📘 Chapter 3: Dynamics – Short 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 questions from Chapter 3 Dynamics including dynamics, force, types of forces, friction, drag, thrust, normal force, air resistance, tension, elastic force, gravitational force, electromagnetic force, magnetic force, fundamental forces, Newton's laws of motion, inertia, mass and weight, gravitational field, force meter, dissipative effect of friction, shooting stars, tread pattern on tyres, sliding friction, terminal velocity, rolling friction, methods to reduce friction, momentum, impulse, and conservation of momentum. Each question is presented with a precise 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 3: Dynamics
📋 Exercise Short Qs
📝 Short Qs
📖 Comprehensive Qs
🔢 Numerical Problems
✅ MCQs
✏️ Constructed Response
Dynamics covers force, Newton's laws, momentum, friction, and circular motion.
📑 Quick Jump to Questions
📝 Short Questions & Answers (PECTAA 2026)
1 What is dynamics?
Answer: Dynamics is concerned with the forces that produce change in the motions of bodies.
✅ Dynamics deals with forces and the changes they produce in motion.
2 Define force?
Answer: A force is a push or pull that can start, stop, or change the magnitude and direction of a body's velocity. For example, opening a door (pushing or pulling), lifting and moving a wheelbarrow.
✅ Force: a push or pull that changes velocity.
3 What are the two major types of forces?
Answer: There are two major types of forces: (i) Contact forces (ii) Non-contact forces
Contact Forces: A contact force is a force that is exerted by one object on the other at the point of contact. For example, friction, drag, thrust, normal force, air resistance, tension force, elastic force.
Non-contact Forces: A non-contact force is defined as the force between two objects which are not in physical contact. For example, gravitational force, electrostatic force, magnetic force etc.
✅ Contact (friction, normal, tension) and Non-contact (gravitational, electrostatic, magnetic).
4 Define friction.
Answer: It is the force that resists motion when the surface of one object comes in contact with the surface of another.
✅ Friction resists motion between surfaces in contact.
5 What is drag?
Answer: The drag force is the resistant force caused by the motion of a body through a fluid. It acts opposite to the relative motion of any object moving with respect to surrounding fluid.
✅ Drag: resistant force of a fluid against motion.
6 What is thrust?
Answer: It is an upward force exerted by a liquid on an object immersed in it. For example, when we try to immerse an object in water, we feel an upward force exerted on the object.
✅ Thrust: upward force by a liquid on an immersed object.
7 What is normal force?
Answer: It is the force of reaction exerted by the surface on an object lying on it. This force acts outward and perpendicular to the surface.
✅ Normal force: reaction force perpendicular to the surface.
8 What is air resistance?
Answer: It is the resistance (opposition) offered by air when an object falls through it.
✅ Air resistance: opposition by air to a falling object.
9 What is tension force?
Answer: It is the force experienced by a rope when a person or load pulls it.
✅ Tension: force in a rope when pulled.
10 What is elastic force?
Answer: It is a force that brings certain materials back to their original shape after being deformed. For examples, rubber bands, springs, and trampolines.
✅ Elastic force: restores shape after deformation.
11 What is gravitational force? State Newton's law of gravitation.
Answer: Gravitational force is an attractive force that exists among all bodies which have mass. It is a long-range force given by Newton's law of gravitation:
\[F = G\frac{m_1m_2}{r^2}\]
where \(m_1\) and \(m_2\) are the masses, \(r\) is the distance between them, and \(G = 6.673 \times 10^{-11} Nm^2kg^{-2}\) is the gravitational constant. For example, it is the force that pulls an apple to the ground and keeps planets in orbit around the Sun.
✅ Gravitational force: attractive force between masses. \(F = G\frac{m_1m_2}{r^2}\).
12 How does gravitational force keep celestial bodies in their positions?
Answer: The Sun's gravitational force keeps the Earth and all other planets of our solar system in fixed orbits. Similarly, the gravitational force of the Earth keeps the moon in its orbit. It also keeps the atmosphere and oceans fixed to the surface of the Earth.
