A2L Item 073

• Description: Specify the impulse needed to reverse an objects direction of motion.
• Goal: Hone the vector nature of impulse and contrast impulse to kinetic energy.
• Source: UMPERG-ctqpe82
• Keywords: Impulse, Mechanics, Momentum

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The question for students:

A block having mass M travels along a horizontal frictionless surface with speed v_x. What impulse must be delivered to the mass to reverse its direction?

1. -mv_x
2. -2mv_x
3. 0
4. 2mv_x
5. mv_x
6. None of the above
7. Cannot be determined

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Commentary for teachers:

Answer

(1) or (2) or (6) are all defensible answers depending upon how students interpret the question. This is a good question for stressing to students that it is their reasoning not their answer that is important.

Background

Impulse is a vector. Using the impulse-momentum relation, the change in momentum must be at least mv_x in the direction opposite to the motion to reverse direction.

Questions to Reveal Student Reasoning

What impulse would be needed to make the mass travel parallel to the y-axis?

Suppose a constant force F_x acts for 4 seconds causing the mass to stop. What force would be needed to stop the mass in 2 seconds.

Suggestions

Have students make a concept map showing the relationships among the quantities mass, velocity, momentum, impulse, time, force, and average force.

Does it bother students that, in 10 seconds gravitation provides an impulse of 10mg to a book whether it is dropped or sitting on a table?

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