GRADE 12: MECHANICS
Compiled by T. Doyle
Acknowledgements:
C. Fairman; S. Murray
and
IEB SAGs
, 2
Background…
Mechanics is the branch of physics dealing with the behaviour of bodies (objects) when forces
are exerted on them.
Kinematics is the branch of mechanics dealin...
Acknowledgements:
C. Fairman; S. Murray
and
IEB SAGs
, 2
Background…
Mechanics is the branch of physics dealing with the behaviour of bodies (objects) when forces
are exerted on them.
Kinematics is the branch of mechanics dealing with changes in motion without reference to the
forces causing the changes.
Dynamics is the branch of mechanics dealing with the relationships between the changes in
motion of bodies and the forces which cause them.
1. SCALAR & VECTOR QUANTITIES
Definition: A scalar quantity is a physical quantity that has magnitude only.
Examples of scalar quantities are:
Quantity Symbol Unit
Distance distance m
Time t s
Mass m kg
Speed v m.s1
Energy E J
Work W J
Power P W
Definition: A vector is a physical quantity that has both magnitude and direction.
Examples of vector quantities are:
Quantity Symbol Unit
Displacement x m
Velocity v m.s1
Acceleration a m.s2
Force F N
Weight Fg N
Momentum p kg.m.s1
Impulse F. t N.s
Definition: A resultant vector is the single vector which has the same effect as the original
vectors acting together
, 3
2. NAMES AND SYMBOLS OF SOME SPECIAL FORCES
Whenever describing a force it is useful to use the prepositions “by” and “on”.
The names and symbols of some commonly encountered forces are explained below:
i. The downward gravitational force on a body is called the weight (Fg) of the body.
ii. For a body on a solid horizontal surface, the upward force on the body, which is
perpendicular to the horizontal surface, is called the normal (FN).
iii. For a floating body, the upward force on the body is called the upthrust (Fupthrust).
iv. A surface which offers no resistance to a body moving on it is called a smooth surface. A
surface which resists the motion of a body on it is called a rough surface. The resistance
offered by a rough surface on a body is called a friction force (Ff). A friction force acts in
a direction opposite to that of the motion of a body. The friction force created when an
object moves through air or water may be known as drag.
v. A string, or cable, etc. which is pulled taut, exerts a force known as tension (T). (An equal
but opposite tension is exerted on two bodies simultaneously, one at each end of the taut
string, or cable, etc.)
vi. A net force (Fnet) is a force which is not balanced by other forces acting on a body
(object). It may be thought of as the “left-over” force on a body after all the forces acting
on the body have been taken into account. It can also be called the unbalanced force or
the resultant force. A net force acting on a body always causes the body to accelerate.
3. FORCE-DIAGRAMS & FREE-BODY DIAGRAMS
In a “free-body diagram” a dot is used to represent the body and all the forces acting on the body
are shown with labelled arrows pointing outwards from the dot.
A labelled diagram showing the body and the directions of all the forces acting on a body is
called a force-diagram. A force-diagram is like a free-body diagram but without the restrictions
that the forces must all point outwards and that the body must be represented with a dot.
Eg 1: A book rests on a table
FN
FN Fg
Fg
Eg 2: A buoy floats on the sea
buoy
Fupthrust
Fupthrust Fg
Fg
, 4
Eg 3: A flower basket hangs from a garden lamp post
Fmetal bar
Fmetal bar
Fstrut Fstrut
Fg
Fg
Eg 4: A jet flying through the air
Flift Fpropulsion
Fdrag Fpropulsion Fdrag
Flift Fg
Fg
Eg 5: A car drives up a hill
Fengine
Fengine FN
Ff
Ff
Fg FN
Fg
Eg 6: A crate is lifted using a pulley system
T
FRAGILE
Fg
T Fg
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