Question 1

Rotational Kinetics — Energy of a Rotating Body A rigid body with mass moment of inertia I about a fixed axis rotates at angular velocity ω. What is its rotational kinetic energy?

Accepted Answer

Introduction / Context: The kinetic energy of rotation is central to dynamics, energy methods, and power computations in machines (flywheels, rotors, gears). It parallels the translational form (1/2 m v^2) with angular analogs I and ω. Given Data / Assumptions: Rigid body rotating about a fixed axis with angular speed ω. Mass moment of inertia about that axis is I. Concept / Approach: For an elemental mass dm at radius r with tangential speed v = ω r, the elemental kinetic energy is dK = (1/2) dm * v^2 = (1/2) dm * (ω^2 r^2). Integrating over the body yields the total rotational kinetic energy. Step-by-Step Solution: Start from dK = (1/2) ω^2 r^2 dm. Integrate: K = (1/2) ω^2 ∫ r^2 dm. Recognize ∫ r^2 dm = I ⇒ K = (1/2) I ω^2. Verification / Alternative check: For a point mass m at radius r, I = m r^2 and K = (1/2) m (ω r)^2 = (1/2) I ω^2, confirming the general form. Why Other Options Are Wrong: I ω has units of angular momentum; I ω^2 lacks the 1/2 factor; 0.5 I ω is dimensionally inconsistent (energy would require ω^2). Common Pitfalls: Confusing torque power P = T ω with kinetic energy; omitting the square on ω; mixing mass and area moments of inertia. Final Answer: 0.5 I ω^2.

Question 2

Bridge steel trusses — support condition and purpose

Most steel bridge trusses are supported with a hinge (pin) at one end and a roller at the other end. The main advantage of this hinge–roller arrangement is that the truss remains stable under l…

Accepted Answer

Introduction / Context: Bridge trusses are typically supported by a hinge (pin) at one end and a roller at the other. This classical arrangement appears in steel, concrete, and composite bridges because it guarantees stability while accommodating temperature changes, shrinkage, and support settlements. Given Data / Assumptions: One bearing provides a pin (hinge) that restrains translation in two directions but allows rotation. The other bearing provides a roller that restrains one direction and allows longitudinal movement. Loads include dead load, live load, wind, and thermal effects. Concept / Approach: For a determinate and stable support system, the structure needs just enough reactions to prevent rigid-body motion without over-restraining it. The hinge–roller pair supplies three reaction components in total (two at the hinge, one at the roller). This prevents sway and uplift while permitting expansion/contraction. Step-by-Step Solution: Identify required restraints: prevent vertical and lateral translation; allow thermal expansion.A hinge resists two translations; a roller resists one, supplying three independent reactions.Three independent reactions in planar statics ensure determinacy and stability against rigid-body motion.Roller freedom in the bridge’s longitudinal direction avoids locked-in thermal forces. Verification / Alternative check: If both ends were fixed against longitudinal movement, temperature rise would induce large axial forces. Conversely, two rollers would not prevent lateral instability. The hinge–roller combination is the balanced solution. Why Other Options Are Wrong: False: Ignores the stabilizing and movement-accommodating role of the supports. Both ends fixed: Over-restraint, leading to thermal stresses and indeterminacy. Only for wooden trusses / only short spans: The principle is material-agnostic and applies to a wide range of spans. Common Pitfalls: Confusing “stability” with “full fixity.” Stability means no rigid-body motion; it does not require restraining all degrees of freedom. Final Answer: True

Question 3

Newton’s second law application — find the force needed

What constant force must be applied to a body of mass 100 kg to produce an acceleration of 5 m/s^2?

Accepted Answer

Introduction / Context: This is a direct application of Newton’s second law of motion, which relates force, mass, and acceleration. It is a staple calculation in engineering mechanics and physics. Given Data / Assumptions: Mass m = 100 kg. Desired acceleration a = 5 m/s^2. Motion in a straight line; neglect friction and other resistances. Concept / Approach: Newton’s second law states that the net force F required to produce an acceleration a on a mass m is F = m * a. This relationship is linear: doubling the mass or the acceleration doubles the required force. Step-by-Step Solution: Write the governing equation: F = m * a.Substitute m = 100 kg and a = 5 m/s^2.Compute: F = 100 * 5 = 500 N.Therefore, the required constant force is 500 N. Verification / Alternative check: If the same 500 N acted on a 50 kg body, the acceleration would be 10 m/s^2, consistent with the linear relationship. Scaling checks help confirm arithmetic. Why Other Options Are Wrong: 20 N, 50 N, 100 N: Too small to generate the specified acceleration for a 100 kg mass. None of these: Incorrect because 500 N is available as an option and is correct. Common Pitfalls: Confusing weight with mass (weight = m * g). Here we are asked for net force to create a given acceleration, not the weight of the object. Final Answer: 500 N

Question 4

Force systems — can two equal and opposite parallel forces with distinct lines of action be replaced by a single force?

