Overview
This lesson covers how to read and analyze displacement-time graphs, interpret motion using direction conventions, and calculate key quantities like distance, speed, and velocity.
Displacement-Time Graph Basics
- Displacement-time graphs show an object's position relative to a starting point over time.
- Positive displacement means movement in the defined positive direction (e.g., right or east); negative means opposite.
- A horizontal line on the graph means the object is stationary.
- Changes in the direction of movement are reflected by changes in the sign of displacement.
Calculating Distance and Speed
- Total distance walked is the sum of all individual movement segments, regardless of direction.
- Average speed = total distance traveled divided by total time.
- Speed is a scalar quantity (no direction).
Calculating Velocity
- Average velocity = change in displacement divided by change in time (final - initial).
- Velocity is a vector quantity and has direction (sign indicates direction based on given convention).
- A negative velocity means movement in the negative direction (e.g., left, west, or south).
Describing Position and Motion
- Displacement at a specific time shows the object's location relative to the start.
- Being at zero displacement means returning to the starting point.
- Not moving (flat graph segment) means displacement does not change.
Worked Examples & Typical Questions
- Determining total distance: sum absolute values of each travel segment.
- Finding average speed: divide total distance by the total time span.
- Describing location at a given time: read the displacement value at that time.
- Calculating velocity between two times: use only the start and end points for that interval.
- If the object doesn’t move during an interval, velocity is zero for that interval.
Key Terms & Definitions
- Displacement — the shortest straight-line distance from the starting point, with direction.
- Distance — total length of the path traveled, regardless of direction.
- Speed — rate at which distance is covered; scalar (no direction).
- Velocity — rate of change of displacement; vector (has direction, positive/negative).
Action Items / Next Steps
- Review displacement-time graphs and practice identifying intervals of motion and rest.
- Practice calculating distance, speed, and velocity from sample graphs.
- Complete any assigned problems or exercises on displacement and velocity calculations.