Motion and Forces: Distance, Displacement, Speed and Velocity
Distance and Displacement
- Distance is a scalar quantity that refers to "how much ground an object has covered" during its motion.
- Displacement, in contrast, is a vector quantity that refers to "how far out of place an object is"; it is the object's overall change in position.
- Whilst distance refers to the total route travelled, displacement represents the straight line distance in a specific direction from the starting point to the endpoint of a journey.
Calculating Distance and Displacement
- Distance is calculated by the actual path taken, whereas displacement involves the shortest distance in a specific direction.
- To calculate displacement you subtract the initial position from the final position.
Speed
- Speed is a scalar quantity that refers to "how fast an object is moving" or the rate at which an object covers distance.
- It is usually measured in metres per second (m/s) or kilometres per hour (km/h).
- Average speed can be calculated using the formula: speed = distance travelled / time taken.
Velocity
- Velocity is the speed of something in a given direction. It's a vector quantity, meaning it has both magnitude (speed) and direction.
- Just as with speed, velocity is usually measured in metres per second (m/s) or kilometres per hour (km/h).
- You can calculate average velocity with the following formula: velocity = displacement / time taken.
Differences between Speed and Velocity
- Although speed and velocity are often used interchangeably in everyday language, they mean different things scientifically.
- Speed refers to how fast something is going, but velocity also tells you the direction of the movement.
- An object's speed can remain constant while its velocity changes (for example if it changes direction), and vice versa.
Graphical Representation of Motion
- Graphs can be used to represent both speed and velocity; A distance-time graph shows how far an object has travelled at any given moment, whilst a velocity-time graph shows both speed and direction.
- On a distance-time graph, a horizontal line represents an object not moving. A diagonal line represents a constant speed, and a curved line indicates changing speed.
- In a velocity-time graph, the gradient of the slope is equal to the acceleration, whilst the area under the graph represents the displacement. The steeper the gradient, the greater the acceleration. A falling gradient signifies deceleration.