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Formative assessment questions for teaching physics

A2L Item 030

  • Description: Order cases by increasing tension in string attached to hanging block.
  • Goal: Reasoning with 2nd law and honing of the concept of tension.
  • Source: UMPERG
  • Keywords: Mechanics, Forces, Dynamics, Tension

The question for students:

Consider the three cases presented below. Assume the friction force between the table and block in situations (B) and (C) can be ignored.

Item030_fig1.gif

Which of the following statements about the tensions in the strings is true?

  1. TA = TB = TC
  2. TB = TC < TA
  3. TA < TB < TC
  4. TB < TC < TA
  5. TA = TC < TB
  6. None of the above
  7. The relationship of the tensions cannot be determined

Commentary for teachers:

Answer

(4) By applying Newton’s second law to the hanging block one obtains a relationship between the tension in the string and the acceleration of the hanging block: The larger the acceleration the smaller the tension force. The acceleration is determined by the total mass of the system.

Background

This is a good problem for challenging students to reason without resorting to writing down a lot of equations. As one of the procedure forces (tension, normal, static friction), the value of tension requires application of the 2nd law.

Questions to Reveal Student Reasoning

What is the tension in situation (A)? Explain. Is the tension equal to the weight in situation (B)? (If some students think so explore what the net force is on the hanging mass, which will lead to a net force of O, and a contradiction since this implies O acceleration.) Of systems (A) and (B), which has the larger acceleration?

Suggestions

Ask students to consider limiting cases. What if the string was not attached to a block on the table (or if the block had almost no mass)? What would happen if the block on the table had a very large mass ?