An effort P applied at one end of a crowbar just overcomes the resistance W at the lid of a tin. The mechanical advantage of the crowbar is expressed as

Analyze the question: A crowbar is a lever. The effort P is the force you apply, and the resistance (load) W is the force the lid puts up against you. We need the expression for its mechanical advantage.

Concept: Mechanical advantage (M.A.) is the ratio of the load to the effort:

It has no units because it is a ratio of two forces. It tells you how many times the machine multiplies your effort.

Solution:

• Load = resistance at the lid = W
• Effort = force applied = P

For a lever, this also equals the ratio of the effort arm to the load arm (taking moments about the fulcrum, P × effort arm = W × load arm). A long effort arm gives a large M.A., which is why a long crowbar makes it easier to open a tin.

Checking the other options:

• A and B (W + P, P − W): mechanical advantage is a ratio, not a sum or a difference.
• D (P/W): this is the inverse of M.A.
• E (wp): a product has no meaning here.

Answer: C

Tip: Remember "M.A. = Load over Effort", or "Load on top" (M.A. = L/E). Velocity ratio is the distance ratio (effort distance ÷ load distance), and efficiency = M.A. ÷ V.R. × 100%. These three ideas appear often in JAMB, WASSCE and NECO.