📘 Chapter 6: Mechanical Properties of Matter – Short Answer Questions
Prepared by Muhammad Tayyab, Subject Specialist Physics, Govt Christian High School Daska. Based on PECTAA 2026 syllabus (National Curriculum 2023).
📖 What's Inside: This section covers short answer questions from Chapter 6 Mechanical Properties of Matter including Elasticity, Pressure, Hooke's Law, Pascal's Law, Barometer, Manometer, and Hydraulic Systems. Each question is presented with a detailed answer as per the official PECTAA 2026 Physics curriculum. Perfect for Punjab Boards (Lahore, Gujranwala, Multan, etc.) and all BISE boards across Pakistan.
📚 Related Resources – Chapter 6: Mechanical Properties of Matter
📋 Exercise Short Qs
📝 Short Qs
📖 Comprehensive Qs
🔢 Numerical Problems
✅ MCQs
✏️ Constructed Response
Mechanical Properties of Matter covers elasticity, pressure, Hooke's law, Pascal's law, and atmospheric pressure.
📖 Short Answer Questions & Answers (PECTAA 2026)
1 What is a deforming force?
Answer:
An external force applied on an object can change its size or shape. Such a force is known as a deforming force.
✅ External force that changes size or shape
2 What is meant by elasticity?
Answer:
An object is said to be elastic, if after removal of the deforming force, it restores to its original size and shape. This property of the material is known as elasticity.
✅ Ability to regain original shape after deforming force is removed
3 What is elastic limit?
Answer:
Most of the materials are elastic up to a certain limit known as elastic limit. Beyond the elastic limit, the change becomes permanent, and the object does not regain its original shape or size even after the removal of the deforming force.
✅ Maximum limit up to which material remains elastic
4 What are inelastic materials?
Answer:
Some materials such as clay dough or plasticine do not return to their original shape after the removal of the deforming force. They are known as inelastic materials.
✅ Materials that do not regain original shape (e.g., clay, plasticine)
5 State Hooke's Law and define spring constant.
Answer:
Hooke's Law: If force \(F\) is applied on a spring to stretch or compress it, the extension or compression \(x\) has been found directly proportional to the applied force within the elastic limit. Thus,
\[ F \propto x \]
\[ F = kx \]
\[ k = \frac{F}{x} \]
OR
Within the elastic limit of a helical spring, the extension or compression in it is directly proportional to the applied force. This is known as Hooke's law.
Spring Constant: Spring constant is defined as the ratio of applied force to the change in length of spring. Spring constant written as
\[ k = \frac{F}{x} \]
✅ Hooke's Law: \(F \propto x\) | Spring constant: \(k = \frac{F}{x}\)
6 What is spring constant?
Answer:
Spring constant is defined as the ratio of applied force to the change in length of spring. Spring constant written as
\[ k = \frac{F}{x} \]
In fact, it is a measure of stiffness of the spring. The greater the value of spring constant, the greater will be the stiffness or strength of the spring. Its unit is \(\text{Nm}^{-1}\).
✅ \(k = \frac{F}{x}\) | Unit: \(\text{Nm}^{-1}\) | Measure of stiffness
7 What does the gradient or slope of a force-extension graph represent?
Answer:
The gradient or slope of force-extension graph is a measure of spring constant \(k\).
✅ Slope = spring constant \(k\)
8 How does a galvanometer make use of Hooke's law?
Answer:
Galvanometer is a current detecting device. It makes use of a tiny spring called hair spring which provides electrical connections to the galvanometer coil and also restores the pointer back to zero position. The deflection of the pointer is proportional to the current flowing through it within the range.
✅ Hair spring provides restoring torque; deflection \(\propto\) current
9 What is meant by density? What is its SI unit?
Answer:
Density of a substance is defined as its mass per unit volume.
\[ \rho = \frac{m}{v} \]
SI unit of density is kilogram per cubic meter (\(\text{kgm}^{-3}\)). Other unit also in use is \(\text{gcm}^{-3}\).
✅ \(\rho = \frac{m}{V}\) | SI unit: \(\text{kgm}^{-3}\)
10 Define pressure and write its formula and unit.
Answer:
Pressure is defined as the force exerted normally on unit area of an object.
\[ P = \frac{F}{A} \]
Pressure is a scalar quantity. In SI units, the unit of pressure is \(\text{Nm}^{-2}\) also called pascal (\(\text{Pa}\)).
\[ \text{Nm}^{-2} = 1\,\text{Pa} \]
✅ \(P = \frac{F}{A}\) | SI unit: \(\text{Pa}\) (\(\text{Nm}^{-2}\))
11 Why is the edge of a chopper made very sharp?
Answer:
The edge of the blade of a chopper is made very sharp because its small contact area produces very high pressure when force is applied on the handle. Therefore, the object is easily cut.
✅ Small area → high pressure → easy cutting
12 Why does a thumb pin pierce easily into a wooden board?
Answer:
The end of a thumb pin is made very sharp so that its contact area is very small. When force is applied at the top, the pressure at the end of the pin becomes so high that it pierces into the wooden board.
