Here is the trap that costs marks on "force and inertia" questions: confusing the direction of force with the direction of motion. NEET 2023 tested exactly this — a body changes direction, and the question asks for the direction of the net force. Most aspirants pick the direction the body is moving after the turn. The correct answer is the direction of Δp (change in momentum), which points along v_final − v_initial, not along either velocity alone.
What force actually means (Newton's Second Law). The net external force on a body equals the rate of change of its linear momentum: F = dp/dt. For constant mass, this reduces to F = ma — the net force equals mass times acceleration (NCERT Class 11 Physics, Chapter 4, page 54). Both F and a are vectors. The acceleration points in the direction of the net force, not necessarily in the direction of motion.
Inertia is resistance to acceleration. Newton's First Law (NCERT Class 11 Physics, Chapter 4, page 51) states that a body at rest stays at rest and a body in uniform motion stays in uniform motion unless acted on by a net external force. Mass quantifies inertia — a heavier body requires more force for the same acceleration.
Newton's Third Law — the action-reaction trap. Every force has an equal and opposite reaction, but the two forces act on different bodies (NCERT Class 11 Physics, Chapter 4, page 51). A common confusion: the weight of a book on a table and the normal force from the table are NOT an action-reaction pair — both act on the book. The actual pair to the book's weight is the gravitational pull the book exerts on the Earth.
Two-block contact force. When force F pushes block A into block B on a frictionless surface, the system acceleration is a = F/(m_A + m_B). The contact force on B is m_B × a — not the full applied force F. Treating each block independently with the full F is a high-frequency error.
Watch out: whenever a question asks "direction of force," compute Δp = m(v_f − v_i) as a vector subtraction. Do not default to the direction of motion.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Newton's First Law of Motion defines which of the following quantities?
Answer: C. Newton's First Law provides the qualitative definition of force (that which changes the state of rest or uniform motion) and of inertia (the tendency of a body to resist that change). NCERT Class 11 Physics, Chapter 4, page 51.
Why A is wrong: A is wrong because acceleration is quantified by Newton's Second Law (F = ma), not the First Law. The First Law defines force qualitatively, not quantitatively through acceleration.
Why B is wrong: B is wrong because momentum (p = mv) is defined separately, and impulse (J = FΔt = Δp) is derived from the Second Law. The First Law does not define either.
Why D is wrong: D is wrong because work (W = F·d) and energy are concepts from the work-energy framework (Chapter 5), not Newton's First Law.
Which of the following is the correct mathematical form of Newton's Second Law for a body of constant mass?
Answer: A. For constant mass, F = dp/dt reduces to F = ma, where F is the net external force (vector) and a is the acceleration (vector). NCERT Class 11 Physics, Chapter 4, page 54.
Why B is wrong: B is wrong because mv is momentum (p), not force. Force is the rate of change of momentum, not momentum itself.
Why C is wrong: C is wrong because mv² has dimensions of kg·m²/s² (energy, specifically twice kinetic energy), not force (kg·m/s²).
Why D is wrong: D is wrong because m/a has dimensions of kg·s²/m, which is not a standard physical quantity and certainly not force.
A book of mass 2 kg rests on a table. The weight of the book is 19.6 N downward and the normal force from the table is 19.6 N upward. These two forces are:
Answer: B. Both the weight and the normal force act on the same body (the book). They balance each other, keeping the book in equilibrium. They are NOT an action-reaction pair — action-reaction pairs always act on different bodies. NCERT Class 11 Physics, Chapter 4, page 51.
Why A is wrong: A is wrong because Newton's Third Law pairs act on DIFFERENT bodies. Both weight and normal force act on the book (same body). The third-law pair to the book's weight is the gravitational pull the book exerts on the Earth. (Trap: mistake: action reaction same body)
Why C is wrong: C is wrong because the two forces are equal in magnitude and opposite in direction, so they cancel. The net force is zero and there is no acceleration.
Why D is wrong: D is wrong because both forces act on the book. Option D contradicts itself — it says 'different bodies' but the scenario describes both forces on the same body.
