Unit 8A: Motion Section 1: Distance and Displacement Indicator PS-5.1: Explain the relationship among distance, time, direction, and the velocity of an object.

Objectives Compare and contrast distance and displacement. Read a graph to determine distance and displacement. Use correct units to express distance and displacement. Demonstrate a scenario where the distance is not zero, but the displacement is.

What is Motion? Motion is a change in the position of one object compared to the position of another object. Distance and Time are used to describe changes in motion.

Reference Points A reference point is needed to determine the position of an object. Motion occurs when an object changes it’s position relative to a reference point. Example ◦ You are riding in a car at 35 miles per hour. You look out of your window at a tree. The tree looks like its moving. ◦ A dog standing beside the tree sees the moving car and starts to chase it. To the dog, the car is moving. ◦ Which object is moving? The car or the tree? WHY?

Relative Motion You are not moving relative to your desk or your school building, but you are moving relative to the other planets in the solar system and the Sun. Motion depends on your point of view.

Distance Length of a path between two points Distance is the actual path from starting to ending point Distance tells you how far an object has moved. It does NOT depend on direction. Distance is a scalar quantity ◦ Magnitude (how far) ◦ No direction required SI unit for measuring distance = meter (m)

Displacement Displacement is the measurement and direction of a straight line from starting to ending point Tells you how far away and in what direction an object is in relation to the starting point A type of vector ◦ Magnitude (distance) ◦ Direction

The distance and direction of an object’s change in position from the starting point. It depends on the starting and stopping points not on the path taken.

Check for Understanding A physical science student runs in the path shown above. How far did he run? What was his final displacement?

Check for Understanding The diagram shows the path of a cross country skier. Point A is the starting point. According to the diagram, what distance did she travel while skiing? What was her final displacement?

Measuring Displacement Not along a straight line ◦ When 2 or more displacements have different directions, the total displacement can be found by graphing a resultant vector ◦ Resultant Vector – Straight line going directly from starting point to ending point

Measuring Displacement Not along a straight line Home School Resultant Vector

Check for Understanding What is the difference between distance and displacement? What units would you use to express distance? Displacement? Can you think of a scenario where an object moves a certain distance, but has a displacement of zero?