**Goal:** Hone the vector nature of impulse and contrast impulse to kinetic energy.

**Source:** UMPERG-ctqpe82

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?

- -mv
_{x} - -2mv
_{x} - 0
- 2mv
_{x} - mv
_{x} - None of the above
- Cannot be determined

## Commentary:

## 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 tothe 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 themass 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?