IGCSE Physics 0625

1.2 Motion

Velocity: Definition and Understanding

Velocity is a fundamental concept in physics that describes how quickly an object is moving and in what direction. It's a vector quantity, meaning it has both magnitude (speed) and direction.

Defining Velocity

Velocity is defined as the rate of change of displacement with respect to time. In simpler terms, it tells us how much an object's position changes over a specific time period and the direction of that change.

Mathematically, velocity ($v$) is represented as:

$$v = \frac{\Delta x}{\Delta t}$$

Where:

• $v$ = Velocity (measured in m/s)
• $\Delta x$ = Change in displacement (change in position) (measured in meters)
• $\Delta t$ = Change in time (measured in seconds)

The displacement is the straight-line distance between the initial and final positions of an object, considering both distance and direction. It's different from the total distance travelled, which accounts for any changes in direction.

Speed vs. Velocity

It's important to distinguish between speed and velocity:

• Speed: Speed is a scalar quantity representing how fast an object is moving. It only considers the magnitude of the motion (how quickly it's covering ground).
• Velocity: Velocity is a vector quantity representing how fast an object is moving *and* in what direction.

An example to illustrate the difference: A car travelling around a circular track at a constant speed has a changing velocity because its direction is constantly changing.

Velocity as a Vector

Because velocity is a vector, it's represented by an arrow. The length of the arrow represents the speed, and the direction of the arrow represents the direction of motion.

Table Summarizing Key Differences

Property	Speed	Velocity
Definition	Rate of covering distance	Rate of change of displacement
Type of Quantity	Scalar	Vector
Includes Direction?	No	Yes
Unit	m/s	m/s (with direction)

Suggested diagram: A vector arrow representing velocity, showing both magnitude (length) and direction. The arrow should point from one position to another.