CELE Engineering Mechanics — Equilibrium of Particles and Rigid BodiesExam Answer Templates

Examiner tip

This is a recall question — examiners expect all three equations stated explicitly in symbolic form. Writing them as words alone ('sum of forces equals zero') without the sigma notation typically earns only partial credit.

Model answer

A rigid body acted upon by a coplanar force system is in equilibrium when the following three conditions are simultaneously satisfied:
ΣFx = 0 (sum of all horizontal force components is zero)
ΣFy = 0 (sum of all vertical force components is zero)
ΣM = 0 (sum of moments about any point is zero)

Answer structure

• Line 1: State that three scalar equations govern 2D equilibrium [1 mark — must name all three: ΣFx = 0, ΣFy = 0, ΣM = 0]

Scoring breakdown

Common mark deductions

• Writing only two equations (ΣFx = 0 and ΣFy = 0) and omitting ΣM = 0
• Writing 'net force = 0' without resolving into components — too vague for full credit
• Using 'ΣF = 0' without specifying x and y directions

Key phrases to include

• ΣFx = 0
• ΣFy = 0
• ΣM = 0
• coplanar
• rigid body
• simultaneously satisfied

Examiner tip

Truss members are the classic example of two-force members — mentioning this in your answer signals to the examiner that you understand the practical application.

Model answer

A two-force member is a structural element that is loaded only at two points with no other forces or couples applied along its length. For such a member to be in equilibrium, the two forces must be equal in magnitude, opposite in direction, and collinear — i.e., they act along the line joining the two points of application. A two-force member therefore carries only axial force: pure tension or pure compression.

Answer structure

• Line 1: Define the two-force member (loaded at two points only) [0.5 mark]
• Line 2: State the force direction (along the line joining the two points) and the internal force type (axial — tension or compression) [0.5 mark]

Scoring breakdown

Common mark deductions

• Saying 'a member connected at two ends' without specifying no intermediate loads
• Omitting that the force acts ALONG the line joining the two points
• Confusing a two-force member with a beam (beams carry shear and moment, not two-force members)

Key phrases to include

• loaded at two points only
• along the line joining the two points
• pure axial force
• tension or compression
• equal, opposite, and collinear

Examiner tip

Draw a quick sketch showing the three forces meeting at a point and mark the three included angles. Examiners immediately recognise that you understand the angle definition correctly.

Model answer

Lami's Theorem applies when exactly three concurrent coplanar forces act on a particle in equilibrium. The theorem states:

F₁/sin α₁ = F₂/sin α₂ = F₃/sin α₃

where F₁, F₂, and F₃ are the magnitudes of the three forces, and α₁, α₂, α₃ are the angles between each pair of the OTHER two forces (i.e., α₁ is the angle between F₂ and F₃, α₂ is the angle between F₁ and F₃, and α₃ is the angle between F₁ and F₂).

Condition for application: The theorem is valid only when exactly three concurrent forces hold a particle in equilibrium.

Answer structure

• Line 1: State the condition — exactly three concurrent coplanar forces in equilibrium [1 mark]
• Line 2: Write the formula F₁/sin α₁ = F₂/sin α₂ = F₃/sin α₃ and define what each angle represents (angle between the OTHER two forces) [1 mark]

Scoring breakdown

Common mark deductions

• Defining the angles as the angle each force makes with the horizontal — this is the most common and costliest error
• Using the formula as a sum instead of a ratio (e.g., F₁ sin α₁ + F₂ sin α₂ = 0)
• Not stating the condition that there must be exactly three forces

Key phrases to include

• three concurrent forces
• particle in equilibrium
• F₁/sin α₁ = F₂/sin α₂ = F₃/sin α₃
• angle between the other two forces
• coplanar

Examiner tip

In board problems, always check the total number of unknowns against the available equations before solving. Pin (2) + Roller (1) = 3 unknowns = 3 equations → statically determinate. This check takes 5 seconds and prevents wasted effort on indeterminate structures.

Model answer

Pin Support: A pin (hinge) support prevents translation in both the horizontal and vertical directions but allows rotation. It therefore provides TWO reaction components: a horizontal reaction (Hₐ) and a vertical reaction (Vₐ). Total: 2 unknowns.

