SPM Physics: AHA! Free Fall – Signs Matter More Than Instinct!

 

In free fall problems, many students rely on instinct: “ball goes up → acceleration must be positive.” That’s wrong. Physics follows sign conventions, not feelings.

KSSM Sign Convention:

• Upward = Positive (+)

• Downward = Negative (–)

• Gravity always acts downward →
  $a=-g=-9.81m\mathrm{/}{s}^2$

Three situations:

1️⃣ Thrown upward →

$v=+ve$

,

$a=-9.81$

2️⃣ At highest point →

$v=0$

,

$a=-9.81$

3️⃣ Falling downward →

$v=-ve$

,

$a=-9.81$

Formula reminders:

• $v=u-gt$

• $s=ut-\frac{1}{2}g{t}^2$

• $v^2=u^2-2gs$

Exam traps:

• At the highest point, velocity = 0 but acceleration ≠ 0.

• Always choose the positive direction first, then stay consistent throughout.

AHA! Same g, different motion, same acceleration. Not by sight, but by INSIGHT!

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