Accepted Answer

Introduction / Context: This question probes the concept of a couple in engineering mechanics. Two equal and opposite parallel forces whose lines of action are separated form a pure moment (a couple), not a single resultant force. Given Data / Assumptions: Two forces, equal in magnitude and opposite in direction. Forces are parallel, with a finite perpendicular distance between their lines of action. Rigid body assumption. Concept / Approach: When two equal and opposite parallel forces are separated by a distance, their net force is zero, but they create a nonzero moment. This moment is called a couple, with magnitude M = F * d, where F is the magnitude of either force and d is the perpendicular distance between their lines of action. Step-by-Step Solution: Compute net force: F_net = +F + (−F) = 0.Compute moment: M = F * d about any point (independent of reference point for a couple).Interpretation: The system reduces to a pure couple, not a single force.Hence, the statement claiming replacement by a single force is incorrect. Verification / Alternative check: Try to represent a couple by a single force at any point; you will always introduce an unintended net force, which contradicts the original zero-resultant condition. Only a free vector couple correctly represents the system. Why Other Options Are Wrong: Correct / variants: Misrepresent the mechanics; a single force cannot replicate a pure couple. Lines of action intersect: They are parallel; if they intersected, the system would be concurrent, not a couple. Common Pitfalls: Confusing a couple with two non-parallel forces; believing every force system reduces to a single force plus a moment at the same point (here the net force is strictly zero). Final Answer: Incorrect

Question 5

Centre of gravity of a rectangle — location statement

The centre of gravity (centroid) of a uniform rectangle lies at the intersection point of its two diagonals.

Accepted Answer

Introduction / Context: The centroid (centre of gravity for uniform density) of standard shapes is fundamental in structural analysis, bending, and area-moment calculations. For rectangles, the centroid is famously at the intersection of diagonals. Given Data / Assumptions: Uniform thickness and density. Planar, axis-aligned rectangle of width b and height h. No cut-outs or holes. Concept / Approach: By symmetry, the centroid must lie midway between parallel sides. Therefore, it is located at coordinates (b/2, h/2) measured from a corner, which is exactly where the diagonals intersect. Step-by-Step Solution: Use symmetry about the rectangle’s midlines.Horizontal symmetry → x̄ = b/2.Vertical symmetry → ȳ = h/2.Intersection of diagonals also occurs at (b/2, h/2), confirming the centroid location. Verification / Alternative check: Apply the first-moment-of-area definition: x̄ = (∫ x dA)/A and ȳ = (∫ y dA)/A. For a uniform rectangle, both evaluate to mid-spans. Why Other Options Are Wrong: Disagree / unless square: The result holds for any rectangle, not only squares. Density varies linearly: The statement assumes uniform density; non-uniformity would shift the centroid. Common Pitfalls: Confusing “centre” with any geometric center point even when cut-outs exist; the centroid changes if holes or non-uniform density are present. Final Answer: Agree

Question 6

Projectile on a downward inclined plane — time of flight dependence

The time of flight of a projectile launched on a downward inclined plane depends on which parameters?

Accepted Answer

Introduction / Context: Projectile motion on an inclined plane is a classic kinematics problem. Unlike level-ground trajectories, the landing condition depends on the plane’s slope as well as the projectile’s launch parameters. Given Data / Assumptions: Initial speed u, launch angle α measured from the horizontal. Plane inclined downward at angle β from the horizontal (β > 0). Uniform gravity g; no air resistance; point-mass projectile. Concept / Approach: Resolve motion into horizontal and vertical components, then impose the landing condition that the projectile meets the inclined plane. The intersection occurs when the projectile’s parametric equation satisfies y = x * tan(−β), which links both α and β in the time-of-flight expression. Step-by-Step Solution: Horizontal: x(t) = u cos α * t.Vertical: y(t) = u sin α * t − (1/2) g t^2.Inclined plane line through origin: y = −x tan β.Set u sin α * t − (1/2) g t^2 = −(u cos α * t) tan β and solve for the nonzero root t.Rearrange terms to obtain t = (2u (sin α + cos α * tan β)) / g.Because tan β appears, both α and β influence the time of flight; u and g also matter. Verification / Alternative check: When β = 0 (level ground), the expression reduces to t = 2u sin α / g, the standard formula, confirming consistency. Increasing downward slope (β larger) generally increases the time of flight for fixed α and u until geometric limits apply. Why Other Options Are Wrong: Angle of projection only / plane angle only: Each alone is insufficient; the intersection condition couples both angles. None of these / initial speed only: Incorrect; both geometric angles matter besides u and g. Common Pitfalls: Forgetting to change the landing condition to match the inclined plane; mixing angle definitions (from horizontal vs. from plane). Final Answer: Both (angle of projection) and (angle of inclination of the plane)

Question 7

Simple harmonic motion terminology — is motion from one extreme to the other called a “beat”?

