Motion

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Speed

Speed

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.

This equation shows the relationship between speed, distance travelled and time taken:

Speed is distance divided by time taken. (http://www.bbc.co.uk/staticarchive/544226c5be16686efa6851f1d76c839d8fcd98f0.gif)

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Distance/Time Graphs

Features of the graphs

The vertical axis of a distance-time graph is the distance travelled from the start. The horizontal axis is the time 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 graph is straight, but sloped.

The diagram on the next page shows some typical lines on a distance-time graph.

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

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

The gradient of a line on a distance-time graph represents the speed of the object

To calculate the gradient of the line on a graph, divide the change in the vertical axis by the change in the horizontal axis.

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Example Distance/Time Graph

time (s) on x axis, distance (m) on y axis (http://www.bbc.co.uk/staticarchive/4290b412e1210381d2a5f21b2fbd4a08259a4e59.gif)

Speed of blue = Gradient = 10m / 2s = 5m/s

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Velocity/Time Graphs

The velocity of an object is its speed in a particular direction. .

Features of the graphs

The vertical axis of a velocity-time graph is the velocity of the object. The horizontal axis is the time 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 constant acceleration, the line on the graph is straight, but sloped.

The diagram on the next page shows some typical lines on a velocity-time graph.

The steeper the line, the greater the acceleration of the object. The blue line is steeper than the red line because it represents an object with a greater acceleration.

A line sloping downwards represents an object with a constant deceleration - slowing down.

The gradient of a velocity-time graph represents the acceleration

The area under a velocity-time graph represents the distance covered

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Example Velocity/Time Graph

(http://www.bbc.co.uk/staticarchive/6029ff54b931d005d2a59cce9aa9f24f88d80b9a.gif)

Acceleration = Gradient = 8m/s / 4s = 2m/s^2

Distance covered = (0.5 * 8 * 4) + (6 *8) = 64m

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Acceleration

Acceleration : The rate of change of velocity, measured in metres per second squared.

When an object moves in a straight line with a constant acceleration, you can calculate its acceleration if you know how much its velocity changes and how long this takes.

Equation

Acceleration is change in velocity divided by time taken (http://www.bbc.co.uk/staticarchive/db1cd2adc51047fe2892f95b57640d9bbdf03f9a.gif)

Stopping Distances

Stopping distance = thinking distance + braking distance

  • Braking distance increases with speed.
  • Icy/wet roads, poor brakes or worn tyres will also increase braking distance
  • Thinking distance increases with speed. If the diver is tired, distracted, has drunk alcohol or taken certain types of drugs the thinking distance will increase.
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