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Explaining Motion - How we can describe motion?

The speed of a moving object can be calculated if the distance travelled and the time taken are known. The faster an object moves, the steeper the line representing it on a distance-time graph.

The velocity of an object is its speed in a particular direction. In velocity-time graphs sloping sloping lines represent steadily increasing or decreasing velocities. Horizontal lines represent movement at constant velocities.

When an object moves in a straight line at a steady speed you can calculate its speed if you know how far it travels and how long it takes. 

Speed (metre per second, m/s) = distance travelled (metre, m)
time taken (second,s) 

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Explaining Motion - Distance-Time Graphs

The verticalk axis of a distance time graph is the distance travelled from the start, and the horizontal axis is the time taken from the start.

When an object is stationary, the line on the graph is horizontal. When an object is moving at a steady speed, the line on the graphy is straight but sloped.


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Explaining Motion - Distance Time Graphs

time (s) on x axis, distance (m) on y axis (http://www.bbc.co.uk/schools/gcsebitesize/science/images/ph_forces01.gif)

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Explaining Motion - Distance Time Graphs

Note that the steeper the line, the greater the speed of the object. The blue line is steeper than the red line because it represents an object moving faster than the object represented by the red line.

The red lines on the graph represent a typical journey where an object returns to the start again. Notice that the line representing the return journey slopes downwards. 

Changes in distances in one direction are positive, and negative in the other direction. If you walk 10m away from that can be written as +10m; if you walk 3m towards me, that can be written as -3m.

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Explaining Motion - Velocity Time Graphs

The velocity of an object is its speed in a particular direction. This means that two cars travelling at the same speed, but in opposite directions, have different velocities. One velocity will be positive, and the velocity in the other direction will be negative.

The vertical axis of a velocity-time graph is the velocity of the object and the horizontal axis is the time taken from the start.

When an object is moving with a constant velocity, the line on the graph is horizontal. When an object is moving with a steadily increasing velocity or a steadily decreasing velocity, the line on the graph is straight, but sloped.


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Explaining Motion - Velocity Time Graphs

time (s) on x axis, velocity (m/s) on y axis (http://www.bbc.co.uk/schools/gcsebitesize/science/images/ph_forces02.gif)

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Explaining Motion - Velocity Time Graphs

The steeper the line, the more rapidly the velocity of the object is changing. The blue line is steeper than the red line because it represents an object that is increasing in velocity much more quickly than the one represented by the red line.

Notice that the part of the red line between 7 and 10 seconds is a line sloping downwards (with a negative gradient). This represents an object that is steadily slowing down.

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