Explanation (Complete Step-by-Step Derivation)

Angular momentum of a particle about a point is:

\[ L = m (\vec{r} \times \vec{v}) \]

Magnitude form:

\[ L = mrv\sin\theta \]

Here, motion is along parallel tracks separated by 10 m.

So perpendicular distance between velocity of A and car B = 10 m

Step 1: Convert speeds into m/s

\[ 72 \text{ km/h} = \frac{72 \times 1000}{3600} \]

\[ = 20 \text{ m/s} \]

\[ 36 \text{ km/h} = 10 \text{ m/s} \]

Step 2: Relative velocity of A with respect to B

\[ v_{AB} = 20 - 10 \]

\[ v_{AB} = 10 \text{ m/s} \]

Step 3: Use angular momentum formula

\[ L = m v_{rel} \times d \]

\[ L = (10^3)(10)(10) \]

\[ L = 10^5 \text{ J·s} \]

Final Answer: 10⁵ J·s

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