Roller Support: A roller support prevents translation only in the direction perpendicular to the rolling surface (typically vertical) but allows both horizontal translation and rotation. It therefore provides ONE reaction component: a vertical reaction (V_B) normal to the surface. Total: 1 unknown.

Key difference: A pin restrains 2 degrees of freedom; a roller restrains 1 degree of freedom. Together (pin + roller), a simply supported beam has exactly 3 unknowns, matching the 3 available equilibrium equations — making it statically determinate.

Answer structure

• Line 1: Define pin support — prevents translation in x and y, allows rotation — provides Hₐ and Vₐ (2 reactions) [1 mark]
• Line 2: Define roller support — prevents translation in one direction (normal), allows horizontal translation and rotation — provides 1 reaction [1 mark]

Scoring breakdown

Common mark deductions

• Saying a roller provides zero reactions — it always provides one normal reaction
• Saying a pin prevents rotation — pins always allow free rotation
• Not specifying the number of unknown reactions for each support type

Key phrases to include

• pin prevents translation in both directions
• pin allows rotation
• two reaction components
• roller provides one normal reaction
• statically determinate

Examiner tip

This type of problem can also be solved using Lami's Theorem as a quick check: the three angles between force pairs are 240°, 120°, and... wait — always verify that the three Lami angles sum to 360° as a built-in check before applying the formula.

Model answer

GIVEN:
Weight W = 400 N (acting downward)
Cord 1: θ₁ = 60° above horizontal (left)
Cord 2: θ₂ = 30° above horizontal (right)

FBD of the junction point:
Three forces: T₁ (up-left at 60°), T₂ (up-right at 30°), W = 400 N (downward)

SIGN CONVENTION: Rightward (+x), Upward (+y)

EQUILIBRIUM EQUATIONS:
ΣFx = 0: −T₁ cos 60° + T₂ cos 30° = 0
→ T₂ (0.866) = T₁ (0.500)
→ T₂ = 0.5774 T₁ … (1)

ΣFy = 0: T₁ sin 60° + T₂ sin 30° − 400 = 0
→ 0.866 T₁ + 0.5774 T₁ (0.500) = 400
→ 0.866 T₁ + 0.2887 T₁ = 400
→ 1.1547 T₁ = 400
→ T₁ = 346.4 N

From (1): T₂ = 0.5774 × 346.4 = 200.0 N

CHECK (ΣFy = 0):
T₁ sin 60° + T₂ sin 30° = 346.4(0.866) + 200.0(0.500)
= 300.0 + 100.0 = 400 N ✓

ANSWER:
T₁ = 346.4 N
T₂ = 200.0 N

Answer structure

• Step 1: List given data and draw the FBD of the junction point showing all three forces with angles [1 mark]
• Step 2: Write ΣFx = 0 and ΣFy = 0, substitute values, and solve simultaneously for T₁ and T₂ [1 mark]
• Step 3: State final answers with units and perform verification check [1 mark]

Scoring breakdown

Common mark deductions

• Using wrong angle (measuring from vertical instead of horizontal or vice versa) — corrupts both equations
• Forgetting the weight W = 400 N in the ΣFy equation
• Not showing the FBD — loses the first mark even if the calculation is correct
• Not including units in the final answer

Key phrases to include

• FBD of junction point
• ΣFx = 0
• ΣFy = 0
• cos 60°, sin 60°, cos 30°, sin 30°
• T₁ = 346.4 N, T₂ = 200.0 N
• verification check

Examiner tip

Always verify that your three Lami angles sum to exactly 360°. If they do not, you have measured the angles incorrectly. This self-check takes 5 seconds and prevents losing all 3 marks.

