A2L Item 126

• Description: Find the mass of the bottom block in a stack.
• Goal: Problem solving
• Source: UMPERG-ctqpe23
• Keywords: Forces, Mechanics, Newton's Third Law

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The question for students:

If m₂ = 2m₁ , m₃ = 3m₁ and the force that the surface exerts on the bottom block is 120N, the mass of the large block is

Use g = 10 N/kg.

1. 2 kg
2. 4 kg
3. 6 kg
4. 8 kg
5. 12 kg
6. 20 kg
7. none of the above

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Commentary for teachers:

Answer

(3) the most common other answers are #1 and #6, each of which represents a typical mistake students make. This problem presents a good opportunity to encourage students to check their answer.

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