developed a hypothesis.
Figure 7 .2 The gravitational potential 
energy of the driver head when it is 
lifted up is used to drive the pole into 
the ground.
apparatus
 ? three steel balls (from recycled 
ball bearings), of different sizes 
(bigger than 8 mm in diameter) 
and of known mass
 ? a metre rule
 ? a plank and strips of card, 
or recycled curtain rail about  
1 m long
 ? a few books
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   180 2015/12/17   10:02 AM
 technical science GRADE 10 181
•	 You tested the hypothesis in an experiment.
•	 You drew a conclusion.
When you follow a scientifi c process in an investigation, you are doing good science.
calculate gravitational potential energy.
The gravitational potential energy of an object is dependent on two variables:
•	 the mass of the object – the greater the mass, the greater its gravitational potential energy
•	 the height to which the object is raised – the higher the object is raised, the greater its 
gravitational potential energy
These two statements are combined in the formula for gravitational potential energy:
 gravitational potential energy = mass × acceleration of gravity × height
or 
 E
p
 = m g h  or U = m g h
where:
•	 E
p
 or U is the symbol for gravitational potential 
energy measured in Joules (J)
•	 m is the symbol for the mass of the object 
measured in kilograms (kg)
•	 g is 9,8 m/s
2
•	 h is the symbol for the height of the object above 
a reference posit ion measured in metres (m)
NOTE: The Joule (J) is the standard unit for the measurement of energy in the SI system.
Worked examples: calculate gravitational potential energy
1. An object that has a mass of 4 kg is held 2 m above the ground. Calculate its gravitational 
potential energy relative to the ground.
 Solution
 Given mass is 4 kg; height above ground is 2 m
 Unknown gravitational potential energy relative to the ground
 Formula E
p
 = m g h
  = 4 × 9,8 × 2   (substitute)
  = 78,4 J
E
P
 = m g h
Potential
energy (J)
Mass (kg)
Acceleration 
of gravity (m/s
2
)
Height (m)
Figure 7 .4
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   181 2015/12/17   10:02 AM
Page 4


 technical science GRADE 10 179
c hapter 7 energy 
Part of this course is based on what you learnt about energy in Natural Sciences, including 
the following:
•	 Forms of energy: potential energy and kinetic energy 
Potential energy can take various forms, such as gravitational potential energy, strain energy 
and chemical energy.
•	 Energy in transfer: electrical energy, heat energy, light energy and sound energy
You can summarise what you learnt about energy in Natural Sciences with a simple statement: 
Energy is needed for life and to make things work.
We will study forms of energy in this chapter, and we will study temperature, heat and energy 
in transfer in Chapter 16 on Heat and Thermodynamics.
unit 7.1 Gravitational potential energy
Because of the force of gravity, everything on Earth has weight – a boat on the sea, 
the international space station orbiting the Earth, a particle of gas – everything that you can see 
or cannot see has weight!
Because an object has weight (and mass), it takes energy to lift it up. As an object is lifted up 
from one position to a higher position, the energy that is used to lift it up is transferred to the 
object. The object gains energy as it is lifted up.
The energy that an object gains as it is physically lifted up from one position to a higher 
position is gravitational potential energy.
definition:  The gravitational potential energy of an object is the energy it has 
because of its position in the gravitational field.
The description “gravitational potential” is full of meaning.
•	 The word “gravitational” shows that the energy is the result of the gravitational 
attraction of the Earth for the object. 
•	 The word “potential” shows that the energy can be used later to do work.
The box is
gaining energy
The man is using energy
The box has gained gravitational
potential energy
The man has
less chemical
potential energy  
The man has a lot of chemical potential
energy available in his muscles
The box has a
little gravitational
potential energy 
Figure 7 .1
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   179 2015/12/17   10:02 AM
180 c hapter 7 ENERGY
For example, the heavy head of the pole-driver in  
Figure 7.2 gains potential energy when it is lifted up.  
When the heavy head is dropped down, the energy of  
the head drives the pole into the ground.
activity 1  investigate the effect of height and 
mass when a ball rolls down a ramp
In this investigation, balls will roll down a ramp and across a 
smooth, level floor. You will investigate the effect of (a) the 
mass of the ball or (b) the height from which it is released, on 
the distance that the ball rolls.
Figure 7 .3 Investigate the effect of height and mass.
A. Divide the class into two groups, one to investigate the 
effect of mass and the other to investigate the effect 
of height. Your teacher will lead you through the 
investigations.