✅ Gravitational force keeps planets in orbits and atmosphere attached to Earth.
13 What is electromagnetic force?
Answer: An electrostatic force acts between two charged objects. Opposite charges attract each other and similar charges repel each other. Note: Like gravitational force, electrostatic force is also a long-range force.
✅ Electromagnetic force: between charged objects (attraction/repulsion).
14 What is magnetic force?
Answer: It is a force which a magnet exerts on other magnets and magnetic materials like iron, nickel, and cobalt. Magnetic force between the poles of two magnets can be either attractive or repulsive.
✅ Magnetic force: between magnets and magnetic materials.
15 What are the four fundamental forces in nature?
Answer: There are four fundamental forces in nature: (i) Gravitational force (ii) Electromagnetic force (iii) Strong nuclear force (iv) Weak nuclear force
✅ Gravitational, Electromagnetic, Strong nuclear, Weak nuclear.
16 What is gravitational force in terms of fundamental forces?
Answer: The gravitational force is an attractive force that exists between all objects with mass. Although it is the weakest among the four fundamental forces. Being a long-range force, it extends to infinite distance although it becomes weaker and weaker.
✅ Gravitational force: weakest but long-range attractive force.
17 What is electromagnetic force?
Answer: The electromagnetic force causes the interaction between electrically charged particles. Both electrostatic and magnetic forces fall under this category. It is a long-range force and is much stronger than gravitational and weak nuclear forces.
✅ Electromagnetic force: interaction between charged particles.
18 What is strong nuclear force?
Answer: It holds the atomic nucleus together by binding protons and neutrons, overcoming the repulsive electromagnetic force between positively charged protons. The strong nuclear force is the strongest of the four forces but acts only over a very short range (\(10^{-14}\) meters).
✅ Strong nuclear force: binds nucleus, strongest but short-range.
19 What is weak nuclear force?
Answer: Weak nuclear force is responsible for the disintegration of a nucleus. For example, the weak nuclear force executes the \(\beta\)-decay (beta decay) of a neutron. It is a short-range force of the order \(10^{-17}\) meters.
✅ Weak nuclear force: responsible for nuclear disintegration (beta decay).
20 Who unified weak nuclear and electromagnetic forces?
Answer: A Pakistani scientist Dr. Abdus Salam along with Sheldon Glashow and Steven Weinberg were awarded in 1979 Nobel Prize in Physics for their contributions to the unification of the weak nuclear force and electromagnetic force as electroweak force.
✅ Dr. Abdus Salam (Pakistani) unified weak nuclear and electromagnetic forces.
21 What is a free-body diagram?
Answer: Free-body diagrams are used to show the relative magnitudes and directions of all the forces acting on an object in a given situation. In other words, a free-body diagram is a special example of the vector diagrams.
✅ Free-body diagram: shows all forces acting on an object.
22 Define Newton's First Law of Motion.
Answer: A body continues its state of rest or of uniform motion in a straight line unless acted upon by some external force. For example, book placed on a table remains there unless a force is applied to move it.
✅ Newton's First Law: inertia, a body continues its state unless acted upon by a force.
23 Define force according to Newton's First Law of Motion.
Answer: Force is an agency which changes or tends to change the state of rest or of uniform motion of a body.
✅ Force: agency that changes or tends to change state of rest or motion.
24 What is inertia? OR Why Newton's First Law of Motion is called law of inertia?
Answer: The property of a body to maintain its state of rest or of uniform motion in a straight line is called inertia. As a result, Newton's First Law of Motion is sometimes called the Law of Inertia because it describes how objects resist changes in their state of motion unless acted upon by an external force. Demonstration of Property of Inertia: When a tablecloth is pulled abruptly from under dishes, the objects remain in their original positions due to inertia.
✅ Inertia: tendency to maintain state of rest or uniform motion.
25 How is mass related to inertia?
Answer: The mass of an object is a measure of its inertia. The greater the mass of an object, the greater is its inertia.
✅ Mass is a measure of inertia; greater mass = greater inertia.
