Lever: 4 kg at 0.5 m, mass at 0.2 m

A 4 kg object hangs 0.5 m to the left of the fulcrum of a lever. What mass must hang 0.2 m to the right of the fulcrum to balance the lever? (Take g = 10 N/kg.)

Step-by-step

1. Weight on the left F₁ = mg = 4×10 = 40
   The object's weight provides the effort on the left
2. F₁·L₁ = F₂·L₂ → F₂ = F₁×L₁/L₂ = 40×0.5/0.2 = 100
   Lever balance: effort × effort arm = load × load arm
3. Equivalent mass = 100/10 = 10 kg
   Convert to the everyday "how heavy"

Why it works

A lever balances when the two moments are equal: F₁ × L₁ = F₂ × L₂. On the shorter arm a larger force is needed, so the required mass is larger than 4 kg.

Common mistakes

• Multiplying by 0.2 ÷ 0.5 instead of dividing by it.
• Forgetting to turn the 4 kg mass into a weight of 40 N first.

Related

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