A2L Item 079
- Description: Determine the velocity of a car from information about its displacement.
- Goal: Distinguish average velocity from velocity.
- Source: UMPERG
- Keywords: Displacement, Mechanics, Velocity
The question for students:
A car is initially located at the 109 mile marker on a long straight highway. Two and one half minutes later the car is located at the 111 mile marker.
What is the velocity of the car?
- 24 mph
- 32 mph
- 40 mph
- 48 mph
- 55 mph
- 64 mph
- Cannot be determined
Commentary for teachers:
Answer
The correct answer is (7) because only the average velocity can be determined. However, students who respond (4) should not be disconfirmed but prodded to be more discriminating when interpreting questions. They have assumed that the car is traveling with a uniform speed.
Background
Students should be able to extract kinematical quantities from everyday situations. They should also have a sense of the size of these quantities.
Questions to Reveal Student Reasoning
What is the speed of the car when it is at the 109 mile marker? How do you know?
Is it possible for the car to be at rest initially and reach the 111 mile marker two and one half minutes later? If it had constant acceleration, what would its speed be when it reached the 111 mile marker?
Suggestions
Have students make a sketch of position vs. time. They probably assume that the speed is uniform throughout the time interval. Have them consider other paths that still connect the two known points on the position vs. time plot. Draw some reasonable path and have the students describe what the car is doing during that interval.