Class 11 Basic Mechanical Engineering — Exercise Solutions
Key Concepts Quick Recap
| Concept | Key Fact |
|---|
| Lever classes | Class 1: F between L and E; Class 2: L between F and E; Class 3: E between F and L |
| MA (Mechanical Advantage) | Load / Effort = Effort arm / Load arm |
| First Law of Thermodynamics | Energy is conserved: Q = ΔU + W |
| Ductility | Ability to be drawn into wires (Al, Cu) |
| Malleability | Ability to be beaten into sheets (Au, Ag, Cu) |
| Hardness | Resistance to scratching/indentation (diamond = hardest) |
| Welding | Joining metals by melting and fusing |
I. Choose the Best Answer (MCQs)
1. In a Class 2 lever, the load is: (a) Between effort and fulcrum (b) At one end, effort at other, fulcrum in middle (c) Between fulcrum and effort (d) At the same position as the effort
Answer(c) Between fulcrum and effort Class 2 lever: Fulcrum — Load — Effort (FLE). Load between F and E. Always has mechanical advantage > 1 (effort arm > load arm). Examples: wheelbarrow, nutcracker, bottle opener, door (hinge = fulcrum, effort at handle = far end).
2. The property of a metal that allows it to be drawn into thin wires is called: (a) Malleability (b) Ductility (c) Hardness (d) Brittleness
Answer(b) Ductility Ductility: ability to be drawn into wires without breaking. Measured by: elongation and reduction in cross-section. Most ductile metals: gold, silver, platinum, copper, aluminium. Malleability: ability to be hammered/rolled into sheets. Most malleable: gold, silver, copper, aluminium.
3. The First Law of Thermodynamics is based on: (a) Conservation of momentum (b) Conservation of mass (c) Conservation of energy (d) Conservation of temperature
Answer(c) Conservation of energy First Law: Q = ΔU + W. Q = heat added to system; ΔU = change in internal energy; W = work done by system. In a closed cycle: ΔU = 0; so Q_net = W_net (net heat = net work). Energy is neither created nor destroyed.
4. Which type of welding uses an electric arc between an electrode and the work piece? (a) Gas welding (b) Arc welding (c) Spot welding (d) Forge welding
Answer(b) Arc welding Arc welding: electric arc (3000-3500°C) generated between electrode and base metal → melts both → fuses on cooling. Types: SMAW (Shielded Metal Arc Welding — stick welding), MIG (Metal Inert Gas), TIG (Tungsten Inert Gas). Gas welding: oxyacetylene flame. Spot welding: resistance welding; used for thin sheets (car bodies).
5. Mechanical advantage (MA) is defined as: (a) Effort / Load (b) Load / Effort (c) Effort × Load (d) Speed × Force
Answer(b) Load / Effort Mechanical Advantage (MA) = Load / Effort = Effort arm / Load arm. MA > 1: machine multiplies force (makes it easier to lift heavy loads). MA < 1: machine multiplies speed/distance (e.g., tweezers). Velocity ratio (VR) = distance moved by effort / distance moved by load. Efficiency = (MA/VR) × 100%.
6. The process of producing components by pressing molten metal into a mould is called: (a) Forging (b) Casting (c) Rolling (d) Extrusion
Answer(b) Casting Casting: molten metal poured into a mould → solidifies into desired shape. Types: sand casting, die casting, investment casting. Advantage: complex shapes; any size. Forging: shaping by compressive force on hot/cold metal (stronger parts). Rolling: metal passed through rollers (sheets, rods). Extrusion: metal forced through a die (tubes, bars).
7. The hardest known natural material is: (a) Steel (b) Iron (c) Diamond (d) Titanium
Answer(c) Diamond Diamond: hardest natural material (Mohs scale: 10). Hardness scale (Mohs): 1 (talc) to 10 (diamond). Diamond: used for cutting tools, abrasives, drill bits. Steel hardness: ~5-8 (varies with composition and heat treatment). Tungsten carbide (WC): next hardest commonly used engineering material (~9).
II. Short Answer Questions
Q1. What are the different types of fits and tolerances used in engineering?
Answer
In manufacturing, components must fit together correctly. Dimensions are never perfect — tolerances define acceptable deviation.
Tolerance: The permissible variation from the basic (nominal) dimension. Example: a shaft diameter specified as 50 ± 0.05 mm means 49.95 to 50.05 mm is acceptable.
Types of fits: 1. Clearance fit: Shaft is always smaller than hole; there is always a gap (clearance). Used for: sliding/rotating parts (bearings, pistons). Minimum clearance ≥ 0. 2. Interference fit: Shaft is always larger than hole; must be pressed or shrink-fit. Used for: permanent assemblies (wheel hubs, press-fit bearings). Maximum clearance is negative. 3. Transition fit: Either clearance or interference may occur depending on actual dimensions. Used for: semi-permanent assemblies (gear hubs, pulleys with keys).
Tolerance grade: indicated by IT numbers (IT01, IT0, IT1...IT16); lower IT = tighter tolerance = more expensive to manufacture.
Q2. Explain the different types of stresses in engineering materials.
Answer
Stress (σ): Internal resisting force per unit area = Force/Area. Unit: N/m² (Pascal) or N/mm² (MPa).
Types of stress: 1. Tensile stress: Pulling force → material tries to elongate. Example: rope being pulled at both ends. σ_tensile = F/A. 2. Compressive stress: Pushing force → material tries to shorten. Example: pillar supporting load. σ_compressive = F/A. 3. Shear stress (τ): Force parallel to cross-section → sliding layers. Example: bolt being cut. τ = F/A (where A = shear plane area). 4. Bending stress: Non-uniform stress across cross-section; tensile on one side, compressive on other. Example: beam under transverse load. 5. Torsional stress: Twisting. Example: drive shaft. τ = T × r / J (J = polar moment of inertia).
Strain (ε): Deformation per unit length = ΔL/L (dimensionless). Hook's Law: σ = E × ε (E = Young's modulus, material property).
Exam Checklist
- Lever Class 1: F in middle (seesaw, scissors). Class 2: L in middle (wheelbarrow, MA>1). Class 3: E in middle (tweezers, MA<1).
- MA = Load/Effort = Effort arm/Load arm.
- Ductility: drawn into wire. Malleability: beaten into sheets.
- First Law: Q = ΔU + W (energy conservation).
- Arc welding: electric arc ~3000-3500°C. Spot welding: resistance welding.
- Casting: molten metal poured into mould. Forging: compressive force.
- Fits: Clearance (gap), Interference (press), Transition (either).
- Stress = Force/Area (Pa or MPa); Strain = ΔL/L. Hooke's Law: σ = Eε.