Model answer

GIVEN:
W = 600 N (downward)
Cord 1: 45° to the vertical → 45° from vertical = 45° from horizontal on the left side
Cord 2: horizontal (rightward along the ceiling)

IDENTIFY THE THREE FORCES AND ANGLES BETWEEN THEM:
Force W = 600 N acts downward (270° from positive x-axis)
Force T₁ acts up-left at 45° from vertical = 135° from positive x-axis
Force T₂ acts horizontally to the right = 0° from positive x-axis

Angle between T₂ and W (α₁, opposite T₁):
From 0° to 270° going counterclockwise = 270° — but we take the interior angle
α₁ = 360° − 270° = 90°...
Re-approach using the standard Lami convention:
α₁ (opposite T₁) = angle between T₂ (0°) and W (270°): |270° − 0°| = 270° → reflex; use 360° − 270° = 90°...
Correct approach: draw all three vectors tip-to-tail in the direction they ACT.
T₁ acts at 135° from +x, T₂ acts at 0° from +x, W acts at 270° from +x.
α₁ (between T₂ and W) = 270° − 0° = 270°? No — use the angle between the two force DIRECTIONS:
angle between T₂ and W = 270° − 0° = 270° (reflex) → actual angle = 360° − 270° = 90°
Wait — Lami angles are the angles BETWEEN adjacent forces measured going around the point.
Going clockwise: T₂(0°) → T₁(135°): gap = 135°
T₁(135°) → W(270°): gap = 135°
W(270°) → T₂(360°=0°): gap = 90°
Check: 135° + 135° + 90° = 360° ✓

APPLY LAMI'S THEOREM:
W/sin(α_W) = T₁/sin(α_T₁) = T₂/sin(α_T₂)
where α_W = angle opposite W = 135°, α_T₁ = 90°, α_T₂ = 135°

600/sin 135° = T₁/sin 90° = T₂/sin 135°
600/0.7071 = T₁/1.0000 = T₂/0.7071
848.5 = T₁ = T₂/0.7071

T₁ = 848.5 × 1.0000 = 848.5 N
T₂ = 848.5 × 0.7071 = 600.0 N

CHECK (ΣFy = 0):
T₁ sin 45° − W = 848.5(0.7071) − 600 ≈ 600 − 600 = 0 ✓

ANSWER:
T₁ = 848.5 N
T₂ = 600.0 N

Answer structure

• Step 1: Identify the three forces, their directions in degrees, and calculate the three Lami angles (must sum to 360°) [1 mark]
• Step 2: Apply Lami's Theorem ratio and substitute the known weight to find T₁ and T₂ [1 mark]
• Step 3: State answers with units and perform equilibrium check [1 mark]

Scoring breakdown

Common mark deductions

• Using the angle the force makes with the x-axis as the Lami angle instead of the angle between the other two forces
• Lami angles not summing to 360° — automatic red flag for examiner
• Applying Lami's Theorem to a system with more than three forces

Key phrases to include

• angle between the other two forces
• three Lami angles sum to 360°
• F/sin α = constant
• T₁ = 848.5 N
• T₂ = 600.0 N

Examiner tip

Always take moments about the support with the most unknown reactions (the pin, in this case) to solve for the roller reaction in a single equation. Then use ΣFy = 0 for the remaining vertical reaction. This two-step approach is what PRC board examiners consider the 'standard' solution path.

Model answer

GIVEN:
Span L = 8 m
Point load P = 40 kN at x = 3 m from A
UDL w = 15 kN/m over full span (0 to 8 m)
Support A: pin (Hₐ, Vₐ)
Support B: roller (V_B)

FBD:
Draw beam AB (8 m). At A: reactions Hₐ (horizontal) and Vₐ (vertical upward). At B: reaction V_B (vertical upward). Applied: 40 kN downward at x = 3 m; UDL 15 kN/m across full 8 m.

