Two Piggy Banks

Why it matters

A stretched bow is silent, still — and dangerous. A flying arrow is the same danger, moved from one piggy bank to the other.

The idea in plain words

The two accounts. Tap them.

PE = mgh · KE = ½mv²

A 2 kg brick on a 5 m ledge (g = 10). Its PE?

100 J, waiting

• the waiting bank: potential — stored by position or state: raised water, stretched rubber, wound spring
• the moving bank: kinetic — carried by motion: wind, flowing river, a struck cricket ball
• the deposits: m, h, v — heavier, higher, faster — each grows its bank (speed counts double-double: v²)
• PE = mgh = 2 × 10 × 5

Predict first

Energy stored in a raised hammer is…

Raised = stored by position (gravitational PE). The moment it swings down, the account transfers to motion (KE).

• potential energy — stored by its position — correct
• kinetic energy
• electrical energy

What you do

Fill both banks: lift the block for potential, roll the ball for kinetic — then make the two accounts exactly equal.

Check yourself

Which stores POTENTIAL energy?

Wound, stretched, raised, compressed = stored. Rolling and blowing = kinetic.

Double a body's speed and its kinetic energy becomes…

KE goes as v² — the square is why speeding cars are disproportionately dangerous.

Water stored high in a dam has…

The reservoir is a giant PE bank; the penstock is the withdrawal slip.

• a wound-up toy spring — correct
• a rolling marble
• the wind
• four times — correct
• double
• half
• potential energy that becomes kinetic as it falls — correct
• kinetic energy while stored
• no energy until electrified