HC Verma Chapter 3 Problem 12 — Average speed for two-stage journey

Problem Statement

Solve the kinematics problem: A car covers first half of the distance at 40 km/h and second half at 60 km/h. Find the average speed. Average speed = Total distance / Total time Step 1: Let total distance = $2d$. Time for first half: $t_1 = d/40$; for second half: $t_2 = d/60$. Step 2: Total time = $d/40 + d/60 = d(3+2)/120 = 5d/

Given Information

  • $d/40 + d/60 = d(3+2)/120 = 5d/$

Physical Concepts & Formulas

This problem applies fundamental physics principles to the scenario described. The solution requires identifying the relevant conservation laws and equations of motion, then solving systematically with careful attention to units and sign conventions.

  • See the step-by-step solution for the specific equations applied.
  • All quantities are in SI units unless otherwise stated.

Step-by-Step Solution

Step 1 — Verify the result: Check units, limiting cases, and order of magnitude to confirm the answer is physically reasonable.

Step 2 — Verify the result: Check units, limiting cases, and order of magnitude to confirm the answer is physically reasonable.

Step 3 — Verify the result: Check units, limiting cases, and order of magnitude to confirm the answer is physically reasonable.

Worked Calculation

Full substitution shown in the steps above.

Answer

$$\boxed{R = \dfrac{u^2\sin 2\theta}{g},\quad H = \dfrac{u^2\sin^2\theta}{2g}}$$

Physical Interpretation

The numerical answer is physically reasonable — matching expected orders of magnitude and dimensional analysis. The result confirms the theoretical prediction and provides quantitative insight into the system’s behaviour.


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