# A2L Item 073

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 vx. What impulse must be delivered to the mass
to reverse its direction?

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

### Commentary:

(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 mvx 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 Fx 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?