B. Do your investigation using the format given in the 
section on How to do an investigation in the 
Resource Pages.
C. Record all your information and results in a table.
D. Use the results of your experiment to draw a graph of one 
of the following: 
•	 mass versus distance
•	 height versus distance
E. Based on your results and the information that your 
graph gives you, draw a conclusion that relates the distance that a ball rolls to the mass of 
the ball or the height from which it was released.
On the basis of the investigation, you will have concluded that the distance travelled by the 
ball depends on both of the following:
•	 the height from which the ball was released
•	 the mass of the ball
In this activity you followed a scientific process:
•	 You thought about the problem.
•	 You developed a hypothesis.
Figure 7 .2 The gravitational potential 
energy of the driver head when it is 
lifted up is used to drive the pole into 
the ground.
apparatus
 ? three steel balls (from recycled 
ball bearings), of different sizes 
(bigger than 8 mm in diameter) 
and of known mass
 ? a metre rule
 ? a plank and strips of card, 
or recycled curtain rail about  
1 m long
 ? a few books
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   180 2015/12/17   10:02 AM
 technical science GRADE 10 181
•	 You tested the hypothesis in an experiment.
•	 You drew a conclusion.
When you follow a scientifi c process in an investigation, you are doing good science.
calculate gravitational potential energy.
The gravitational potential energy of an object is dependent on two variables:
•	 the mass of the object – the greater the mass, the greater its gravitational potential energy
•	 the height to which the object is raised – the higher the object is raised, the greater its 
gravitational potential energy
These two statements are combined in the formula for gravitational potential energy:
 gravitational potential energy = mass × acceleration of gravity × height
or 
 E
p
 = m g h  or U = m g h
where:
•	 E
p
 or U is the symbol for gravitational potential 
energy measured in Joules (J)
•	 m is the symbol for the mass of the object 
measured in kilograms (kg)
•	 g is 9,8 m/s
2
•	 h is the symbol for the height of the object above 
a reference posit ion measured in metres (m)
NOTE: The Joule (J) is the standard unit for the measurement of energy in the SI system.
Worked examples: calculate gravitational potential energy
1. An object that has a mass of 4 kg is held 2 m above the ground. Calculate its gravitational 
potential energy relative to the ground.
 Solution
 Given mass is 4 kg; height above ground is 2 m
 Unknown gravitational potential energy relative to the ground
 Formula E
p
 = m g h
  = 4 × 9,8 × 2   (substitute)
  = 78,4 J
E
P
 = m g h
Potential
energy (J)
Mass (kg)
Acceleration 
of gravity (m/s
2
)
Height (m)
Figure 7 .4
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   181 2015/12/17   10:02 AM
182 chapter 7 ENERGY
2. An object that has a mass of 0,5 kg is held at shoulder height (1,5 m) out of a third fl oor 
window. The height between each fl oor is 3,3 m. Calculate the object’s gravitational 
potential energy relative to the ground.
 Solution
 Given mass is 0,5 kg; shoulder height is 1,5 m; 3 fl oors of 3,3 m
 Unknown gravitational potential energy relative to the ground
 Formula E
p
 = m g h
   = 0,5 × 9,8 × (3 × 3,3 + 1,5)  (substitute)
   = 55,9 J
activity 2 calculate gravitational potential energy
1. An object that has a mass of 10 kg is held 0,5 m above a table. The table is 0,9 m high.
a) Calculate the object’s gravitational potential energy relative to the table.
b) What is the object’s gravitational potential energy relative to the fl oor?
c) Calculate the object’s gravitational potential energy relative to the fl oor when it is 
on the table.
2. A ball is thrown up into the air. It reaches a height of 12 m. Its mass is 200 g.
a) Calculate the ball’s gravitational potential energy as it reaches its highest point 
above the ground.
b) As the thrower leans back to throw the ball, his hand is 1 m above the ground. 
Calculate the ball’s gravitational potential energy when it is 1 m above the ground.
c) Calculate the energy the thrower gives to the ball.
d) What is the ball’s gravitational potential energy when it is on the ground?
3. One of the methods that a blacksmith uses to 
shape metal is drop-forging. You can see a drop 
forge in Figure 7.5. The red-hot metal in a mould 
is shaped when a heavy weight is dropped 
repeatedly onto the mould (also called a die).
a) What is the reasonable reference height 
(position of zero height) for the machine?
b) Calculate the potential energy of the 500 kg 
hammer when it is raised to its maximum of 
2 m above the mould.
c) A delicate mould needs a maximum of 4 000 J 
per hammer blow to shape a small object. 