26 Define Newton's Second Law of Motion.
Answer: If a net external force acts upon a body, it accelerates the body in the direction of force. The magnitude of acceleration is directly proportional to the magnitude of force and is inversely proportional to the mass of the body. Mathematically \(F = ma\).
✅ Newton's Second Law: \(F = ma\). Acceleration is proportional to force and inversely to mass.
27 Define SI unit of force.
Answer: One newton is the force which produces an acceleration of \(1 ms^{-2}\) in a body of mass \(1 kg\). \(1N = 1 kg \times 1 ms^{-2}\).
✅ Newton (N): force for \(1 m/s^2\) acceleration of \(1 kg\).
28 What is effect of force on velocity of body? OR What is the effect of force on velocity according to Newton's second law?
Answer: A force can change the velocity of a body by producing acceleration or deceleration in it. As velocity is a vector quantity, so the change may be in its magnitude, direction or in both of them.
✅ Force changes velocity (magnitude, direction, or both).
29 State Newton's Third Law of Motion with an example.
Answer: For every action, there is always an equal and opposite reaction. For example, when we press a spring, the force applied by our hand on the spring is the action. The spring pushes back on our hand with an equal and opposite force, which is the reaction.
✅ Newton's Third Law: action and reaction are equal and opposite.
30 What are the limitations of Newton's laws of motion?
Answer: Newton's laws of motion have the following limitations: 1. High Velocities: They fail for objects moving at speeds close to the speed of light, where relativistic mechanics is used. 2. Elementary Particles: They are not suitable for describing the motion of elementary particles at atomic or subatomic scales.
✅ Limitations: fail at speeds near light and for elementary particles.
31 What is difference between mass and weight?
Answer:
| Mass | Weight |
|---|---|
| Mass of a body is the quantity of matter in it. It determines the magnitude of acceleration produced when a force acts on it. | The weight of an object is equal to the force with which the Earth attracts the body towards its centre. |
| It is a scalar quantity (has magnitude only). | It is a vector quantity (has both magnitude and direction). |
| Mass remains constant everywhere. | Weight varies with the gravitational field strength (g) at different locations. |
| The SI unit of mass is kilogram (kg). | The SI unit of weight is newton (N). |
| Mass is measured using an ordinary balance. | Weight is measured using a spring balance. |
✅ Mass: scalar, constant, kg. Weight: vector, varies with g, N.
32 What is a gravitational field? Explain gravitational field strength and how it affects the weight of an object.
Answer: Gravitational Field: The gravitational field is the space around a mass where another mass experiences a force due to gravitational attraction. Gravitational Field Strength: The gravitational field strength is defined as the gravitational force acting on a unit mass. The weight of an object is given by \(w = mg\) where \(g\) is the gravitational field strength, with a standard value of \(10 N kg^{-1}\). Since the value of \(g\) varies with location and altitude, the weight of an object also changes accordingly.
✅ Gravitational field: space around mass where force is felt. Weight = mg.
33 What is a force meter?
Answer: A force meter is a scientific instrument used to measure force. It is also called a newton meter or a spring balance. Force meter measures force directly in newtons (N). Digital force meters are now available for more precise measurements.
✅ Force meter: measures force in newtons (also called newton meter).
34 What is the dissipative effect of friction?
Answer: Friction is a dissipative force due to which the energy is wasted in doing work to overcome against friction. The lost energy appears in the form of heat.
✅ Friction dissipates energy as heat.
35 How does friction cause shooting of stars seen in the sky at night?
Answer: These are actually asteroids that enter the Earth's atmosphere. As they are moving, air resistance causes generation of heat. Their temperature becomes so high that they start burning and ultimately disintegrate.
✅ Asteroids burn due to air resistance (friction) in the atmosphere.
36 How does the tread pattern on tyres reduce the chances of skidding on wet roads?
Answer: On a wet road, the water does not form wet layer between the tyre surface and the road surface due to the spaces in the tread pattern on the tyre.
✅ Tread pattern allows water to escape, preventing a wet layer and skidding.
