Two long trains and of lengths and respectively are moving on parallel straight tracks.

PHYSICS · Multi-part · A Level · Physics · Mechanics and materials · Mechanics and materials

Problem

Two long trains

A

and

B

of lengths

150\,\text{m}

and

120\,\text{m}

respectively are moving on parallel straight tracks. (i) Train

A

moves due east with a speed of

72\,\text{km h}^{-1}

while train

B

moves due east with a speed of

54\,\text{km h}^{-1}

. Find the speed of

A

as observed from

B

. (ii) Using the result of part (i), compute the time taken for the entire length of train

A

to pass completely by an observer seated in train

B

. (iii) If both trains now move with the same magnitudes of velocity as before but in opposite directions, determine the new passing time for the whole of train

A

to clear train

B

. Comment briefly on the difference between the two times obtained.

Related theory

Relative Motion & Reference Frames
Reference frames and point mass
Motion in a plane
Scalars and vectors

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