Modelling Motion in a Straight Line
Set up a motion problem before calculating anything: choose which direction counts as positive, treat the object as a particle, and keep displacement, velocity and acceleration apart from distance, speed and everyday words like slowing down.
What a learner can do afterwards
- Give both the displacement and the distance travelled for an out-and-back journey
- Say what a negative velocity and a negative acceleration each mean once a direction is chosen
- State the assumptions hidden in the words particle, smooth and light
1 · Read
You already work out speed as distance divided by time. Motion adds one thing: direction. Turn the journey into numbers before calculating. Displacement runs straight from start to finish with direction attached. Distance is the full path length. Velocity pairs with displacement the way speed pairs with distance. Pick a positive direction first and stick to it.
A runner goes 8 m forward and 8 m back. Finish minus start is zero, so displacement is 0 m. The path covered is 16 m of distance. Average velocity is zero while average speed is positive. Keep the pair you were asked for.
Signs carry the story. Negative velocity means moving against the chosen positive direction. Negative acceleration means velocity is decreasing. Slowing down means velocity and acceleration point opposite ways. No single sign means slow on its own. Flip one sign by mistake and a braking car speeds up in your answer.
Models confess what they ignore. A particle has no size or spin. Smooth means no friction. Light means no mass worth counting. State these assumptions, since they bound how far the answer can be trusted.
Choose a direction, split displacement from distance, read the signs, and state the assumptions.
Includes a reading credited to the academy collection. Rewritten readings were written for this dataset.
2 · Watch
به انگلیسیTake it off screen
Where it sits
Learn first
Optional connections
14 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.