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Speed, Distance and Time

Speed, Distance and Time. Speed and Velocity. An object has speed when it travels a distance in a time interval. At any moment in time, a moving object has instantaneous speed . Since this is difficult to calculate, we usually use average speed. Speed and Velocity.

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Speed, Distance and Time

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  1. Speed, Distance and Time

  2. Speed and Velocity • An object has speed when it travels a distance in a time interval. • At any moment in time, a moving object has instantaneous speed. Since this is difficult to calculate, we usually use average speed.

  3. Speed and Velocity • Since velocity is speed in a given direction, • the equation for average velocity is: final distance – initial distance Velocity = final time – initial time ∆d Vav= ∆t • The unit for speed or velocity is metres per second (ms-1) or kilometers per hour (kmh-1).

  4. Distance and Time • Distance is a measurement of how far apart two points are. • The unit for distance is the metre (m), or kilometer (km). • Displacement is a measurement of how far an object has moved from its starting point.

  5. Distance and Time • The equation is: displacement = final – initial distance distance • Time is a measurement of the duration of an event. • The unit for time is the second (s), or hour (h).

  6. Distance-Time Graphs • Distance is plotted on the vertical axis and Time on the horizontal axis. • The distance-time graph for an object moving at a constant speed is always a straight line with a slope.

  7. d t Interpreting Distance/time graphs • the gradient of a distance/time graph is the velocity Stationary Constant (v=0) (the distance remains the same)

  8. d t Interpreting Distance/time graphs Faster constant velocity Slow constant velocity Constant velocity (the distancechanges at a constant rate)

  9. d t Interpreting Distance/time graphs Slowing down (-ve acceleration) Speeding up (+ve acceleration) Changing velocity (the distance moved per second changes)

  10. d t Interpreting Distance/time graphs An object that is returning to its starting point. (at a constant speed)

  11. B C d D A t Interpreting Distance/time graphs RC Car Journey • A -fast constant speed (steep gradient and line straight) • B –slower constant speed (shallow gradient and line straight) • C –stationary (line horizontal) • D –Very fast constant speed backwards (steep gradient, straight line, sloping down)

  12. Calculating Gradient • The slope or gradient of a line can be calculated using the formula: • Gradient = Increase in vertical height (of line) Increase in horizontal length

  13. Velocity (speed) Velocity (speed) is a measure of how quickly the position of an object is changing (units ms-1) • If the object is a constant velocity (balanced forces) the formula is;

  14. Velocity (speed) • If the velocity of a journey changes (some slowing and speeding up) the formula for the average velocity is;

  15. Velocity Calculations • A cyclist travels 50km in 1.5 hours. Calculate her velocity. • An athlete records a time of 64s for a 400m race. What speed was he running at? • An Otago student takes 2.5 hours to return to Dunedin from Timaru(190km), with a short stop at Hampden from fish and chips. What was her average velocity for the journey?

  16. Acceleration • An object changing itsspeed is said to be accelerating. If the acceleration is: • positive (eg. 2ms-2) = object speeding up • negative (eg. -2ms-2) = object slowing down or decelerating

  17. Acceleration • The equation is: final speed – initial speed acceleration = time taken ∆v a = ∆t • The unit for acceleration is metres per second squared (or ms-2).

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