A2L Item 125

• Description: Identify the force on one block pushed by another.
• Goal: Problem solving with dynamics
• Source: UMPERG-ctqpe21
• Keywords: Dynamics, Forces, Mechanics, Newton's Third Law

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The question for students:

Two blocks rest on a frictionless surface. Both blocks move to the right with acceleration of 2 m/s². The force on the big block due to the small block is

1. 14N to the right
2. 10N to the left
3. 8N to the right
4. 6N to the left
5. 4N to the right
6. 2N to the left
7. none of the above
8. cannot be determined

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Commentary for teachers:

Answer

(7) The force on the small block must cause the specified acceleration. The 3rd law requires that the force on the big block be equal and opposite. The magnitude is 4N but it is directed to the left.

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