The Turning Trick

Why it matters

A mechanic who can't open a bolt doesn't push harder — she reaches for a LONGER spanner. She's not stronger. She's multiplying.

The idea in plain words

The turning power of a force. Tap each term.

τ = F × d

A 60 cm steering wheel is turned with 15 N on its rim. Moment about the centre?

τ = 4.5 N·m

• N·m: τ — moment of the force (torque) — its turning effect about the axis
• newton (N): F — the applied force
• metre (m): d — PERPENDICULAR distance from the axis to the force's line of action
• convention: sign — anticlockwise moments count positive, clockwise negative (or vice versa — pick and stick)
• d = radius = 0.3 m
• τ = F × d = 15 × 0.3

Predict first

A bolt needs 20 N·m to open. With a 0.5 m spanner, the force needed is…

τ = F × d → F = 20 ÷ 0.5 = 40 N. Double the arm, halve the force — distance is strength you don't have to supply.

• 40 N — half the arm of a 0.25 m spanner would need 80 N — correct
• 20 N — the bolt decides the force
• 10 N

What you do

Open the 20 N·m bolt twice: once by brute force on a stubby arm, once the smart way — small force, long arm.

Check yourself

The S.I. unit of moment of force is…

Numerically like the joule but NOT energy — torque is force × perpendicular distance, a turning quantity.

Why are door handles fixed at the edge farthest from the hinge?

τ = F × d. Near the hinge d ≈ 0 — try it once and your arm will file a complaint.

A force's moment is zero when…

Through the axis, d = 0 → τ = 0, however huge the force — exactly your slide-vs-spin discovery.

• N·m (newton metre) — correct
• N/m
• joule
• maximum d gives maximum moment for the least force — correct
• it looks better
• the door is strongest there
• its line of action passes through the axis — correct
• the force is applied slowly
• the body is heavy