REPLACE UDL WITH RESULTANT:
F_UDL = 15 kN/m × 8 m = 120 kN
Acts at centroid = 8/2 = 4 m from A

SIGN CONVENTION: Upward (+), Rightward (+), CCW moments (+), moments taken about A

STEP 1 — Horizontal equilibrium:
ΣFx = 0: Hₐ = 0 (no horizontal applied load)
→ Hₐ = 0

STEP 2 — Moment about A (eliminates Vₐ):
ΣMₐ = 0:
+V_B(8) − 40(3) − 120(4) = 0
8 V_B = 120 + 480 = 600
V_B = 75.0 kN ↑

STEP 3 — Vertical equilibrium:
ΣFy = 0:
Vₐ + V_B − 40 − 120 = 0
Vₐ = 160 − 75.0
Vₐ = 85.0 kN ↑

VERIFICATION — ΣM_B = 0 (check):
−Vₐ(8) + 40(8−3) + 120(8−4) = 0?
−85(8) + 40(5) + 120(4) = −680 + 200 + 480 = 0 ✓

FINAL ANSWERS:
Hₐ = 0
Vₐ = 85.0 kN (upward)
V_B = 75.0 kN (upward)

Answer structure

• Step 1: Draw labeled FBD showing all loads, reactions, and dimensions [1 mark]
• Step 2: Replace UDL with its resultant (F = wL = 120 kN at 4 m from A) [1 mark]
• Step 3: Apply ΣFx = 0 → Hₐ = 0 [0.5 mark]
• Step 4: Apply ΣMₐ = 0 → V_B = 75.0 kN [1.5 marks]
• Step 5: Apply ΣFy = 0 → Vₐ = 85.0 kN [0.5 mark]
• Step 6: Verify using ΣM_B = 0 and state final answers with units [0.5 mark]

Scoring breakdown

Common mark deductions

• Not converting the UDL to a resultant force — using wL without placing it at the centroid loses 1–2 marks
• Taking the UDL resultant at the wrong position (using 8 m from A instead of 4 m from A)
• Not drawing the FBD at all — automatic loss of FBD mark
• Sign errors in the moment equation (adding a load moment instead of subtracting)
• Forgetting Hₐ — always check ΣFx = 0 even when the answer is zero

Key phrases to include

• FBD with all reactions labeled
• UDL resultant = wL at L/2
• ΣMₐ = 0 to find V_B
• ΣFy = 0 to find Vₐ
• Vₐ = 85.0 kN, V_B = 75.0 kN
• verification ΣM_B = 0

Examiner tip

The fixed-end moment is the most commonly forgotten reaction in PRC board problems. Always draw three arrows at a fixed support: one for the vertical force, one for the horizontal force, and one curved arrow for the moment. This habit will never let you forget Mₐ.

Model answer

GIVEN:
Cantilever length L = 4 m
Point load P = 25 kN at free end B (downward)
UDL w = 8 kN/m over full 4 m length (downward)
Fixed support at A: provides Vₐ (vertical), Hₐ (horizontal), Mₐ (moment)

FBD:
Draw beam AB fixed at A. At A: Vₐ upward, Hₐ rightward, Mₐ CCW (assumed). Applied: 25 kN downward at B (x = 4 m from A); UDL 8 kN/m over full length.

REPLACE UDL:
F_UDL = 8 × 4 = 32 kN downward at centroid = 4/2 = 2 m from A

SIGN CONVENTION: Upward (+y), Rightward (+x), CCW (+M)

STEP 1 — Horizontal equilibrium:
ΣFx = 0: Hₐ = 0 (no horizontal loads applied)

STEP 2 — Vertical equilibrium:
ΣFy = 0: Vₐ − 25 − 32 = 0
Vₐ = 57 kN ↑

STEP 3 — Moment equilibrium about A:
ΣMₐ = 0: Mₐ − 25(4) − 32(2) = 0
Mₐ = 100 + 64 = 164 kN·m (CCW / hogging at fixed end)

VERIFICATION — ΣM about B:
Mₐ − Vₐ(4) + F_UDL(4 − 2) = 164 − 57(4) + 32(2) = 164 − 228 + 64 = 0 ✓

FINAL ANSWERS:
Hₐ = 0
Vₐ = 57 kN (upward)
Mₐ = 164 kN·m (CCW / hogging)

Answer structure

• Step 1: Draw labeled FBD showing fixed-end reactions Vₐ, Hₐ, Mₐ and all applied loads [1 mark]
• Step 2: Compute UDL resultant 32 kN at 2 m from A [0.5 mark]
• Step 3: ΣFx = 0 → Hₐ = 0 [0.5 mark]
• Step 4: ΣFy = 0 → Vₐ = 57 kN [1 mark]
• Step 5: ΣMₐ = 0 → Mₐ = 164 kN·m [1.5 marks]
• Step 6: Verify and state all answers with correct units [0.5 mark]

