A2L Item 074

Goal: Problem solving using momentum conservation.

Source: UMPERG-ctqpe90

On an icy road, an automobile traveling east with speed 50 mph collides
head-on with a sports car of half the mass traveling west with speed 60
mph. If the vehicles remain locked together, the final speed is:

  1. 10 mph, West
  2. 20 mph, West
  3. 30 mph, West
  4. 10 mph, East
  5. 20 mph, East
  6. 30 mph, East
  7. The vehicles remain stationary.
  8. None of the above
  9. Cannot be determined



(8) None of the above. This is a straightforward totally inelastic
collision situation.


Students are frequently bothered by the idea of a totally or perfectly
inelastic collision. They are inclined to think of inelasticity as
imperfection, so the idea of perfect imperfection is distressing.
Consequently the scale shifts and they label collisions when objects
stick together as inelastic, the general collision as elastic, and
collisions conserving kinetic energy as perfectly elastic.

Questions to Reveal Student Reasoning

How fast would the car have to be traveling for the combined vehicles to
remain at rest after the collision?

If the collision was elastic, in which direction would the sports car
travel after the collision?


By relating the general collision problem to that of two masses
colliding with a spring between them, it is possible to get students to
realize that all two body collisions pass through the state with both
objects traveling with the CM velocity. This helps unify the concepts
of elastic, inelastic and perfectly inelastic collisions.