📘 Chapter 3: Dynamics – Constructed Response 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 constructed response questions from Chapter 3 Dynamics including Newton's third law application with calculations, advantage of air bags over seatbelts, horse-cart reasoning error, cricket ball catching technique, jumping from a boat scenario, and the scenario if friction vanishes. 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 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.
✏️ Constructed Response Questions & Answers (PECTAA 2026)
3.1 Two ice skaters weighing \(60kg\) and \(80kg\) push off against each other on a frictionless ice track. The \(60kg\) skater gains a velocity of \(4ms^{-1}\). Considering all the relevant calculations involved, explain how Newton's third law applies to this situation.
Answer: They push each other with equal and opposite force (Newton's Third Law).
The total momentum stays zero:
\[60 \times 4 + 80 \times v_{2} = 0\]
\[240 + 80v_{2} = 0\]
\[80v_{2} = -240\]
\[v_{2} = -\frac{240}{80}\]
\[v_{2} = -3ms^{-1}\]
Both move in opposite directions to conserve momentum.
✅ Equal and opposite forces (Newton's Third Law). Total momentum remains zero. The 80kg skater moves at \(-3ms^{-1}\).
3.2 Inflatable air bags are installed in the vehicles as safety equipment. In terms of momentum, what is the advantage of air bags over seatbelts?
Answer: Airbags increase the time of impact during which the momentum changes. Since the change in momentum occurs over a longer time, the force on the body is reduced, causing less injury.
✅ Airbags increase impact time, reducing force (impulse-momentum theorem).
3.3 A horse refuses to pull a cart. The horse argues, "according to Newton's third law, whatever force I exert on the cart, the cart will exert an equal and opposite force on me. Since the net force will be zero, therefore, I have no chance of accelerating (pulling) the cart." What is wrong with this reasoning?
Answer: The horse's reasoning is wrong because the action and reaction forces act on different bodies. The force exerted by the horse on the cart and the force exerted by the cart on the horse do not cancel each other.
✅ Action and reaction act on different bodies, so they cannot cancel each other.
3.4 When a cricket ball hits high, a fielder tries to catch it. While holding the ball he/she draws hands backward. Why?
Answer: The fielder draws his/her hands backward to increase the time of catching the ball. This reduces the force exerted by the ball on the hands and prevents injury.
✅ Increasing catching time reduces force (impulse-momentum theorem).
3.5 When someone jumps from a small boat onto the river bank, why does the jumper often fall into the water? Explain.
Answer: When the jumper pushes against the boat, the boat moves backward due to Newton's Third Law. The backward motion of the boat may cause the jumper to lose balance and fall into the water.
✅ Boat moves backward (reaction), causing loss of balance.
3.6 Imagine that if friction vanishes suddenly from everything, then what could be the scenario of daily life activities?
Answer: If friction suddenly vanished, we would not be able to walk or stop moving objects. Vehicles could not brake, and everyday activities would become difficult and unsafe because there would be no grip between surfaces.
✅ Without friction: no walking, no braking, no grip.
📐 Key Concepts – Dynamics (Constructed Response)
Newton's Third Law: Action and reaction are equal and opposite, acting on different bodies.
Impulse-Momentum Theorem: \(F \Delta t = \Delta p\). Increasing time reduces force.
Conservation of Momentum: Total momentum remains constant if no external force acts.
Friction: Essential for walking, braking, and everyday grip.
💡 Exam Tip:
For constructed response questions, write clear, logical answers. Show calculations where needed (like in Q1). Explain concepts like Newton's Third Law, impulse-momentum theorem, and conservation of momentum. These questions test your understanding of real-world applications of physics. 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