Scoring breakdown

Common mark deductions

• Drawing only two reactions at a fixed support (forgetting the fixed-end moment Mₐ) — loses FBD mark immediately
• Taking UDL resultant at 4 m from A (end) instead of 2 m (centroid) — wrong moment arm
• Not including units for the moment (writing '164' without 'kN·m')
• Assuming Mₐ = 0 as if the fixed end is a pin

Key phrases to include

• fixed support provides Vₐ, Hₐ, and Mₐ
• F_UDL = wL at L/2
• ΣMₐ = 0
• Vₐ = 57 kN
• Mₐ = 164 kN·m
• hogging moment at fixed end

Examiner tip

In board exams, the instruction 'Draw the FBD' is worth marks even in numerical problems. A complete FBD — isolation, correct reactions, all loads, all dimensions — is worth at least 1 mark separate from the numerical solution.

Model answer

FREE-BODY DIAGRAM (FBD): An FBD is a simplified sketch of a body (or portion of a structure) that has been isolated from all surrounding constraints, with every external force and reaction clearly shown. It is the foundational tool for applying equilibrium equations.

STEPS TO DRAW AN FBD FOR A SIMPLY SUPPORTED BEAM:

1. Isolate the beam: draw the beam as a single free line segment, detached from all supports.
2. Remove supports and replace with reactions:
   — At the pin (A): draw Hₐ (horizontal) and Vₐ (vertical).
   — At the roller (B): draw V_B (vertical, perpendicular to the rolling surface).
3. Apply all external loads in their correct positions and directions (point loads as single arrows, UDL as distributed arrows with the resultant labeled).
4. Show all dimensions (span, load positions) to enable moment arm calculations.
5. Assume a positive direction convention and label all unknown reactions with assumed directions (if the solved value is negative, the force acts opposite to the assumed direction).

Answer structure

• Part 1: Define FBD — isolated body with all external forces and reactions shown [1 mark]
• Part 2: List at least 4 correct steps in drawing an FBD for a simply supported beam (isolation, replace supports with reactions, apply loads, show dimensions) [2 marks]

Scoring breakdown

Common mark deductions

• Defining FBD without mentioning that the body is isolated (detached from supports)
• Only listing applied loads and forgetting to mention support reactions in the FBD
• Not specifying the difference in reactions between pin and roller supports
• Omitting dimensions from the FBD description

Key phrases to include

• isolated from constraints
• every external force shown
• pin: two reactions (Hₐ, Vₐ)
• roller: one normal reaction (V_B)
• dimensions for moment arms
• sign convention

Examiner tip

The phrase 'statically determinate' signals to the examiner that you understand the deeper implication of the three-equation limit. Always include it in your answer.

Model answer

In a 2D (coplanar) rigid body equilibrium problem, the maximum number of unknowns that can be solved is THREE. This is because there are exactly three independent scalar equilibrium equations available: ΣFx = 0, ΣFy = 0, and ΣM = 0. Three equations can uniquely solve three unknowns — making the structure statically determinate. If the number of unknowns exceeds three, the structure is statically indeterminate and requires compatibility equations beyond statics.

Answer structure

• Line 1: State the answer — 3 unknowns — and name the three equations (ΣFx = 0, ΣFy = 0, ΣM = 0) [1 mark]

Scoring breakdown

Common mark deductions

• Saying '2 unknowns' — confusing particle equilibrium (2 equations for a point) with rigid body equilibrium (3 equations)
• Not justifying by naming the three equilibrium equations

Key phrases to include

• three unknowns maximum
• ΣFx = 0, ΣFy = 0, ΣM = 0
• three independent equations
• statically determinate

Examiner tip

The three-force member principle is a powerful shortcut on PRC board problems — it lets you determine the direction of an unknown reaction from geometry alone, converting a two-unknown problem into a one-unknown problem solvable by Lami's Theorem.

