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1.4 Relative motion of two moving object calculations

(1) Two objects travelling in the same direction (but on the same straight line)

=== object A (velocity a) ===>      === object B (velocity b) ===>

The velocity of relative motion = a - b (using the same units)

If the green car is travelling at 25 m/s and is overtaken by the blue car travelling at 40 m/s, the relative speed or velocity is 40 - 25 = 15 m/s. So the blue car overtakes at a relative speed of 15 m/s.

 

(2) Two objects travelling in opposite directions (but not quite on the same straight line, to avoid collision!)

=== object C (velocity c) ===>      <=== object D (velocity d) ===

The velocity of relative motion = c + d (using the same units).

If the green is travelling at 30 m/s and the blue car at 20 m/s, their relative speed is 30 + 20 = 50 m/s.

This is why head-on collisions of two road vehicles is so deadly because 20 or 30 m/s becomes a collision speed of 50 m/s !!!

(3) If one of the objects is stationary, the relative speed or velocity is that of the moving object.

 

INDEX physics notes: Speed calculations and distance-time graphs

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Keywords, phrases and learning objectives for speed, motion and graphs

Know what is meant by the relative motion of two moving object.

Be able to calculate relative speed of two objects whether they are travelling in the same direction or opposite directions.

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INDEX physics notes: Speed calculations and distance-time graphs