Calculate the maximum height to which the 
500 kg hammer should be raised.
Figure 7 .5 A drop forge 
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   182 2015/12/17   10:02 AM
Page 5


 technical science GRADE 10 179
c hapter 7 energy 
Part of this course is based on what you learnt about energy in Natural Sciences, including 
the following:
•	 Forms of energy: potential energy and kinetic energy 
Potential energy can take various forms, such as gravitational potential energy, strain energy 
and chemical energy.
•	 Energy in transfer: electrical energy, heat energy, light energy and sound energy
You can summarise what you learnt about energy in Natural Sciences with a simple statement: 
Energy is needed for life and to make things work.
We will study forms of energy in this chapter, and we will study temperature, heat and energy 
in transfer in Chapter 16 on Heat and Thermodynamics.
unit 7.1 Gravitational potential energy
Because of the force of gravity, everything on Earth has weight – a boat on the sea, 
the international space station orbiting the Earth, a particle of gas – everything that you can see 
or cannot see has weight!
Because an object has weight (and mass), it takes energy to lift it up. As an object is lifted up 
from one position to a higher position, the energy that is used to lift it up is transferred to the 
object. The object gains energy as it is lifted up.
The energy that an object gains as it is physically lifted up from one position to a higher 
position is gravitational potential energy.
definition:  The gravitational potential energy of an object is the energy it has 
because of its position in the gravitational field.
The description “gravitational potential” is full of meaning.
•	 The word “gravitational” shows that the energy is the result of the gravitational 
attraction of the Earth for the object. 
•	 The word “potential” shows that the energy can be used later to do work.
The box is
gaining energy
The man is using energy
The box has gained gravitational
potential energy
The man has
less chemical
potential energy  
The man has a lot of chemical potential
energy available in his muscles
The box has a
little gravitational
potential energy 
Figure 7 .1
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   179 2015/12/17   10:02 AM
180 c hapter 7 ENERGY
For example, the heavy head of the pole-driver in  
Figure 7.2 gains potential energy when it is lifted up.  
When the heavy head is dropped down, the energy of  
the head drives the pole into the ground.
activity 1  investigate the effect of height and 
mass when a ball rolls down a ramp
In this investigation, balls will roll down a ramp and across a 
smooth, level floor. You will investigate the effect of (a) the 
mass of the ball or (b) the height from which it is released, on 
the distance that the ball rolls.
Figure 7 .3 Investigate the effect of height and mass.
A. Divide the class into two groups, one to investigate the 
effect of mass and the other to investigate the effect 
of height. Your teacher will lead you through the 
investigations.
B. Do your investigation using the format given in the 
section on How to do an investigation in the 
Resource Pages.
C. Record all your information and results in a table.
D. Use the results of your experiment to draw a graph of one 
of the following: 
•	 mass versus distance
•	 height versus distance
E. Based on your results and the information that your 
graph gives you, draw a conclusion that relates the distance that a ball rolls to the mass of 
the ball or the height from which it was released.
On the basis of the investigation, you will have concluded that the distance travelled by the 
ball depends on both of the following:
•	 the height from which the ball was released
•	 the mass of the ball
In this activity you followed a scientific process:
•	 You thought about the problem.
•	 You developed a hypothesis.
Figure 7 .2 The gravitational potential 
energy of the driver head when it is 
lifted up is used to drive the pole into 
the ground.
apparatus
 ? three steel balls (from recycled 
ball bearings), of different sizes 
(bigger than 8 mm in diameter) 
and of known mass
 ? a metre rule
 ? a plank and strips of card, 
or recycled curtain rail about  
1 m long
 ? a few books
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   180 2015/12/17   10:02 AM
 technical science GRADE 10 181
•	 You tested the hypothesis in an experiment.
•	 You drew a conclusion.
When you follow a scientifi c process in an investigation, you are doing good science.
calculate gravitational potential energy.
The gravitational potential energy of an object is dependent on two variables:
•	 the mass of the object – the greater the mass, the greater its gravitational potential energy
•	 the height to which the object is raised – the higher the object is raised, the greater its 
gravitational potential energy
These two statements are combined in the formula for gravitational potential energy:
 gravitational potential energy = mass × acceleration of gravity × height
or 
 E
p
 = m g h  or U = m g h
where:
•	 E
p
 or U is the symbol for gravitational potential 
energy measured in Joules (J)
•	 m is the symbol for the mass of the object 
measured in kilograms (kg)
•	 g is 9,8 m/s
2
•	 h is the symbol for the height of the object above 
a reference posit ion measured in metres (m)
NOTE: The Joule (J) is the standard unit for the measurement of energy in the SI system.