Model answer

THREE-FORCE MEMBER PRINCIPLE:
A rigid body acted upon by exactly three forces (and no couples) is in equilibrium only if the three forces are either:
(a) Concurrent — all three forces pass through a single common point, OR
(b) Parallel — all three forces are parallel (a special case where the concurrent point is at infinity).

APPLICATION TO FIND AN UNKNOWN REACTION DIRECTION:
If two of the three forces are known in both magnitude and direction (e.g., the applied load and the reaction at a roller), the line of action of the third force (e.g., the reaction at a pin) must pass through the point where the first two forces intersect. This geometrically determines the direction of the unknown pin reaction without resolving into components, saving significant calculation time.

Example: A bar loaded at one point with a vertical force and supported by a pin at A and a cable at B — the cable direction and the load direction intersect at a point P; the pin reaction at A must pass through P.

Answer structure

• Part 1: State the three-force member principle — concurrent or parallel [1 mark]
• Part 2: Explain how to use it to find the direction of an unknown reaction (trace the intersection of the known forces) [1 mark]

Scoring breakdown

Common mark deductions

• Saying 'three-force member' carries only three types of internal forces — this confuses it with a three-force body principle
• Not mentioning the parallel case as a valid alternative
• Not explaining how the principle is used to find an unknown direction

Key phrases to include

• three forces in equilibrium
• concurrent at a single point
• or parallel
• line of action of unknown passes through intersection
• geometrically determines direction

Examiner tip

For uniform bars and beams, the self-weight always acts at the geometric centroid (midpoint for a prismatic member). This is a classic PRC board trap — always check if 'uniform' appears in the problem statement.

Model answer

GIVEN:
Bar weight W = 300 N (acts at midpoint, x = 1 m from A)
Length L = 2 m
Cable at B: T at 35° above horizontal
Pin at A: Hₐ (horizontal), Vₐ (vertical)

FBD:
Horizontal bar from A (left, pin) to B (right, cable). W = 300 N downward at x = 1 m. Cable tension T at B at 35° above horizontal. Pin at A: Hₐ rightward (assumed), Vₐ upward (assumed).

SIGN CONVENTION: Rightward (+x), Upward (+y), CCW (+M)

STEP 1 — ΣMₐ = 0 (eliminates Hₐ and Vₐ):
T sin 35°(2) − 300(1) = 0
T(0.5736)(2) = 300
1.1472 T = 300
T = 261.6 N

STEP 2 — ΣFx = 0:
Hₐ − T cos 35° = 0
Hₐ = 261.6 × cos 35° = 261.6 × 0.8192 = 214.3 N (rightward → actually leftward to balance cable)
Correct: cable pulls B to the right; horizontal component of T at the pin must resist.
Hₐ = T cos 35° = 261.6(0.8192) = 214.3 N ← (leftward, away from wall)
SIGN CHECK: ΣFx = Hₐ − T cos35° = 0 → Hₐ = 214.3 N directed toward the right (if cable goes up-right, its horizontal component is rightward; pin reacts leftward at the wall — but here pin is at A and bar is free). Standard: Hₐ = 214.3 N (the reaction on the bar at A from the pin is to the right if T pulls bar at B to the right).
Re-state: Hₐ = T cos 35° = 214.3 N (directed away from wall / to the right).

STEP 3 — ΣFy = 0:
Vₐ + T sin 35° − 300 = 0
Vₐ = 300 − 261.6(0.5736) = 300 − 150.0 = 150.0 N ↑

STEP 4 — Magnitude of pin reaction Rₐ:
Rₐ = √(Hₐ² + Vₐ²) = √(214.3² + 150.0²) = √(45924 + 22500) = √68424 = 261.6 N

Direction: θ = arctan(Vₐ/Hₐ) = arctan(150.0/214.3) = arctan(0.6999) = 35.0°
So Rₐ acts at 35° above the horizontal.

INTERPRETATION: The pin reaction magnitude equals T — a classic result confirming the three-force member geometry.

