Why is fiberglass preferred over metal for making high jump bars?

Correct — B. The crossbar rests loosely on its supports and is meant to fall when a jumper hits it, so it must be light and must not injure the athlete.

World Athletics excludes metal for the bar and caps its weight at 2 kg. Fibreglass, the usual material, is light, flexes under a blow and is safer on impact → option (b).

Why the others are wrong

• (a)
  Metal is too flexible for accurate performance — Too flexible turns the comparison around. Flexibility is fibreglass's advantage in the keyed option, and metal is the harder, less forgiving material when a jumper strikes the bar.
• (c)
  Metal bends permanently; fibreglass is rigid — Fibreglass is rigid is the false half. The bar's value lies in flexing under a blow, and a rigid bar would lose the give that makes it safer to hit.
• (d)
  Fibreglass is cheaper and easier to shape — Cheaper and easier to shape answers a manufacturing question the stem does not ask. What decides the material is how the bar behaves when a jumper hits it.

In the high jump the athlete clears a horizontal crossbar resting on supports between two uprights. A clearance counts only if the bar stays up, so the bar must fall readily when touched and must not injure anyone who hits or lands on it.

World Athletics fixes its specification: about 4 m long, circular in cross-section except for its end pieces, at most 2 kg, and not made of metal.

Key facts

• World Athletics does not allow a metal high-jump crossbar, and fibreglass is the usual material.
• The crossbar is about 4 m long and weighs at most 2 kg.
• Its circular part is 30 mm ± 1 mm in diameter.

Common traps

• Choosing cost because it sounds practical. The question asks why the material suits the bar, and that is about behaviour on impact.
• Swapping the properties. Fibreglass is the flexible one, so a statement calling it rigid is false.

No directly related past PYQ was found.