Physics 9702/11 — May/June 2013

Background Concept

A scalar quantity has magnitude only (e.g. distance, time, energy, power).

A vector quantity has magnitude and direction (e.g. displacement, velocity, acceleration, force, momentum).

A quick test is: if changing direction changes the meaning of the quantity (even if the size is the same), it is a vector.

Understanding the Question

You are given four pairs of physical quantities. You must choose the pair that contains exactly one vector and exactly one scalar.

Approach

For each option:

1. Decide whether each quantity is a vector or scalar.
2. Reject pairs that are both vectors or both scalars.
3. Pick the only pair with one of each.

Step-by-Step Reasoning

• A displacement (vector) and force (vector) → two vectors → not correct.
• B kinetic energy (scalar) and power (scalar) → two scalars → not correct.
• C acceleration (vector) and momentum (vector) → two vectors → not correct.
• D velocity (vector) and distance (scalar) → one vector + one scalar → correct.

Key Takeaways

• Learn a core list: displacement/velocity/acceleration/force/momentum are vectors; distance/speed/energy/power are scalars.
• The defining feature of a vector is the need for a direction.

Common Mistakes

• Confusing distance (scalar) with displacement (vector).
• Confusing speed (scalar) with velocity (vector).
• Thinking energy or power are vectors because they involve motion; they are still scalars.

Things to Be Careful About

• Many quantities come in scalar/vector pairs with similar names (distance vs displacement, speed vs velocity): read the words precisely.
• For MCQs, check all four options: three here are “both scalar” or “both vector”, only one has a mix.