# Many students think that an object's acceleration is always in the direction in which the object is moving

Forces and Motion

Misconception

Some studies have found that students commonly think that the direction of an object's acceleration will always be in the same direction as that object's velocity. Students may, for instance, struggle to grasp that a car coming to a stop at a red light will have an acceleration that is opposite in direction to its velocity.

### Diagnostic Resources

• Acceleration and Deceleration

This resource introduces the idea of acceleration in directions that oppose velocity.

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• Acceleration Changes Velocity

This resource helps explain the relationship between velocity and acceleration more clearly for students.

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• Non-uniform acceleration with a ticker-timer

The experiment shows that acceleration is not always constant and can itself have a rate of change. The rate of change of distance (or, more strictly, 'displacement') is called velocity. The rate of change of velocity is called acceleration. The rate of change of acceleration has no name so it's hard to get hold of the concept.

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## References

The following studies have documented this misconception.

• Trowbridge, D. E. and McDermott, L. C. () Investigation of student understanding of the concept of acceleration in one dimension. American Journal of Physics, 49 (3), 242-253.

US-based researchers investigated 19-year-olds ability to apply the concept of acceleration when interpreting the motions of real objects. They found that students struggled to grasp acceleration in the time usually allotted, suggesting the need to dedicate more time to basic kinematic concepts at the introductory level, even at the cost of advanced topics.

Paper digest

• Saltiel, E. and Malgrange, J. L. () 'Spontaneous' ways of reasoning in elementary kinematics. European Journal of Physics, 1 (2), 73-80.

This study aimed to investigate spontaneous reasoning in elementary motion among 80 eleven-year-old children and 700 university students (first and fourth years). It revealed common correct and incorrect ideas shared by both groups. These results may not solely stem from school learning but align with the "natural model" proposed by the authors, distinct from physicists' kinematic model. It comprises two interacting components: descriptive motion and causal motion explanations.