Understanding Speed and Velocity

In physics, quantities can be classified based on whether they have only magnitude or both magnitude and direction. This is the difference between scalar quantities and vector quantities.

• Scalar quantities: These are physical quantities that are completely described by only their magnitude. Examples include speed, distance, mass, time, and temperature.
• Vector quantities: These are physical quantities that require both magnitude and direction for their complete description. Examples include velocity, displacement, momentum, force, and acceleration.

Identifying the Quantity with Speed and Direction

The question asks which quantity specifies its speed with direction. Let's look at the options provided:

• Displacement: This is the change in position of an object. It has both magnitude (the shortest distance between the initial and final points) and direction. It is a vector quantity. While related to position and movement, it's not directly "speed with direction".
• Momentum: This is the product of an object's mass and its velocity. Since velocity is a vector, momentum is also a vector quantity having both magnitude and direction. It describes the 'quantity of motion'. It's not simply "speed with direction".
• Velocity: This is defined as the rate of change of displacement with respect to time. It has both magnitude and direction. The magnitude of velocity is known as speed. Therefore, velocity precisely represents speed coupled with a specific direction.
• Force: This is a push or pull that can cause an object to accelerate. It has both magnitude and direction. It is a vector quantity describing an interaction, not the motion itself in terms of speed and direction.

Comparing Options and Velocity

Based on the definitions, velocity is the term specifically used to describe "speed with direction". Speed is the magnitude of velocity, telling us how fast something is moving. Velocity tells us how fast it's moving and in what direction.

The quantity that combines the concept of 'how fast' (speed) with 'in what direction' is Velocity.

Conclusion: Why Velocity is the Correct Answer

The question asks for the quantity that specifies its speed with direction. Speed is the magnitude of velocity. Velocity is defined as a vector quantity whose magnitude is speed and whose direction is the direction of motion. Therefore, velocity is the quantity that includes both speed and direction.

Revision Table: Key Concepts

Additional Information: Scalars vs. Vectors in Physics

Understanding the difference between scalar and vector quantities is fundamental in physics. Scalars are easy to work with mathematically (simple addition/subtraction), but vectors require more complex operations that account for direction (vector addition, subtraction, multiplication).

For example, if you walk 5 meters East and then 5 meters West, your total distance travelled is 10 meters (a scalar sum). However, your displacement is 0 meters because you ended up back where you started (vector addition accounts for direction).

Similarly, knowing an object's speed (e.g., 60 km/h) tells you how fast it's going. Knowing its velocity (e.g., 60 km/h North) tells you both how fast and where it's headed. This direction component is crucial for predicting future positions or analyzing interactions like collisions.