#21.1
Understand the language relating to forces.
Weight, tension, thrust or compression, normal reaction (or normal contact force), frictional force, resistance, driving force.
Understand that the value of the normal reaction depends on the other forces acting.
Understand that there may be frictional force when the surface is not smooth (i.e. is rough).
#21.2
Know that the acceleration due to gravity is not a universal constant but depends on location in the universe. Know that on earth, the acceleration due to gravity is often modelled to be a constant, \(g~m~s^{-2}\).
\(g ≈ 10\), \(g ≈ 9.8\)
Unless otherwise specified, in examinations the value of \(g\) should be taken to be \(9.8\).
Notation: Acceleration due to gravity, \(g~m~s^{-2}\).
[Excludes: Inverse square law for gravitation.]
#21.3
Be able to identify the forces acting on a system and represent them in a force diagram. Understand the difference between external and internal forces and be able to identify the forces acting on part of the system.
#21.4
Be able to find the resultant of several concurrent forces when the forces are parallel or in two perpendicular directions or in simple cases of forces given as 2-D vectors in component form.
#21.5
Understand the concept of equilibrium and know that a particle is in equilibrium if and only if the vector sum of the forces acting on it is zero in the cases where the forces are parallel or in two perpendicular directions or in simple cases of forces given as 2-D vectors in component form.
#21.6
Be able to resolve a force into components and be able to select suitable directions for resolution. Be able to find the resultant of several concurrent forces by resolving and adding components.
e.g. Horizontally and vertically, or parallel and perpendicular to an inclined plane.
#21.7
Know that a particle is in equilibrium if and only if the resultant of the forces acting on it is zero. Know that a body is in equilibrium under a set of concurrent forces if and only if their resultant is zero.
#21.8
Know that vectors representing a set of forces in equilibrium sum to zero. Know that a closed figure may be drawn to represent the addition of the forces on an object in equilibrium.
#21.9
Be able to formulate and solve equations for a particle in equilibrium: by resolving forces in suitable directions; by drawing and using a polygon of forces.
For example, a triangle of forces.
[Excludes: Non-coplanar forces.]
#21.10
Understand that the overall contact force between surfaces may be expressed in terms of a frictional force and a normal contact force and be able to draw an appropriate force diagram.
Understand that the normal contact force cannot be negative.
Understand the following modelling assumptions.
• Smooth is used to mean that friction may be ignored.
• Rough indicates that friction must be taken into account.
Notation: Normal reaction.
#21.11
Understand that the frictional force may be modelled by \(F \le \mu R\) and that friction acts in the direction to oppose sliding.
Model friction using \(F = \mu R\) when sliding occurs.
Notation:
Coefficient of friction \(= \mu\)
Limiting friction,
Static equilibrium
The term angle of friction.