Speed
Table of Contents
What is Speed?
Speed measures how fast something moves, indicating the distance it can travel over a given period. As a scalar quantity, speed only concerns the magnitude of movement, not its direction. This distinguishes speed from velocity, a vector quantity that describes how fast something moves and in what direction.
To calculate speed, you divide the distance traveled by the time it takes to cover that distance.
The formula for speed (S) is given as: \large S=\frac{\text{distance}}{\text{time}}
For example, if a car travels 100 kilometers in 2 hours, its speed is 100 km÷2 h=50 km/h.
Meters per second (m/s) and kilometers per hour (km/h) are used to measure speed. The choice of units depends on the context, such as using m/s for scientific research and km/h for road travel speeds.
Instantaneous Speed vs. Average Speed
Instantaneous Speed
Instantaneous speed is the speed of an object at a particular moment in time. Unlike average speed, which measures the total distance traveled over the total travel time, instantaneous speed focuses on how fast an object moves at a specific time.
To calculate instantaneous speed, you consider the distance an object travels over an extremely small, or infinitesimal, time interval. This concept is often used in physics to understand the dynamics of moving objects when their speeds are constantly changing, such as a car accelerating or decelerating on the road.
Average Speed
Average speed represents an object’s overall movement rate over a specific period. It is calculated by dividing the total distance the object has traveled by the total time it took to travel that distance. This measurement provides a simple way to understand how fast an object has been moving on average without accounting for variations in speed at different times during the journey.
To determine the average speed, add all the distances traveled during the trip and divide this total by the sum of the time intervals in which the distances were covered. For example, if a car travels 300 kilometers in 4 hours, its average speed would be \frac{300 \text{ km}}{4 \text{ h}} = 75 \text{ km/h}.
Average speed helps get a general sense of the speed of a trip or journey. Still, it doesn’t reflect changes in speed that might occur along the way, such as stops, accelerations, or decelerations. It’s a practical way to measure speed in everyday contexts, like determining how fast a vehicle traveled on a road trip or calculating an athlete’s speed over a distance race. It helps plan and assess travel times, vehicle fuel efficiency, and athletic event performance.
Units of Speed
In the International System of Units (SI), speed is most commonly measured in meters per second (m/s) or kilometers per hour (km/h). The choice between these units often depends on the context; m/s is frequently used in scientific and engineering calculations because it directly relates to the base SI units. Meanwhile, km/h is more common in everyday situations, particularly road transport, to indicate vehicle speeds.
Other speed units are prevalent outside the SI system, especially in countries like the United States. Miles per hour (mph) is the standard unit for road vehicle speeds, reflecting the distance traveled in miles in one hour. Feet per second (ft/s) is another unit used, often in more technical or scientific contexts within the U.S. system, to measure speeds in various situations.
Velocity vs. Speed
Speed
Speed, as a scalar quantity, measures how fast an object is moving but does not indicate the direction of the movement. It provides the magnitude of motion, or the rate at which an object covers distance, without any reference to the direction in which the object is traveling. This distinguishes speed from velocity, which is a vector quantity that specifies both the magnitude and the direction of motion.
For instance, if a car is traveling at 60 kilometers per hour, this information tells you the speed of the car but not where it is going. The speed value alone (60 km/h) gives you an idea of how quickly the car is covering ground, but without the directional component, you wouldn’t know if the car is moving north, south, east, or west.
Velocity
Velocity is a vector quantity that provides a comprehensive description of an object’s motion, encompassing both its speed and direction. While speed tells us how fast an object is moving, velocity specifies the speed along with the direction of the movement. This dual attribute of magnitude and direction allows velocity to give a complete picture of how an object is moving through space.
For example, if a car is moving at 60 kilometers per hour to the east, its velocity would be described as “60 km/h east.” This information is more informative than just the speed because it tells us the rate of motion and where the car is heading. If the car changes direction but maintains the same speed, its velocity changes because the directional component of velocity has changed, even though the speed remains constant.
Types of Speed
Uniform Speed
Uniform speed refers to motion at a constant speed, where an object covers equal distances in equal intervals of time, and there is no change in its velocity in terms of magnitude. However, there’s a slight nuance here regarding velocity: if the direction of the motion changes while maintaining constant speed, then the velocity would still change due to its directional component. Therefore, for true uniform motion in the sense of velocity, both speed and direction must remain constant.
In the context of speed alone, an object moving at a uniform speed will have a distance-time graph that is a straight line, indicating a steady rate of distance covered over time. This straight-line graph shows that the object’s speed is constant, without any acceleration or deceleration.
Non-Uniform Speed
Non-uniform speed occurs when an object changes its speed over time, accelerating or decelerating, and potentially altering its direction as well. In this type of motion, the object does not cover equal distances in equal time intervals, leading to variations in speed and possibly in velocity if the direction of motion changes.
The distance-time graph for an object moving at non-uniform speed will not be a straight line. Instead, it will display curves or sections with varying slopes, reflecting the changes in speed. A steeper slope on the graph indicates a faster speed, while a shallower slope signifies a slower speed. Curves in the graph represent acceleration or deceleration, showing how the speed of the object increases or decreases over time.
For instance, a car that starts moving from rest and picks up speed is accelerating, exhibiting non-uniform speed. If the car then slows down to stop at a traffic light, it decelerates, further demonstrating non-uniform speed. The distance-time graph of this journey would show a curved line that starts flat (when the car is at rest), then curves upwards as the car accelerates, and flattens again as the car stops.
Related Links
Acceleration
Momentum
Torque
Velocity