37 What is sliding friction? Define its two categories.
Answer: The friction between two solid surfaces is called sliding friction, which can be divided into two categories: (i) Static Friction: The resisting force between the two surfaces before the motion starts is called the static friction. The maximum value of the static friction is called a limiting friction. (ii) Kinetic Friction: The friction during motion is called kinetic friction.
✅ Sliding friction: static (before motion) and kinetic (during motion).
38 Why does a Styrofoam piece fall more slowly than a cricket ball in air?
Answer: The Styrofoam having larger surface would face greater opposing force of the air and thus moves slowly.
✅ Styrofoam faces greater air resistance due to larger surface area.
39 Define terminal velocity of an object.
Answer: The constant velocity achieved by an object when the upward force of air resistance balances the downward force of gravity is called terminal velocity.
✅ Terminal velocity: constant velocity when air resistance balances weight.
40 What is rolling friction?
Answer: When an object rolls over a surface, the friction produced is called rolling friction.
✅ Rolling friction: friction when an object rolls.
41 How can you show that rolling friction is lesser than the sliding friction?
Answer: The reason for the rolling friction to be less than the sliding friction is that there is no relative motion between the wheel and the surface over which it rolls. The wheel touches the surface only at a point. It does not slide. The rolling friction is about one hundred times smaller than the sliding friction.
✅ Rolling friction is about 100 times smaller than sliding friction.
42 What are the methods used to reduce friction?
Answer: The following methods are used to reduce friction: (i) The parts which slide against each other are highly polished. (ii) Oil or grease is applied between the moving parts of the machinery. (iii) Sliding friction is converted into rolling friction by the use of ball bearings in the machines and wheels under the heavy objects. (iv) The bodies moving through air or water are streamlined to minimize air or water friction.
✅ Methods: polishing, lubrication, ball bearings, streamlining.
43 Define momentum and Newton's second law of motion in terms of change in momentum.
Answer: Momentum: The momentum of a moving body is the product of its mass and velocity. Mathematically, \(p = m \times v\). Momentum is a vector quantity, and its SI unit is \(kgms^{-1}\) or Ns. Newton's Second Law in Terms of Momentum: According to Newton's second law of motion, the rate of change of momentum of a body is equal to the force acting on it. Mathematically, \(F = \frac{\Delta p}{\Delta t}\).
✅ Momentum: \(p = mv\). Newton's second law: \(F = \Delta p / \Delta t\).
44 Define impulse of force.
Answer: When a large force \(F\) acts on an object for a short time \(\Delta t\), the product \(F \times \Delta t\) is called Impulse, which is equal to the total change in momentum of the object.
\[\text{Impulse} = F \times \Delta t\]
\[\text{Impulse} = \frac{\Delta p}{\Delta t} \times \Delta t\]
\[\text{Impulse} = \Delta p\]
\[\text{Impulse} = \text{change in momentum}\]
✅ Impulse = Force × Time = Change in momentum.
45 State the principle of conservation of momentum.
Answer: If no external force acts on an isolated system, the final total momentum of the system is equal to the initial total momentum of the system.
✅ Conservation of momentum: total momentum remains constant if no external force acts.
📐 Key Concepts – Dynamics
Dynamics: Study of forces and their effect on motion.
Force: Push or pull that changes velocity.
Newton's Laws: 1st (inertia), 2nd (\(F = ma\)), 3rd (action-reaction).
Momentum: \(p = mv\). Impulse: \(F \Delta t = \Delta p\).
Friction: Static, kinetic, rolling (smallest).
Terminal Velocity: Constant velocity when air resistance equals weight.
💡 Exam Tip:
For short questions, always write precise and to-the-point answers. Use key terms like "dynamics", "contact forces", "non-contact forces", "Newton's laws", "inertia", "momentum", "impulse", "friction", and "terminal velocity" as they are commonly tested in exams. These questions follow the PECTAA 2026 pattern and are prepared by Subject Specialist Muhammad Tayyab.
Chapter 1Physical Quantities and Measurements
Chapter 2Kinematics
Chapter 3✨ Dynamics
Chapter 4Turning Effect of Forces
Chapter 5Work and Energy
Chapter 6Mechanical 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