Forces

A moving object needs no net forward force to keep a steady speed. By Newton's first law, constant velocity means zero resultant force: driving force and resistive forces (friction + drag) are equal in magnitude and opposite. An object falling at constant (terminal) speed is the same case -- weight equals the total drag. Only a change of motion needs one.

Forces that are equal in magnitude cancel only when they act in opposite directions along the same line. Equal forces acting in different directions still leave a resultant, so the motion changes. Check the directions before concluding that forces balance: "they are the same size, so nothing happens" is true only for a directly opposing pair.

To get the acceleration from $F = ma$ you divide: $a = F/m$. Candidates multiply ($F \times m$), or substitute a single force instead of the resultant. Find the resultant force along the line of motion first, then divide by the mass. The mass must be in kilograms -- convert a weight in newtons to a mass before substituting.

Lowering the centre of gravity does not mean it "moves to the ground". It means the object is more stable: the vertical line through the centre of gravity is less likely to fall outside the base, so the object is harder to topple. Explain stability by the position of the centre of gravity and the width of the base, never by weight alone.

Reserve proportional for the straight part through the origin of a load-extension graph, up to the limit of proportionality. Past that point the line curves -- equal increases in load give larger increases in extension -- and $F = kx$ no longer holds, so the spring constant cannot be read from that region.

The moment is $F \times d$ -- multiply. Candidates divide, and they use the wrong distance: $d$ must be the perpendicular distance from the pivot to the line of action, not the length of a handle held at an angle and not the slanted (hypotenuse) distance. Give the unit as N m.

The spring constant $k = F/x$ uses the extension $x$ -- the increase in length beyond the natural length -- not the total stretched length. Read the extension as the change from the origin, and do not invert the formula: $x/F$ is not the spring constant. The unit of $k$ is N/m.