📘 Chapter 3: Dynamics – Exercise 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 exercise short questions from Chapter 3 Dynamics including force, changes in motion, contact forces, Newton's first law, impulse, friction, inertia, rolling friction, terminal velocity, and action-reaction. 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.
📝 Exercise Short Questions & Answers (PECTAA 2026)
3.1 What kind of changes in motion may be produced by a force?
Answer: A force is a push or pull that can start, stop, or change the magnitude and direction of a body's velocity.
OR
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.
✅ A force can start, stop, or change the magnitude and direction of velocity.
3.2 Give 5 examples of contact forces.
Answer: Five examples of contact forces are:
1. Friction
2. Drag
3. Thrust
4. Normal force
5. Air resistance
✅ Friction, Drag, Thrust, Normal force, Air resistance.
3.3 An object moves with constant velocity in free space. How long will the object continue to move with this velocity?
Answer: According to Newton's first law of motion, in outer space, if an object is thrown where no force acts on it, it would continue to move forever at a constant velocity.
✅ It would continue to move forever at constant velocity.
3.4 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.
3.5 Why has not Newton's first law been proved on the Earth?
Answer: All the bodies moving on the Earth are stopped by the force of friction. Therefore, practically, a body cannot continue its motion without an opposing force.
✅ Friction stops bodies on Earth, so the law cannot be practically proved.
3.6 When sitting in a car which suddenly accelerates from rest, you are pushed back into the seat, why?
Answer: When the bus starts moving quickly from rest, the passengers are pushed back against the seat. This is because the tendency of passengers is to retain their state of rest due to inertia.
✅ Inertia causes passengers to retain their state of rest.
3.7 The force expressed in Newton's second law is a net force. Why is it so?
Answer: If a net external force acts upon a body, it accelerates the body in the direction of force. Newton's second law deals with the acceleration produced in a body when a net force acts upon it.
✅ Newton's second law applies to the net (resultant) force.
3.8 How can you show that rolling friction is lesser than the sliding friction?
Answer: A body with wheels faces less friction as compared to a body of the same size without wheels.
The rolling friction is about one hundred times smaller than the sliding friction because the wheel touches the surface only at a point and does not slide.
✅ Rolling friction is about 100 times smaller than sliding friction.
3.9 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 gravity.
3.10 An astronaut walking in space wants to return to his spaceship by firing a hand rocket. In what direction does he fire the rocket?
Answer: He fires the rocket in the direction opposite to the spaceship. Due to action and reaction, he will move towards the spaceship.
✅ Fire the rocket opposite to the spaceship (action-reaction).
📐 Key Concepts – Dynamics
Force: A push or pull that can start, stop, or change velocity.
Contact Forces: Friction, Drag, Thrust, Normal force, Air resistance.
Newton's First Law: A body continues in its state of rest or uniform motion unless acted upon by an external force.
Impulse: \(F \times \Delta t = \Delta p\) (Change in momentum).
Inertia: The tendency of a body to maintain its state of rest or uniform motion.
Rolling Friction: Much smaller than sliding friction (about 100 times).
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 "force", "contact forces", "Newton's first law", "impulse", "inertia", "rolling friction", "terminal velocity", and "action-reaction" 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