Ch 3 · Forces, Moments and Centre of Gravity · IGCSE Physics 0625
Topic 3 of 5 in Motion, Forces and Energy — IGCSE Physics 0625 — 20 lessons.
Introduction
A force is a push or a pull. Forces make things speed up, slow down, change direction, stretch and turn. In this lesson you will combine forces into a single resultant, link the resultant force to acceleration, investigate how springs stretch, and use moments to explain why a see-saw balances and why a bus with a low centre of gravity is hard to tip over.
Key ideas
1. What forces can do. A force can change an object's speed, its direction of motion, or its shape and size (stretching, squashing, bending).
🧑🏫 Teacher's explanation
Imagine you are playing tug-of-war. If both sides pull with 500 N, nobody moves because the forces cancel out (zero resultant). If one side pulls harder, the rope accelerates. This is the essence of forces: they either change how fast something moves, which way it moves, or how much it stretches/squashes.
📖 Definitions
🔍 Worked example — Easy
A force of 15 N is applied to a door at a perpendicular distance of 0.8 m from the hinges. Calculate the moment of the force.
🔍 Worked example — Standard
A spring has a spring constant k = 250 N/m. Calculate the extension produced by a load of 5.0 N.
🔍 Worked example — Exam-style
A uniform beam of mass 2.0 kg is pivoted at its center. A 5.0 N force is applied downwards at one end (0.5 m from pivot). Find the force required at the other end (0.5 m from pivot) to keep the beam in equilibrium.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the magnitude of the clockwise moment produced by the 4.0 N weight. [2] (b) Determine the magnitude and direction of the force required at the 80 cm mark to keep the ruler in equilibrium. [2] [4 marks]
Practical notes
Load-extension of a spring. Clamp a spring beside a vertical metre rule. Record the position of the bottom of the spring with no load. Add loads (e.g. 0.50 N steps), recording the new position each time; calculate extension. Read the rule at eye level using a fixed pointer to avoid parallax. Remove loads to check the spring returns to its original length. Plot extension against load (or load against extension, as the question requires).
Worked example
Example 1 — resultant force and acceleration. For the car in the diagram, the driving force is 3000 N and the total resistive force is 1200 N. The car has a mass of 1200 kg.
🔍 Worked example — Easy
A car of mass 1200 kg is driven by a force of 3000 N. The total resistive force (friction and air resistance) is 1200 N. Calculate the acceleration of the car.
🔍 Worked example — Standard
A spring has an original length of 5.0 cm. When a load of 2.0 N is applied, its length becomes 9.0 cm. Calculate the spring constant k in N/m.
🔍 Worked example — Exam-style
A uniform metre rule is balanced on a pivot at the 50 cm mark. A 2.0 N weight is placed at the 10 cm mark and a 1.5 N weight is placed at the 30 cm mark. Calculate the magnitude of a single force F required at the 90 cm mark to keep the rule in equilibrium.
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) 45 - 20 = 25\ N west. (2) a = F/m = 2.0/0.50 = 4.0\ m/s^2. (3) x = F/k = 1.5/25 = 0.060\ m = 6.0\ cm. (4) 40 × 0.25 = 10\ N m. (5) 500 × 0.60 = 300\ N m; d = 300/400 = 0.75\ m to the right of the pivot. (6) Towards the centre of the bend (perpendicular to the velocity); a larger force is needed at higher speed. (7) A low centre of gravity and a wide base make the car more stable: it must tilt through a larger angle before its centre of gravity moves outside the base, so it is less likely to topple.