Worked examples: calculate gravitational potential energy
1. An object that has a mass of 4 kg is held 2 m above the ground. Calculate its gravitational 
potential energy relative to the ground.
 Solution
 Given mass is 4 kg; height above ground is 2 m
 Unknown gravitational potential energy relative to the ground
 Formula E
p
 = m g h
  = 4 × 9,8 × 2   (substitute)
  = 78,4 J
E
P
 = m g h
Potential
energy (J)
Mass (kg)
Acceleration 
of gravity (m/s
2
)
Height (m)
Figure 7 .4
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   181 2015/12/17   10:02 AM
182 chapter 7 ENERGY
2. An object that has a mass of 0,5 kg is held at shoulder height (1,5 m) out of a third fl oor 
window. The height between each fl oor is 3,3 m. Calculate the object’s gravitational 
potential energy relative to the ground.
 Solution
 Given mass is 0,5 kg; shoulder height is 1,5 m; 3 fl oors of 3,3 m
 Unknown gravitational potential energy relative to the ground
 Formula E
p
 = m g h
   = 0,5 × 9,8 × (3 × 3,3 + 1,5)  (substitute)
   = 55,9 J
activity 2 calculate gravitational potential energy
1. An object that has a mass of 10 kg is held 0,5 m above a table. The table is 0,9 m high.
a) Calculate the object’s gravitational potential energy relative to the table.
b) What is the object’s gravitational potential energy relative to the fl oor?
c) Calculate the object’s gravitational potential energy relative to the fl oor when it is 
on the table.
2. A ball is thrown up into the air. It reaches a height of 12 m. Its mass is 200 g.
a) Calculate the ball’s gravitational potential energy as it reaches its highest point 
above the ground.
b) As the thrower leans back to throw the ball, his hand is 1 m above the ground. 
Calculate the ball’s gravitational potential energy when it is 1 m above the ground.
c) Calculate the energy the thrower gives to the ball.
d) What is the ball’s gravitational potential energy when it is on the ground?
3. One of the methods that a blacksmith uses to 
shape metal is drop-forging. You can see a drop 
forge in Figure 7.5. The red-hot metal in a mould 
is shaped when a heavy weight is dropped 
repeatedly onto the mould (also called a die).
a) What is the reasonable reference height 
(position of zero height) for the machine?
b) Calculate the potential energy of the 500 kg 
hammer when it is raised to its maximum of 
2 m above the mould.
c) A delicate mould needs a maximum of 4 000 J 
per hammer blow to shape a small object. 
Calculate the maximum height to which the 
500 kg hammer should be raised.
Figure 7 .5 A drop forge 
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   182 2015/12/17   10:02 AM
 technical science GRADE 10 183
d) A certain machine operator prefers to use a smaller hammer raised to the top of the 
machine rather than a heavy hammer raised only part of the way. Calculate the mass of 
the hammer he would use in (c).
4. One hundred and twenty five 100 mm long nails need to be hammered into a long piece of 
pine to fix it to a pine rafter. There is little room to work, so all the carpenter will be able to 
do is lift the hammer and drop it. The hammer he will use weighs 4 kg and he will be able 
to raise it to a maximum of 300 mm above the head of the nail. On average a 100 mm nail 
requires 160 J of energy to drive it into pine.
a) Calculate the minimum number of times the carpenter must hit each nail.
b) Calculate the total energy he will expend doing the job.
c) The carpenter is a big guy, so he wants to use a hammer that is twice as heavy.
•	 Guess the number of hammer blows he will have to give each nail.
•	 Prove your guess by doing a calculation.
d) Criticise this question.
Zero height position 
definition:  The zero height position is the reference position for a particular 
situation. It is the position at which the height is taken to be zero.
To be able to determine the gravitational potential energy of an object, a zero height 
position must be chosen.
•	 The ground surface is often taken as the position of zero height.
•	 In a classroom experiment involving gravitational potential energy, the desktop might be 
chosen as the zero height position.
Look at Figure 7.6a and Figure 7.6b. What zero height position would you choose for these two 
lifting devices?
Figure 7 .6a Mine headgear of an old mine Figure 7 .6b An old-time builder with a crane on a tower
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   183 2015/12/17   10:02 AM