Motion

Distance -time graphs, velocity and acceleration, velocity - time graphs, and using graphs

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Distance - Time Graphs

• We can use graphs to help us describe the motion of an object.
• A distance-time graph shows us the distance of an object from a starting point (y-axis) against the time taken (x-axis)
• The speed of an object is the distance travelled each second
• The graident of the line on a distance-time graph represents the speed. The steeper the graident the greater the speed.
• If an object is stationary, the line on a distance-time graph is horizontal
• If an object is moving at a constant speed, the line on a distance-time graph is a straight line that slopes upwards.
• We can calculate the speed of an object using the equation:

Speed in metres per second, m/s = distance travelled in metres, m

time taken in seconds, s

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Velocity and Acceleration

• The velocity of an object is its speed in a given direction.
• If the object changes direction it changes velocity, even if its speed stays the same.
• If the velocity of an object changes, we say that it accelerates.
• We can calculate acceleration using the equation:

a = v - u

t

a = acceleration ( m/s )

v = final velocity (m/s)

u = initial velocity (m/s)

t = time taken for the change (s)

If the value calculated for acceleration is negative, the body is decelerating. A deceleration is the same as a negative acceleration.

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Velocity - Time Graphs

• A velocity - time graph shows the velocity of an object (y-aixs) against the time taken (x-axis)
• The graident of the line on a velocity-time graph represents acceleartion.
• The steeper the graident, the greater the acceleration
• If the line on a velocity-time graph is a horizontal line, the acceleratiuon is zero. Therefore the object is travelling at a steady speed.
• If the graident of the line is negative, the object is decelerating
• The area under the line on a velocity-time graph represents the distance travelled in a given time.
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Using Graphs...

key points:

• The speed of an object is given by the graident of the line on its distance-time graph
• The acceleration of an object is given by the graident of the line on a velocity-time graph
• The distance travelled by an object is given by the area under the line of its velocity-time graph.
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