Accepted Answer

Introduction / Context: Terminology matters in vibration theory. “Beat” and “oscillation” are distinct concepts. A beat arises from superimposing two close frequencies, whereas motion from one extreme to the other in simple harmonic motion is half an oscillation. Given Data / Assumptions: Simple harmonic motion (SHM) with extremes at ±A. No damping or external forcing. Standard definitions from vibration theory. Concept / Approach: One complete oscillation in SHM is the path from +A to −A and back to +A (or vice versa). Traveling from one extreme to the opposite extreme corresponds to half an oscillation. “Beats” occur when two harmonic motions with close frequencies are combined, producing amplitude modulation at the difference frequency. Step-by-Step Solution: Define oscillation: one full cycle of periodic motion.From +A to −A = half cycle; −A back to +A = another half, totaling one cycle.Define beats: superposition of frequencies f1 and f2 ≈ f1, producing envelope frequency |f1 − f2|.Therefore, calling “extreme-to-extreme” motion a beat is incorrect. Verification / Alternative check: Observe a tuning fork example: two forks of slightly different frequencies produce audible beats, unrelated to a single oscillator’s traversal between extremes. Why Other Options Are Wrong: Agree (and variants): Conflate half-cycle motion with interference phenomena. Agree only in acoustics: Even in acoustics, beats arise from interference, not single-oscillator excursion. Common Pitfalls: Using “beat” loosely for any repetitive motion; forgetting that beat frequency equals the small difference between two close frequencies. Final Answer: Disagree

Question 8

Second moment of area — hollow rectangular section about centroidal X–X

For a rectangular section of outer breadth B and outer depth H with a concentric rectangular void of breadth b and depth h, what is the second moment of area about the centro…

Accepted Answer

Introduction / Context: The second moment of area (area moment of inertia) of a hollow rectangle is widely used for deflection and bending stress calculations. Engineers often need a general formula independent of a specific diagram. Given Data / Assumptions: Outer rectangle: breadth B, depth H. Inner void (hole): breadth b, depth h, concentric and axis-aligned. Neutral axis X–X is horizontal through the centroid (mid-depth). Concept / Approach: For composite areas, subtract the inner void’s second moment of area from that of the outer area about the same axis. For a solid rectangle about its own centroidal horizontal axis, I_xx = (b * h^3) / 12 (breadth times depth cubed over 12). Step-by-Step Solution: Outer I_xx(out) = (B * H^3) / 12.Inner I_xx(in) = (b * h^3) / 12.Hollow I_xx = I_xx(out) − I_xx(in).Therefore, I_xx = (B * H^3 − b * h^3) / 12. Verification / Alternative check: When b = 0 and h = 0 (no hole), the formula reduces to (B * H^3)/12 for a solid rectangle. When b = B and h = H (no material left), the result becomes zero, as expected. Why Other Options Are Wrong: (B^3 * H − b^3 * h)/12: Swaps breadth and depth exponents; correct for I_yy, not I_xx. Sum form: Should be the difference (subtract the void). (B^3 − b^3)/12, (H^3 − h^3)/12: Incomplete; ignore the other dimension and units. Common Pitfalls: Mixing up I_xx and I_yy; forgetting to keep axes through the common centroid (no parallel-axis shift needed if concentric). Final Answer: (B * H^3 − b * h^3) / 12

Question 9

Velocity ratio — differential wheel and axle

For a differential wheel and axle with effort wheel diameter D and axle diameters d1 (larger) and d2 (smaller), what is the velocity ratio (V.R.)?

