GRADE 9 FORCE, WORK, ENERGY & POWER
IMPORTANT DEFINITIONS:
• Resultant (Net) force: The single force that will have the same effect as the other forces acting
together.
• Frictional force (Ff): The force that opposes the motion of an object.
• Normal force (FN): The perpendicular force exerted by a surface on an object in contact with it.
• Newton's first law: An object continues in a state of rest or uniform (moving with constant)
velocity unless it is acted upon by a net or resultant force.
• Inertia: The property of an object that causes it to resist a change in its state of rest or motion.
• Newton's second law: When a net force is applied to an object of mass, m, it accelerates in the
direction of the net force. The acceleration, a, is directly proportional to the net force and
inversely proportional to the mass.
• Newton's third law: When object A exerts a force on object B, object B simultaneously exerts
an oppositely directed force of equal magnitude on object A.
• Weight: The gravitational force the Earth exerts on any object on or near its surface.
• Work: Work done on an object by a force is the product of the displacement and the force
parallel to the displacement
• Gravitational potential energy: The energy an object possesses due to its position relative to a
reference point
• Kinetic energy: The energy an object has as a result of the object's motion •
Mechanical energy: The sum of gravitational potential and kinetic energy at a point.
• Principle of conservation of mechanical energy: In the absence of air resistance or any external
forces, the mechanical energy of an object is constant.
• Power: The rate at which work is done or the rate at which energy is transferred
Mechanics is about everything that is
happening to things – how heavy they
are, what is pushing and pulling them,
, how they move and what they can do.
This topic is all about mechanics.
1
Unit 1: Force
1.1 Introduction to Force
What is a force? A force is a push or pull upon an object.
This happens due to an object's interaction with another object.
Forces can be considered to be contact forces or non-contact forces.
What is the difference?
Contact forces Non-contact forces
The forces which act on bodies when they The forces experienced by bodies even
are in physical contact. without physically touched.
Types of contact forces: applied force, Types of non-contact forces: gravitational
frictional force, normal force, tension force force, magnetic force, electrostatic force
etc. etc.
There are many different ways in which a force can affect an object.
A force can cause –
a. distortion
b. change in motion
c. change in direction
, 2
Types of forces
We have already described forces as contact or non-contact forces. The table below supplies a
detailed description of the different types of forces as well as the symbols representing them.
Different types of forces
Force Symbol Description
Applied force FA The force applied on an object by another object.
Gravitational force Fg The force with which the Earth attracts an
object on or near its surface. This force is also
called the weight of the object.
On earth: Fg = mg where g = 9,8 m∙s–2.
Normal force FN The perpendicular force exerted by a surface
on an object it is in contact with.
Frictional force Ff The force that opposes the motion of an object.
A frictional force always works in the opposite
direction to the movement of the object. It is
parallel to the motion.
Tension force FT The force exerted by a rope or wire when it is
tightened by forces on both sides. The
tensional force pulls equally strongly on both
objects.
Air resistance Fair A frictional force experienced by objects
moving through air. This is a type of frictional
force.
Free-Body Diagrams
• A free-body diagram is a sketch of an object showing all the forces acting on it. •
The object (which is pushed, pulled, lifted or squashed) is represented by a dot. •
The forces are represented by arrows (vectors) that all point away from the dot
Examples:
a) A man is holding a 100kg mass above his head. Draw a free-body diagram of the forces on the
bar.
Picture of scenario Free Body Diagram
IMPORTANT DEFINITIONS:
• Resultant (Net) force: The single force that will have the same effect as the other forces acting
together.
• Frictional force (Ff): The force that opposes the motion of an object.
• Normal force (FN): The perpendicular force exerted by a surface on an object in contact with it.
• Newton's first law: An object continues in a state of rest or uniform (moving with constant)
velocity unless it is acted upon by a net or resultant force.
• Inertia: The property of an object that causes it to resist a change in its state of rest or motion.
• Newton's second law: When a net force is applied to an object of mass, m, it accelerates in the
direction of the net force. The acceleration, a, is directly proportional to the net force and
inversely proportional to the mass.
• Newton's third law: When object A exerts a force on object B, object B simultaneously exerts
an oppositely directed force of equal magnitude on object A.
• Weight: The gravitational force the Earth exerts on any object on or near its surface.
• Work: Work done on an object by a force is the product of the displacement and the force
parallel to the displacement
• Gravitational potential energy: The energy an object possesses due to its position relative to a
reference point
• Kinetic energy: The energy an object has as a result of the object's motion •
Mechanical energy: The sum of gravitational potential and kinetic energy at a point.
• Principle of conservation of mechanical energy: In the absence of air resistance or any external
forces, the mechanical energy of an object is constant.
• Power: The rate at which work is done or the rate at which energy is transferred
Mechanics is about everything that is
happening to things – how heavy they
are, what is pushing and pulling them,
, how they move and what they can do.
This topic is all about mechanics.
1
Unit 1: Force
1.1 Introduction to Force
What is a force? A force is a push or pull upon an object.
This happens due to an object's interaction with another object.
Forces can be considered to be contact forces or non-contact forces.
What is the difference?
Contact forces Non-contact forces
The forces which act on bodies when they The forces experienced by bodies even
are in physical contact. without physically touched.
Types of contact forces: applied force, Types of non-contact forces: gravitational
frictional force, normal force, tension force force, magnetic force, electrostatic force
etc. etc.
There are many different ways in which a force can affect an object.
A force can cause –
a. distortion
b. change in motion
c. change in direction
, 2
Types of forces
We have already described forces as contact or non-contact forces. The table below supplies a
detailed description of the different types of forces as well as the symbols representing them.
Different types of forces
Force Symbol Description
Applied force FA The force applied on an object by another object.
Gravitational force Fg The force with which the Earth attracts an
object on or near its surface. This force is also
called the weight of the object.
On earth: Fg = mg where g = 9,8 m∙s–2.
Normal force FN The perpendicular force exerted by a surface
on an object it is in contact with.
Frictional force Ff The force that opposes the motion of an object.
A frictional force always works in the opposite
direction to the movement of the object. It is
parallel to the motion.
Tension force FT The force exerted by a rope or wire when it is
tightened by forces on both sides. The
tensional force pulls equally strongly on both
objects.
Air resistance Fair A frictional force experienced by objects
moving through air. This is a type of frictional
force.
Free-Body Diagrams
• A free-body diagram is a sketch of an object showing all the forces acting on it. •
The object (which is pushed, pulled, lifted or squashed) is represented by a dot. •
The forces are represented by arrows (vectors) that all point away from the dot
Examples:
a) A man is holding a 100kg mass above his head. Draw a free-body diagram of the forces on the
bar.
Picture of scenario Free Body Diagram
