In physics, physical quantities that have only magnitude and no associated direction are known by which term?

Introduction / Context:
When studying mechanics and other branches of physics, it is essential to classify physical quantities into scalars and vectors. This classification tells you whether a quantity is completely described by a single number or whether it also has a direction associated with it. The question asks for the correct description of physical quantities that possess only magnitude but no direction. Understanding this distinction is vital for solving problems correctly, especially when adding or subtracting quantities like forces, velocities, and distances.

Given Data / Assumptions:

• Physical quantities can be described by magnitude and possibly direction.
• The question focuses on quantities with only magnitude.
• Options mix up magnitude, direction, speed, and velocity terms.
• We assume the standard physics definitions of scalar and vector quantities.

Concept / Approach:
A scalar quantity is defined as one that is completely described by a single real number, called its magnitude. Examples include mass, temperature, time, distance, and energy. These do not require direction for their specification. A vector quantity, on the other hand, is described by both magnitude and direction, for example displacement, velocity, acceleration, and force. Therefore, quantities with only magnitude and no direction are scalar quantities. The language used in the correct option should capture this definition succinctly, mentioning magnitude and explicitly stating the absence of direction.

Step-by-Step Solution:
Step 1: Recall that scalars have magnitude only, while vectors have both magnitude and direction.Step 2: Identify the option that states magnitude only and no direction, which matches the scalar definition.Step 3: Check that the other options either specify direction only or confuse speed and velocity.Step 4: Confirm that magnitude only and no direction is the correct descriptive phrase for scalar quantities.Step 5: Choose that option as the answer.

Verification / Alternative check:
Consider some examples. Mass of 5 kilograms, time of 10 seconds, and temperature of 30 degrees Celsius all make sense without any directional information. These are clearly scalar quantities. In contrast, a velocity of 10 metres per second north requires a direction to be complete. If you remove direction from velocity, you are left with speed, which is scalar. This exercise shows that the essential property of scalars is magnitude only, which confirms the correctness of the option stating magnitude only and no direction.

Why Other Options Are Wrong:
Option A, direction only and no magnitude, is physically meaningless because a physical quantity without any size cannot be measured or used in equations. Option C, speed and velocity together, is a mix of a scalar and a vector and does not describe a category of quantities. Option D, velocity only and no speed, confuses the definitions, since velocity inherently has both magnitude and direction and cannot exist without magnitude. Only option B correctly captures the property of scalar quantities as having magnitude only and no direction.

Common Pitfalls:
Students sometimes confuse speed and velocity, thinking both are vectors, when actually speed is scalar and velocity is vector. Another mistake is to assume that all quantities in motion, like distance and displacement, are vectors, but distance is scalar. Always ask yourself whether specifying a number alone is enough to describe the quantity. If yes, it is likely scalar. If a direction must be added for the description to make sense, the quantity is vector. This habit helps you avoid errors in classification questions like this.

Final Answer:
Magnitude only and no direction