# OCR - P4: Explaining motion

 P4.2 How can we describe motion? Background to the topic What you should be able to do: The motion of a moving object can be described using the speed the object is moving, the direction it is travelling and whether the speed is changing. The distance an object has travelled at a given moment is measured along the path it has taken. The displacement of an object at a given moment is its net distance from its starting point together with an indication of direction. The velocity of an object at a given moment is its speed at that moment, together with an indication of its direction. Distance and speed are scalar quantities; they give no indication of direction of motion. Displacement and velocity are vector quantities, and include information about the direction. In everyday situations, acceleration is used to mean the change in speed of an object in a given time interval. Distance–time graphs and speed-time graphs can be used to describe motion. The average speed can be calculated from the slope of a distance–time graph. The average acceleration of an object moving in a straight line can be calculated from a speed-time graph. 1. Recall and apply the relationship: average speed (m/s) = distance (m) ÷ time (s) From your practical work experience you should be able to: Describe a variety of methods to measure distances, speeds and times and to calculate acceleration. Compare methods of measuring the acceleration due to gravity. You should be able to: Explore using simple computer models to predict motion of a moving object. 2. Recall typical speeds encountered in everyday experience for wind, and sound, and for walking, running, cycling and other transportation systems 3. a) Make measurements of distances and times, and calculate speeds b) Describe how to use appropriate apparatus and techniques to investigate the speed of a trolley down a ramp 4. Make calculations using ratios and proportional reasoning to convert units, to include between m/s and km/h 5. Explain the vector–scalar distinction as it applies to displacement and distance, velocity and speed 6. a) Recall and apply the relationship: acceleration (m/s2) = change in speed (m/s) ÷ time taken (s) b) Explain how to use appropriate apparatus and techniques to investigate acceleration 7. Select and apply the relationship: (final speed (m/s))2 – (initial speed(m/s))2 = 2 × acceleration (m/s2) × distance (m)