WAEC Physics 2022 Theory — Question 8

Model answer

(a)(i) The law of inertia states that a body continues in its state of rest or uniform linear motion unless acted upon by an external force to act otherwise. (ii) How a safety belt works using the law of inertia: The safety belt acts as an external force which prevents a passenger from an otherwise forward motion after impact or collision (the passenger's body, by inertia, would continue moving forward when the vehicle suddenly stops, and the belt provides the restraining force). (b)(i) Moment of force is defined as the product of a force and perpendicular distance from a reference point or pivot to the line of action of the force. (ii) I. [Force diagram: plank AB, 2m long, with W=54N acting at midpoint (1m from A), knife edge (reaction R) at 0.5m from B (i.e. 1.5m from A), and tension T acting vertically upward at A.] II. Taking moments about the knife edge: T×1.5 = 54×0.5 (sum of clockwise moment = sum of anticlockwise moment about the pivot)... [T=18N per similar worked example] III. At equilibrium, ΣF=0: T+R=54, so R=54−T=54−18=36N. (c) Differences between solid friction and viscosity: Solid friction | Viscosity - Occurs on solid surfaces | Occurs in fluids (liquid and gas) - Independent of area of surfaces in contact | Depends on area of the surface in contact - Depends on the nature of the surfaces in contact | Depends on the speed of motion between the layers of the fluid - Independent of temperature | Depends on temperature (d) Method of increasing the velocity ratio of a pulley system: By increasing the number of pulleys used in the system.