FINAL ANSWERS:
Cable tension T = 261.6 N
Pin reaction Rₐ = 261.6 N at 35° above horizontal (directed away from B)

Answer structure

• Step 1: Labeled FBD showing W at midpoint, cable T at B (35°), pin reactions Hₐ and Vₐ [1 mark]
• Step 2: ΣMₐ = 0 → T = 261.6 N [1.5 marks]
• Step 3: ΣFx = 0 → Hₐ = 214.3 N [0.5 mark]
• Step 4: ΣFy = 0 → Vₐ = 150.0 N [0.5 mark]
• Step 5: Compute Rₐ = √(Hₐ² + Vₐ²) and direction angle; state final answers [1.5 marks]

Scoring breakdown

Common mark deductions

• Placing the weight at the end of the bar instead of at its midpoint (uniform bar → weight at L/2)
• Using cos 35° instead of sin 35° in the moment equation (moment is caused by the vertical component of T)
• Not computing the magnitude and direction of the pin reaction — just leaving Hₐ and Vₐ as separate components

Key phrases to include

• weight acts at midpoint
• ΣMₐ = 0 eliminates pin reactions
• T sin 35° × 2 = 300 × 1
• T = 261.6 N
• Rₐ = √(Hₐ² + Vₐ²)
• direction = arctan(Vₐ/Hₐ)

Examiner tip

Two-body or multi-member problems always require isolating sub-bodies. The key decision: identify WHICH sub-body, when isolated, has the fewest unknowns. Isolating the arm here gives a single-equation solution for T. Always isolate the simpler body first.

Model answer

GIVEN:
Arm length = 1.5 m (horizontal, attached to post at base of arm junction)
Sign weight W = 500 N at free end of arm (1.5 m from post)
Cable: connects free end of arm to post 2 m above arm junction, at 40° above horizontal
Post height = 3 m (fixed base)

FBD — ENTIRE STRUCTURE (arm + post):
Fixed base reactions: H_base (horizontal), V_base (vertical), M_base (moment)
Applied load: W = 500 N downward at the tip of the arm
Cable tension T at 40° (acts on arm at 1.5 m from post)
Note: cable is internal if we consider arm + post as one body; treat cable as internal.

FBD — ARM ONLY (isolate the arm at the junction):
At junction: pin reactions Hⱼ, Vⱼ from post on arm
Cable tension T at 40° at free end (acts upward-left if cable goes up to post)
Cable direction: the cable connects arm tip to a point on the post 2 m above the arm junction.
Cable angle check: tan θ = 2/1.5 → θ = arctan(2/1.5) = arctan(1.333) = 53.1° ≠ 40°.
PROBLEM STATEMENT says θ = 40° — use given angle (40°) for calculations.

SOLVE FOR CABLE TENSION USING ΣM = 0 ABOUT ARM JUNCTION:
ΣM_junction = 0:
T sin 40°(1.5) − 500(1.5) = 0
T(0.6428)(1.5) = 500(1.5)
0.9642 T = 750
T = 777.9 N

FBD — ENTIRE STRUCTURE (post + arm) for base reactions:
External forces: W = 500 N downward at horizontal distance 1.5 m from post axis, Cable tension T = 777.9 N at 40° (cable connects arm tip to post wall — it is an internal force to the whole structure IF the cable is part of the post. If the cable is attached to a wall anchor away from the post, it is external. Assume the cable is anchored to the post — it cancels internally.)

For the overall FBD with cable as INTERNAL: only W = 500 N is external.
ΣFx = 0: H_base = 0
ΣFy = 0: V_base − 500 = 0 → V_base = 500 N ↑
ΣM_base = 0 (CCW+): M_base − 500(1.5) = 0 → M_base = 750 N·m (CCW)

Note: The moment arm for W about the base = 1.5 m (horizontal distance).
The height of the arm above the base: the arm is at some height on the post — 500 N is purely vertical so its moment about the base axis equals 500 × (horizontal offset) = 500 × 1.5 = 750 N·m. The height does not affect this result.

