Basic Mechanical Engineering — Exercise Solutions
Key Concepts Quick Recap
| Topic | Key Points |
|---|
| Thermodynamics laws | Zeroth (thermal equilibrium), First (energy conservation), Second (entropy) |
| Simple machines | Lever, Pulley, Inclined plane, Wedge, Screw, Wheel and axle |
| Mechanical advantage | Load / Effort; >1 means machine multiplies force |
| Efficiency | Useful output / Total input × 100% |
| Casting | Molten metal poured into mould; sand casting most common |
| Welding | Joining metals by heat/pressure; arc welding most common |
| Lathe | Machine for turning (rotating) operations |
| Drilling machine | Makes holes using rotating drill bit |
I. Choose the Best Answer (MCQs)
1. The First Law of Thermodynamics states: (a) Entropy always increases (b) Energy can be created but not destroyed (c) Energy can neither be created nor destroyed (d) Heat flows from cold to hot
Answer(c) Energy can neither be created nor destroyed First Law: Q = ΔU + W. Heat (Q) supplied to a system = increase in internal energy (ΔU) + work done by the system (W).
2. A lever of first class has: (a) Effort between fulcrum and load (b) Fulcrum between effort and load (c) Load between fulcrum and effort (d) No fulcrum
Answer(b) Fulcrum between effort and load Lever classes: Class 1 — Fulcrum in middle (seesaw, crowbar, scissors); Class 2 — Load in middle (wheelbarrow, nutcracker); Class 3 — Effort in middle (tweezers, forearm).
3. Mechanical advantage of a machine is defined as: (a) Efficiency × 100 (b) Load / Effort (c) Effort / Load (d) Output power / Input power
Answer(b) Load / Effort MA = Load / Effort. MA > 1: machine multiplies force. MA < 1: machine multiplies distance/speed. Ideal MA (no friction) = Velocity ratio (VR).
4. In arc welding, the source of heat is: (a) Gas flame (b) Friction (c) Electric arc between electrode and workpiece (d) Laser beam
Answer(c) Electric arc between electrode and workpiece Arc welding: electric arc (~3000-4000°C) melts base metal and electrode (filler). Types: SMAW (Shielded Metal Arc Welding / Stick welding), MIG, TIG, SAW. Most common welding process in industry.
5. Which material is hardest on the Mohs scale? (a) Glass (b) Steel (c) Diamond (d) Corundum
Answer(c) Diamond Mohs hardness scale 1-10: Talc(1), Gypsum(2), Calcite(3), Fluorite(4), Apatite(5), Feldspar(6), Quartz(7), Topaz(8), Corundum(9), Diamond(10). Diamond = hardest naturally occurring material.
6. A lathe machine is primarily used for: (a) Drilling holes (b) Turning cylindrical parts by rotating workpiece against a cutting tool (c) Milling flat surfaces (d) Grinding
Answer(b) Turning cylindrical parts by rotating workpiece against a cutting tool Lathe operations: turning (cylinder), facing (flat end), threading, boring, knurling, taper turning. Work rotates; tool moves.
7. The property of a metal that allows it to be drawn into wires is called: (a) Malleability (b) Hardness (c) Ductility (d) Brittleness
Answer(c) Ductility Ductility: ability to deform under tensile stress without fracture → drawn into wires. Copper, aluminium, gold are highly ductile. Malleability: ability to deform under compressive stress → beaten into sheets (gold, copper, aluminium).
II. Short Answer Questions
Q1. What is the difference between casting and forging?
Answer
| Feature | Casting | Forging |
|---|
| Process | Molten metal poured into mould; solidifies | Metal shaped by compressive forces (hammering/pressing) |
| State of metal | Liquid | Solid (hot or cold) |
| Properties | Good for complex shapes; porosity possible | Better mechanical properties (grain flow); stronger |
| Surface finish | Less smooth; needs machining | Better surface finish |
| Examples | Engine blocks, pump casings (sand casting) | Crankshafts, connecting rods, hand tools |
Q2. List the types of welding processes and their applications.
Answer
| Welding Type | Process | Applications |
|---|
| Arc welding (SMAW) | Electric arc + coated electrode | General fabrication, structural |
| MIG/MAG welding | Wire electrode + shielding gas | Automotive, sheet metal |
| TIG welding | Tungsten electrode + inert gas | Precision work, stainless steel, aluminium |
| Gas welding (OAW) | Oxygen-acetylene flame | Light fabrication, repairs |
| Resistance welding (spot) | Electric resistance + pressure | Automotive body panels |
| Laser beam welding | High-energy laser | Electronics, precision parts |
Exam Checklist
- First Law: Q = ΔU + W (energy conservation).
- Second Law: entropy increases; heat flows hot → cold spontaneously.
- MA = Load/Effort; Efficiency = useful output/input × 100%.
- Lever Class 1: fulcrum middle; Class 2: load middle; Class 3: effort middle.
- Ductility: drawn into wire; Malleability: beaten into sheets.
- Arc welding: electric arc (3000-4000°C); most common industrial welding.
- Lathe: turning (rotating workpiece); Drill: holes; Milling: flat surfaces.
