Module 3: Forces and motion

Learners should be able to demonstrate and apply their knowledge and understanding of:

3.1 Motion

3.1.1 Kinematics

(a) displacement, instantaneous speed, average speed, velocity and acceleration
(b) graphical representations of displacement, speed, velocity and acceleration
(c) Displacement–time graphs; velocity is gradient
(d) Velocity–time graphs; acceleration is gradient; displacement is area under graph.

3.1.2 Linear motion

(a) (i) the equations of motion for constant acceleration in a straight line, including motion of bodies falling in a uniform gravitational field without air resistance
v = u + at
s = \frac{1}{2} (u + v)t
s = ut + \frac{1}{2}at^{2}
v^{2} = u^{2} + 2as

(ii) techniques and procedures used to investigate the motion and collisions of objects
(b) (i) acceleration g of free fall
(ii) techniques and procedures used to determine the acceleration of free fall using trapdoor and electromagnet arrangement or light gates and timer
(c) reaction time and thinking distance; braking distance and stopping distance for a vehicle.

3.1.3 Projectile motion

(a) independence of the vertical and horizontal motion of a projectile
(b) two-dimensional motion of a projectile with constant velocity in one direction and constant acceleration in a perpendicular direction.


3.2 Forces in action 

3.2.1 Dynamics

(a) \mbox{net force} = \mbox{mass} \times \mbox{acceleration}; F = ma
(b) the newton as the unit of force
(c) weight of an object; W = mg
(d) the terms tension, normal contact force, upthrust and friction
(e) free-body diagrams
(f) one- and two-dimensional motion under constant force.

3.2.2 Motion with non-uniform acceleration

(a) drag as the frictional force experienced by an object travelling through a fluid
(b) factors affecting drag for an object travelling through air
(c) motion of objects falling in a uniform gravitational field in the presence of drag
(d) (i) terminal velocity
(ii) techniques and procedures used to determine terminal velocity in fluids.

3.2.3 Equilibrium

(a) moment of force
(b) couple; torque of a couple
(c) the principle of moments
(d) centre of mass; centre of gravity; experimental determination of centre of gravity
(e) equilibrium of an object under the action of forces and torques
(f) condition for equilibrium of three coplanar forces; triangle of forces

3.2.4 Density and pressure

(a) density; \rho = \frac{m}{V}
(b) pressure; p = \frac{F}{A} for solids, liquids and gases
(c) p = h \rho g; upthrust on an object in a fluid; Archimedes’ principle.


3.3 Work, energy and power

3.3.1 Work and conservation of energy

(a) work done by a force; the unit joule
(b) W = Fx \cos{i} for work done by a force
(c) the principle of conservation of energy
(d) energy in different forms; transfer and conservation
(e) transfer of energy is equal to work done.

3.3.2 Kinetic energy and potential energies

(a) kinetic energy of an object; E_{k} = \frac{1}{2}mv^{2}
(b) gravitational potential energy of an object in a uniform gravitational field; E_{p} = mgh
(c) the exchange between gravitational potential energy and kinetic energy.

3.3.3 Power

(a) power; the unit watt; P = \frac{W}{t}
(b) P = Fv
(c) efficiency of a mechanical system; \mbox{efficiency} = \frac{\mbox{useful output energy}}{\mbox{total input energy}} \times 100%


3.4 Materials

3.4.1 Springs

(a) tensile and compressive deformation; extension and compression
(b) Hooke’s law
(c) force constant k of a spring or wire; F = kx
(d) (i) force–extension (or compression) graphs for springs and wires
(ii) techniques and procedures used to investigate force–extension characteristics for arrangements which may include springs, rubber bands, polythene strips.

3.4.2 Mechanical properties of matter

(a) force–extension (or compression) graph; work done is area under graph
(b) elastic potential energy; E = \frac{1}{2}Fx; E = \frac{1}{2}kx^{2}
(c) stress, strain and ultimate tensile strength
(d) (i) \mbox{Young modulus} = \frac{\mbox{tensile stress}}{\mbox{tensile strain}}, E = \frac{\sigma}{\epsilon}
(ii) techniques and procedures used to determine the Young modulus for a metal
(e) stress–strain graphs for typical ductile, brittle and polymeric materials
(f) elastic and plastic deformations of materials.


3.5 Newton’s laws of motion and momentum

3.5.1 Newton’s laws of motion

(a) Newton’s three laws of motion
(b) linear momentum; p = mv; vector nature of momentum
(c) net force = rate of change of momentum; F = \frac{\Delta p}{\Delta t}
(d) impulse of a force; \mbox{impulse} = F \Delta t
(e) impulse is equal to the area under a force–time graph.

3.5.2 Collisions

(a) the principle of conservation of momentum
(b) collisions and interaction of bodies in one dimension and in two dimensions
(c) perfectly elastic collision and inelastic collision.