In basic mechanics, speed of a body in a specified direction is known by which physical term?

Introduction / Context:
This question tests a simple but fundamental idea from mechanics: the distinction between scalar and vector quantities. Speed is a scalar that tells us how fast an object is moving but not the direction, whereas velocity includes both magnitude and direction. Understanding this difference is crucial for correctly analysing motion, forces and kinematic equations in physics.

Given Data / Assumptions:

• The phrase speed in a given direction is used in the question.
• We want the standard physics term that corresponds to this description.
• Options include velocity, torque, acceleration and displacement.
• We assume basic Newtonian mechanics in a straight line or plane.

Concept / Approach:
Speed is defined as the rate of change of distance with respect to time and has magnitude only. When the direction of motion is also specified, the quantity becomes a vector called velocity, which is the rate of change of displacement with time. Torque is related to rotational effect of a force about an axis. Acceleration is the rate of change of velocity with time. Displacement is the change in position of an object. Therefore, the correct term for speed in a given direction is velocity.

Step-by-Step Solution:
Step 1: Recall that speed is a scalar quantity defined as distance travelled per unit time. Step 2: Recognise that when we add direction to speed, we are describing a vector quantity. Step 3: In physics, the vector quantity equal to speed with a specific direction is called velocity. Step 4: Understand that torque measures the turning effect of a force and is unrelated to linear speed or direction. Step 5: Note that acceleration is the rate of change of velocity, not the same as speed in a direction. Step 6: Displacement describes a change in position and is not a rate; it does not refer to speed. Step 7: Therefore, the appropriate physical term for speed in a given direction is velocity.

Verification / Alternative check:
In kinematics, equations like v = u + a * t and s = u * t + (1/2) * a * t^2 use v and u to denote velocity, which has both magnitude and direction. For example, an object moving east at 10 m/s has a velocity of 10 m/s east, which includes direction. If we ignore the east part and only say 10 m/s, then we are talking about speed. This consistent usage across textbooks and problems confirms that velocity is correctly described as speed in a given direction.

Why Other Options Are Wrong:
Torque is a measure of rotational effect and has units of newton metre, not m/s. Acceleration is the rate of change of velocity and has units of m/s^2, not m/s. Displacement is a vector describing change in position, measured in metres, and is not a rate of motion.

Common Pitfalls:
Students sometimes mix up speed and velocity in everyday language because people often use the term speed when they formally mean velocity. In physics, it is important to keep the terms distinct. Speed only tells you how fast you are moving, while velocity tells you both how fast and in what direction. Remembering that the phrase speed in a specified direction is essentially the textbook definition of velocity will help you recall the correct term in problems.

Final Answer:
Speed in a given direction is called Velocity.