FINAL ANSWERS:
Cable tension T = 777.9 N
Base vertical reaction V_base = 500 N (upward)
Base horizontal reaction H_base = 0
Base fixed-end moment M_base = 750 N·m (CCW / resisting the overturning of the arm)

Answer structure

• Step 1: FBD of the arm showing W, cable T at 40°, and junction reactions [1 mark]
• Step 2: ΣM about junction = 0 → T = 777.9 N [1.5 marks]
• Step 3: FBD of entire structure for base reactions (cable is internal) [1 mark]
• Step 4: ΣFx = 0, ΣFy = 0, ΣM_base = 0 → H = 0, V = 500 N, M = 750 N·m [1.5 marks]

Scoring breakdown

Common mark deductions

• Not isolating the arm to solve for T — trying to use the full-body FBD for T and getting lost with too many unknowns
• Using cos 40° instead of sin 40° in the moment equation (vertical component of T causes the moment about the junction)
• Including cable tension as an external load in the full-body FBD (it is internal)
• Forgetting the base moment M_base at the fixed support

Key phrases to include

• FBD of arm in isolation
• ΣM about junction = 0
• T sin 40°
• T = 777.9 N
• cable internal to full structure
• M_base = 750 N·m

Do

• State the answer in the very first line — do not build up to it
• Include all required symbolic notation (ΣFx = 0, ΣFy = 0, ΣM = 0) for equilibrium-related recall questions
• Use the exact technical term (e.g., 'statically determinate', 'two-force member', 'concurrent') — synonyms often do not earn marks
• Keep it brief — 1–3 lines maximum for a 1-mark question

Don't

• Do not spend more than 2 minutes — move on immediately after answering
• Do not write long paragraphs — examiners scan for keywords, not essays
• Do not leave blank — a partially correct answer may still earn partial credit on some PRC board subtests
• Do not use vague language like 'when forces are balanced' without the mathematical statement

Do

• Structure your answer in two numbered or bulleted parts matching the two marks
• Include a labeled sketch or equation where relevant — even a small FBD sketch can earn half the marks
• State the formula before substituting numbers
• Define any symbol you introduce (e.g., 'where α₁ = angle between F₂ and F₃')

Don't

• Do not write one continuous paragraph — break into two distinct parts
• Do not skip units — 2-mark numerical answers without units typically lose 0.5–1 mark
• Do not confuse the Lami angle definition (angle between the other two forces vs. angle with an axis)
• Do not over-explain — 6 lines is the maximum; anything more wastes time

Do

• Draw and label the FBD as Step 1 — it is usually worth its own mark
• Write ΣFx = 0, ΣFy = 0, ΣM = 0 explicitly as numbered equations before substituting
• Show all arithmetic steps (no mental calculations) — the PRC board awards process marks
• Include a one-line verification at the end (e.g., 'Check: ΣFy = 85 + 75 − 60 − 100 = 0 ✓')
• State units at each step (N, kN, m, kN·m)

Don't

• Do not skip the FBD even if the problem seems simple — you risk losing 1 of 3 marks
• Do not mix up sin and cos components — write the angle and identify the correct trigonometric function each time
• Do not forget to convert UDL to resultant before taking moments
• Do not use the Lami angle as the angle a force makes with the x-axis

Do

• Write a 'GIVEN:' section listing all numerical data — this organizes your solution and earns the first partial mark
• Draw a complete, labeled FBD as a separate clear diagram — use a ruler if available
• Convert ALL distributed loads to resultants before writing any equilibrium equation
• Choose your moment center strategically — take ΣM = 0 about the support with the most unknowns to eliminate them
• Perform and show a verification check using a different equation from those used to solve (e.g., ΣM_B = 0 to verify after solving with ΣMₐ = 0)
• Box or underline your final answers clearly

Don't

• Do not start writing equations without first drawing the FBD
• Do not forget the fixed-end moment at a fixed (cantilever) support
• Do not forget Hₐ from pin supports — always include ΣFx = 0 even if the answer is zero
• Do not place a triangular load resultant at the midpoint — it acts at 1/3 from the maximum intensity end (or 2/3 from the zero end)
• Do not multiply a concentrated applied moment by a distance in ΣM — it is added directly with its correct sign
• Do not run out of time by writing unnecessarily long explanations — use equations and labeled diagrams to communicate efficiently