🌟 IGCSE Physics Topic 1.5: Forces

⚙️ 1.5.1 Effects of Forces

🔹 What can forces do?

A force is a push or pull acting on an object.
Forces can cause an object to:

• Change shape (e.g. squeezing a sponge)

• Change size (e.g. stretching a spring)

• Change speed (speed up or slow down)

• Change direction of motion

💡 Example:
When you stretch a rubber band, you apply a force that increases its length.
When you press a soft ball, it changes shape.

🔹 Load–Extension Graphs

When a force (load) is applied to stretch an elastic material like a spring or rubber band, the extension (increase in length) can be measured.

Experiment setup:

1. Hang a spring vertically from a clamp stand.

2. Measure its original length (L₀).

3. Add known weights (e.g. 1 N, 2 N, 3 N...).

4. For each weight, measure the new length (L).

5. Calculate extension (x) = L – L₀.

6. Plot a graph of load (F) on the y-axis vs extension (x) on the x-axis.

Graph features:

• Straight line region → obeys Hooke’s Law:
  ( F = kx )
  where
  F = force (N),
  k = spring constant (N/m),
  x = extension (m).

• Limit of proportionality:
  The point after which the line stops being straight.
  Beyond this, the spring does not obey Hooke’s law.

📘 Exam Tip:
They may ask you to identify the limit of proportionality on a graph or calculate k = F/x from data in the straight-line region.

🔹 Resultant of Forces (in a straight line)

If several forces act along the same line:

• If they act in the same direction, add them:
  

• If they act in opposite directions, subtract:
  

💡 Example:
If a car engine produces a forward force of 600 N and friction opposes it with 200 N,
→ Resultant force = 600 – 200 = 400 N forward.

🔹 Newton’s First Law (No Resultant Force)

“An object remains at rest or moves in a straight line at constant speed unless acted on by a resultant force.”

💡 Example:
A spacecraft in deep space continues moving at constant speed because there’s no air resistance or external force.

🔹 Newton’s Second Law (Resultant Force Causes Acceleration)

A resultant force changes an object’s velocity (speed or direction).

Formula:

where
F = force (N), m = mass (kg), a = acceleration (m/s²)

💡 Example:
A 2 kg object accelerates at 3 m/s².
Force = 2 × 3 = 6 N.

📘 Exam Tip:
Always ensure units are consistent — mass in kg and acceleration in m/s².

🔹 Friction and Drag

Friction is the force that opposes motion between two surfaces in contact.
It causes heating and wear.

Types:

• Solid friction: between solid surfaces (e.g. car tires on road)

• Drag: resistance in liquids or gases

  • In liquids → viscous drag

  • In gases → air resistance

💡 Example:
A skydiver feels strong air resistance opposing downward motion.

📘 Exam Tip:
Always show direction of friction opposite to motion in force diagrams.

🔹 Motion in a Circular Path

When an object moves in a circle, a centripetal force acts towards the center of the circle.

This force changes the direction of motion (not the speed).

Factors affecting circular motion:

(a) If force increases, speed increases (mass and radius constant).
(b) If force increases, radius decreases (mass and speed constant).
(c) If mass increases, more force is needed to maintain same speed and radius.

💡 Examples:

• A stone on a string moving in a circle.

• A car turning a corner.

⚖️ 1.5.2 Turning Effect of Forces (Moments)

🔹 What is a Moment?

The moment of a force is the turning effect of a force about a pivot.

Units: Newton-metre (N m)

💡 Example:
If you push a door 0.5 m from the hinge with 10 N:
Moment = 10 × 0.5 = 5 N m

📘 Exam Tip:
Always use the perpendicular distance from the line of action of the force to the pivot, not just any distance.

🔹 Principle of Moments

For an object in equilibrium,
Total clockwise moments = Total anticlockwise moments

💡 Example:
A seesaw balances when both children produce equal and opposite moments.

🔹 Equilibrium

An object is in equilibrium when:

• The resultant force = 0

• The resultant moment = 0

💡 Example:
A uniform beam balanced on a pivot with equal weights on both sides.

🔹 Experiment: Demonstrating No Resultant Moment

Apparatus: Metre rule, pivot, several known weights, spring balance.

Procedure:

1. Place the rule on a pivot at its center.

2. Hang weights on both sides at different distances.

3. Adjust until the rule is level.

4. Measure forces and distances.

5. Show that sum of clockwise moments = sum of anticlockwise moments.

🎯 1.5.3 Centre of Gravity (C.G.)

🔹 Definition

The centre of gravity is the point through which the entire weight of an object acts.

💡 For regular objects, it’s at the geometrical centre.
💡 For irregular shapes, it must be found experimentally.

🔹 Experiment: Finding the C.G. of a Plane Lamina

Apparatus: Cardboard (irregular shape), plumb line, pin, stand.

Procedure:

1. Suspend the lamina from a pin and let it hang freely.

2. Hang the plumb line from the same pin and mark the line on the card.

3. Repeat by hanging from two other points.

4. The intersection of the lines is the centre of gravity.

🔹 Stability and Centre of Gravity

• A low centre of gravity → more stable

• A high centre of gravity → less stable

💡 Example:

• A racing car has a low, wide base → very stable.

• A tall cupboard can topple easily → less stable.

📘 Exam Tip:
An object topples when its centre of gravity falls outside its base.

🧠 Summary Table

Concept	Key Formula	Unit
Hooke’s Law	F = kx	N, N/m, m
Newton’s Second Law	F = ma	N
Moment of a force	M = F × d	N m
Equilibrium (moments)	Σ Clockwise = Σ Anticlockwise	—

💬 Quick Recap Tips for Exams

✅ Use free-body diagrams to show all forces.
✅ Check if the system is balanced or accelerating.
✅ Watch units: convert grams → kg, cm → m.
✅ Label graphs carefully (load–extension).
✅ When asked for "direction of motion" → think of resultant force direction.
✅ Remember: moment depends on perpendicular distance only!