Physics 5054/12 — October/November 2015

Walkthrough

The question asks which quantity is scalar. A scalar quantity is described completely by its magnitude alone, such as 5 kg or 10 s. A vector quantity needs both magnitude and direction, such as 5 m/s to the north or 10 N downwards.

Mass is the amount of matter in an object and does not involve direction, so it is a scalar. Force is a push or pull and always acts in a direction, so it is a vector. Velocity is speed in a stated direction, so it is also a vector. Weight is the gravitational force acting on an object, and it acts vertically downwards, so it is a vector too.

Therefore the correct option is A.

Key Takeaways

• A scalar has magnitude only: mass, speed, distance, time, energy.
• A vector has magnitude and direction: force, velocity, weight, displacement, acceleration.
• Weight is a force, so it must be treated as a vector even though it is often quoted as a number with a unit.

Common Mistakes

• Choosing weight as scalar because it is often written as a single number. Weight is a force and acts downwards, so it is a vector.
• Confusing mass with weight. Mass is a scalar measured in kg; weight is a force measured in N.
• Thinking velocity is scalar because its speed value is a number. Velocity includes direction, so it is a vector.

Things to Be Careful About

• Remember that the unit of mass is kg, while the unit of weight is N.
• If a quantity has a direction, such as downwards, it cannot be scalar.
• Speed is scalar, but velocity is vector: the two are not the same.