Welcome brilliant students!
In today’s tutorial on AllCBTs, we are solving a top recurring GCE Physics question. This one is all about moments of force — specifically involving a uniform wooden plank balanced on a pivot. This concept appears regularly in GCE Paper 2, so pay close attention.
You can watch the full class in the video below:
The Question Scenario
We’re told:
- A uniform wooden plank is 2 meters long
- It weighs 200 N
- It’s pivoted on a knife edge positioned 0.4 meters from one end
Understanding “Uniform” in Physics
“Uniform” means the weight is evenly distributed — so the center of gravity of the plank lies at its midpoint. For a 2 m plank, that’s exactly 1 meter from either end.
This is where the 200 N force acts — right at the center of the plank.
Setting Up Distances
We are told the pivot is 0.4 m from one end. Let’s call that the left end.
So we have:
- Distance from left end to pivot = 0.4 m
- Distance from pivot to center (1 m) = 1.0−0.4=0.6 m1.0 – 0.4 = 0.6 \text{ m}1.0−0.4=0.6 m
- Distance from pivot to right end = 2.0−0.4=1.6 m2.0 – 0.4 = 1.6 \text{ m}2.0−0.4=1.6 m
Objective of the Question
Calculate the downward force F that should be applied at the left end of the plank (the one closer to the pivot), so that the plank balances horizontally.
Applying the Principle of Moments
The principle of moments says: Clockwise moment=Anticlockwise moment\text{Clockwise moment} = \text{Anticlockwise moment}Clockwise moment=Anticlockwise moment
Let the unknown force be F, acting at 0.4 m from the pivot.
The 200 N weight acts at 0.6 m from the pivot on the opposite side.
Equation:
F×0.4=200×0.6F \times 0.4 = 200 \times 0.6F×0.4=200×0.6 F=200×0.60.4=1200.4=300 NF = \frac{200 \times 0.6}{0.4} = \frac{120}{0.4} = 300 \text{ N}F=0.4200×0.6=0.4120=300 N
Final Answer:
F = 300 Newtons
You need to apply a downward force of 300 N at the left end to keep the plank in perfect horizontal balance.
Watch more educational videos and past questions: https://youtube.com/@allcbts
Why This Matters in GCE Physics
This question covers:
- Centre of gravity
- Principle of moments
- Equilibrium
- Force and distance relationships
It’s one of the most common practical application questions in GCE Physics. Knowing how to draw, label, and apply forces correctly is key to scoring full marks in this section.
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Learn how to solve the GCE Physics 2025 plank balance question using the principle of moments. Step-by-step guide with diagrams, formulas, and answers.
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GCE Physics 2025, principle of moments, balancing a plank, moment of force, equilibrium in physics, how to solve GCE Physics questions, GCE practical physics
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