Physics 9702/13 — October/November 2011

Background Concept

A vector quantity has both magnitude and direction (e.g. velocity: (10\ \text{m s}^{-1}) east). A scalar quantity has magnitude only (e.g. speed: (10\ \text{m s}^{-1})).

Common examples:

• Vectors: displacement, velocity, acceleration, force, electric field strength.
• Scalars: speed, distance, mass, time, work, energy, power.

Understanding the Question

You are given four groups of three quantities. The task is to choose the single group that contains only vectors, meaning every quantity in that group must have a direction as well as a size.

Approach

Go option by option and check whether any listed quantity is a scalar. If an option contains even one scalar, it cannot be correct. The remaining option (if any) with all vectors is the answer.

Step-by-Step Reasoning

• Option A: acceleration (vector), displacement (vector), speed (scalar) (\rightarrow) not all vectors.
• Option B: acceleration (vector), work (scalar), electric field strength (vector) (\rightarrow) not all vectors.
• Option C: displacement (vector), force (vector), velocity (vector) (\rightarrow) all vectors.
• Option D: power (scalar), electric field strength (vector), force (vector) (\rightarrow) not all vectors.

So the only option containing only vectors is C.

Key Takeaways

• Vectors require a direction; scalars do not.
• Remember key scalar quantities that often appear in distractors: speed, work, power.

Common Mistakes

• Confusing speed (scalar) with velocity (vector).
• Thinking work is a vector because force is involved; work is scalar because it is energy transferred.
• Forgetting that electric field strength is a vector (it has a direction: the direction of force on a positive test charge).

Things to Be Careful About

• Always check every item in the group: one scalar makes the whole option wrong.
• Be precise with paired concepts: displacement vs distance, velocity vs speed.