Accepted Answer

Introduction / Context: The differential wheel and axle increases motion advantage by using two axles of slightly different diameters wound in opposite senses. Understanding its velocity ratio is key to predicting lift per turn and selecting geometry. Given Data / Assumptions: Effort wheel diameter D. Two axle diameters: d1 (larger), d2 (smaller), on the same shaft. Rope or chain winds on one axle and unwinds from the other; the load is supported such that rise equals half the net rope take-up. Concept / Approach: In one revolution, rope wound onto the larger axle = π d1; rope unwound from the smaller axle = π d2. The net rope shortening is π(d1 − d2). Since the load is supported by two parts of the rope, the load rises by half the net shortening. Step-by-Step Solution: Effort distance per turn = circumference of effort wheel = π D.Net rope take-up per turn = π(d1 − d2).Load rise per turn = (π(d1 − d2)) / 2.Velocity ratio V.R. = effort distance / load distance = (π D) / (π(d1 − d2)/2) = 2D / (d1 − d2). Verification / Alternative check: If d1 = d2, V.R. → ∞, which reflects no lift (mechanism locks), matching the physical expectation that there must be a diameter difference to produce motion. Why Other Options Are Wrong: D / (d1 − d2): Misses the factor of 2 due to the two supporting rope segments. D / (d1 + d2), 2D / (d1 + d2): Use the sum instead of the difference; incorrect geometry. (d1 − d2) / (2D): Reciprocal of the correct expression. Common Pitfalls: Forgetting the “half” due to two supporting segments; confusing the differential wheel and axle with the plain wheel and axle or the Weston differential pulley. Final Answer: 2D / (d1 − d2)

Question 10

Perfect truss (perfect frame) criterion

A planar framed structure (truss) is perfect if the number of its members m is _____ (2j − 3), where j is the number of joints.

Accepted Answer

Introduction / Context: In planar truss analysis, a “perfect” (or statically determinate and stable) truss has just enough members to maintain shape without redundancy. The classical relation is m = 2j − 3. Given Data / Assumptions: Planar pin-jointed truss. No internal mechanisms; joints are ideal pins; members carry only axial force. External reactions provide overall stability. Concept / Approach: The formula m = 2j − 3 comes from counting equations of equilibrium and unknowns for a planar pin-jointed truss with adequate external restraints. If m is less than this number, the frame is deficient (mechanism). If m exceeds, it is redundant (indeterminate by statics alone). Step-by-Step Solution: For j internal joints, joint equilibrium gives 2j scalar equations (ΣFx = 0, ΣFy = 0 at each joint).Unknown internal member forces = m; external reaction components = 3 for a stable planar structure.Statical determinacy condition: m + 3 = 2j ⇒ m = 2j − 3.Thus, the perfect truss criterion is satisfied when m equals 2j − 3. Verification / Alternative check: Example: A simple triangle truss has j = 3, m = 3. Formula gives m = 2*3 − 3 = 3, which fits the base case. Why Other Options Are Wrong: Less than: Mechanism — insufficient members to prevent distortion. Greater than: Redundant — indeterminate using only statics. Either (b) or (c): Contradicts the exact equality needed for a perfect truss. Common Pitfalls: Applying the formula to space trusses (different relation), or to frames with non-pinned joints and bending members where the count changes. Final Answer: equal to

Question 11

Engineering mechanics – properties of a couple (pure moment)
Which of the following statements correctly describe a force couple in statics and dynamics?

Accepted Answer

Introduction / Context: A couple is a fundamental concept in engineering mechanics. It consists of two equal, parallel, and opposite forces separated by a distance, creating a pure moment with no net resultant force. This question checks your understanding of the defining properties and implications of a couple in equilibrium and motion analysis. Given Data / Assumptions: Two forces are equal in magnitude, opposite in direction, and non-collinear. The perpendicular distance between their lines of action is constant. Rigid-body assumptions; forces are coplanar for simplicity. Concept / Approach: The net force of a couple is zero because forces cancel vectorially. However, they produce a moment M = F * d about any point. Since the resultant force is zero, the moment of a couple is free vector: its magnitude and sense are independent of the moment center. A single force cannot nullify a pure moment; only another couple of equal magnitude and opposite sense can. Step-by-Step Solution: Show force summation: ΣF = +F − F = 0.Compute moment about an arbitrary point O: ΣM_O = F * d (taking proper sign). This value is the same about any O for a given couple.For balance, require ΣF = 0 and ΣM = 0. A single force would violate ΣF = 0; hence, only an opposite couple can balance. Verification / Alternative check: Replace a wrench (force + couple) by an equivalent system. Removing the force leaves a pure couple. To counter it, another couple of opposite sense is required; adding a single force reintroduces a resultant. Why Other Options Are Wrong: Each of A, B, and C is correct individually; the best composite choice is “All of the above”. Common Pitfalls: Assuming the couple’s moment depends on the reference point. Unlike a force’s moment, a couple’s moment is invariant with respect to the moment center. Final Answer: All of the above

Engineering Mechanics Questions