✅ Sharp end → small area → high pressure → pierces easily
13 Why is walking on pebbles painful compared to walking on flat ground?
Answer:
When the ground is flat, this reaction force is spread over the whole area of the foot and the pressure due to reaction force is not painful. But when we walk on pebbles, the contact area is reduced. Then the pressure due to reaction force becomes so high that it becomes painful.
✅ Small contact area → high pressure → painful
14 Why do heavy animals like elephants have thick legs and large flat feet?
Answer:
Heavy animals like elephant have thick legs and large flat feet so that due to large contact area, pressure becomes less otherwise, their bones would not tolerate the pressure.
✅ Large area → less pressure → protects bones
15 Why does liquid pressure increase with depth?
Answer:
Equation \(P = \rho gh\) shows that pressure in a liquid increase with depth.
The value of pressure depends on the depth and density of the liquid.
✅ \(P = \rho gh\) | Pressure increases with depth
16 Define atmospheric pressure.
Answer:
The atmosphere exerts pressure on the surface of the Earth and on everything on the Earth. This pressure is called atmospheric pressure.
✅ Pressure exerted by the atmosphere on Earth's surface
17 Why does the atmosphere exert pressure?
Answer:
The molecules of air are always in motion. They collide with one another and with all other objects coming in their way. Thus, they exert force on the objects. This force per unit area is the atmospheric pressure.
✅ Air molecules collide with objects → exert force → pressure
18 Why do we not feel the enormous atmospheric pressure acting on our bodies?
Answer:
We do not feel it because practically all the bodies have air inside them. As atmospheric pressure acts in all directions, so it balances the pressure inside.
✅ Pressure inside body balances atmospheric pressure
19 How does atmospheric pressure vary with height?
Answer:
As we go up in the air, atmospheric pressure decreases because the density of air decreases continuously with altitude.
✅ Pressure decreases with altitude (density decreases)
20 How does atmospheric pressure indicate changes in weather?
Answer:
Atmospheric pressure becomes low in cloudy regions because water vapours decrease the density of air. A fall in atmospheric pressure often means that rain clouds are on the way and the rain is to follow.
✅ Fall in pressure → rain clouds approaching
21 What is a barometer?
Answer:
Atmospheric pressure is usually measured by the height of mercury column which it can support. Instruments which measure the atmospheric pressure are called barometers.
✅ Instrument used to measure atmospheric pressure
22 What is a manometer?
Answer:
A simple manometer consists of a U-shaped glass tube which contains mercury. It is an instrument used to measure unknown pressure relative to atmospheric pressure.
✅ U-shaped tube instrument to measure gas pressure relative to atmosphere
23 What is Pascal's Law? State its advantages and applications.
Answer:
Pascal's Law: When pressure is applied at one point in an enclosed fluid, it is transmitted equally to all parts of the fluid without loss.
Advantages of hydraulic systems based on Pascal's Law:
- (i) Liquids do not absorb any of the supplied energy.
- (ii) They can move heavy loads and produce great forces due to their incompressibility.
Applications of hydraulic systems:
- (i) Hydraulic press
- (ii) Car lift at service stations
- (iii) Hydraulic brakes in vehicles
✅ Pressure transmitted equally in all directions | Applications: Hydraulic press, car lift, brakes
24 How do hydraulic brakes work?
Answer:
When pedal is pushed down, the piston applies pressure on the liquid in the master cylinder. The liquid pressure is transmitted equally to all the larger pistons of other cylinders. This pressure causes these pistons to move outward pressing the brake pads towards brake discs or brake drums. Force of friction between the pads and discs or drums slows down the vehicle.
✅ Pressure transmitted equally → brake pads press discs/drums → friction slows vehicle
📐 Key Concepts – Mechanical Properties of Matter (Short Questions)
Hooke's Law: \(F = kx\)
Spring Constant: \(k = \frac{F}{x}\)
Density: \(\rho = \frac{m}{V}\)
Pressure: \(P = \frac{F}{A}\)
Liquid Pressure: \(P = \rho gh\)
Pascal's Law: Pressure transmitted equally in all directions
💡 Exam Tip:
For short answer questions, write concise and accurate answers. Use definitions, formulas, and examples where required. These questions test your understanding of fundamental concepts like elasticity, pressure, Hooke's law, Pascal's law, barometer, and manometer. These questions follow the PECTAA 2026 pattern and are prepared by Subject Specialist Muhammad Tayyab.
Chapter 1Physical Quantities and Measurements
Chapter 2Kinematics
Chapter 3Dynamics
Chapter 4Turning Effect of Forces
Chapter 5Work and Energy
Chapter 6✨ Mechanical Properties of Matter
Chapter 7Thermal Properties of Matter
Chapter 8Magnetism
Chapter 9Nature of Science
📖 Complete syllabus coverage for Class 9 Physics (PECTAA 2026) – Chapters 1 to 9
Created by Hira Science Academy | Aligned with PECTAA 2026 Syllabus