A body of mass 0.5 kg moves due south at 4 m/s and is deflected to move due east at 3 m/s. The direction of the net force that caused this change is:
Answer: D. F is along Δp = m(v_f − v_i). Take east as +x and north as +y. v_i = −4ĵ (south), v_f = 3î (east). Δv = 3î − (−4ĵ) = 3î + 4ĵ. This vector points into the first quadrant: north-east. The net force is in the direction of Δv, not along either the initial or final velocity. NCERT Class 11 Physics, Chapter 4, page 54. (Pattern: force as momentum change direction, NEET 2023.)
Why A is wrong: A is wrong because 'due east' is the direction of the FINAL velocity, not the direction of the change in momentum. Force is along Δp, not along v_final. (Trap: trap: force direction deltap vs velocity)
Why B is wrong: B is wrong because 'due north' ignores the eastward component of Δv. The change in velocity has both a northward component (+4ĵ from reversing the southward motion) and an eastward component (+3î from the new eastward velocity).
Why C is wrong: C is wrong because south-east would require a negative y-component in Δv. Since v_i is southward, subtracting it (v_f − v_i) actually adds a northward component, not a southward one.
A horizontal force of 20 N is applied to block A (mass 3 kg) which pushes block B (mass 2 kg) on a frictionless surface. The contact force exerted by A on B is:
Answer: B. System acceleration a = F/(m_A + m_B) = 20/(3 + 2) = 4 m/s². Contact force on B = m_B × a = 2 × 4 = 8 N. The full 20 N acts on the system, not on block B alone. NCERT Class 11 Physics, Chapter 4, page 54. (Pattern: two-block horizontal push, NEET 2024.)
Why A is wrong: A is wrong because 20 N is the external force applied to the entire system (A + B), not the contact force between A and B. Using the full F on B alone ignores Newton's Third Law internal force decomposition. (Trap: trap: two block internal vs external force)
Why C is wrong: C is wrong because 12 N = m_A × a = 3 × 4 N. This is the net force on block A (applied force minus contact reaction), not the contact force on B.
Why D is wrong: D is wrong because 10 N = F/2, implying the force splits equally between the two blocks regardless of mass. Force distribution depends on mass ratio, not equal division.
A cricket ball of mass 0.15 kg moving at 20 m/s is brought to rest by a fielder in 0.1 s. The average force exerted by the fielder on the ball is:
Answer: A. F = Δp/Δt = m(v_f − v_i)/Δt = 0.15 × (0 − 20)/0.1 = −30 N. Magnitude = 30 N. NCERT Class 11 Physics, Chapter 4, page 54.
Why B is wrong: B is wrong because 3 N = m × v = 0.15 × 20 = 3, which is the momentum (in kg·m/s), not the force. Force requires dividing by Δt.
Why C is wrong: C is wrong because 300 N results from using Δt = 0.01 s (an order-of-magnitude error in the time interval) instead of the given 0.1 s.
Why D is wrong: D is wrong because 0.3 N = m × v × Δt = 0.15 × 20 × 0.1, which multiplies by time instead of dividing. Dimensionally this gives kg·m (not force).
Two blocks of masses 4 kg and 6 kg are placed in contact on a frictionless surface. A force of 50 N pushes the 6 kg block toward the 4 kg block. The contact force between the blocks is:
Answer: C. System acceleration a = 50/(6 + 4) = 5 m/s². Contact force on the 4 kg block = m × a = 4 × 5 = 20 N. The force is applied to the 6 kg block; only the contact force accelerates the 4 kg block. NCERT Class 11 Physics, Chapter 4, page 54.
Why A is wrong: A is wrong because 50 N is the external force on the system, not the internal contact force. Applying the full 50 N to the 4 kg block would give it acceleration 12.5 m/s², inconsistent with the system. (Trap: trap: two block internal vs external force)
Why B is wrong: B is wrong because 30 N = 6 × 5, which is the net force on the 6 kg block (50 − 20 = 30 N), not the contact force on the 4 kg block.
Why D is wrong: D is wrong because 25 N = 50/2, implying equal force division regardless of mass. The contact force depends on which block's mass is being accelerated.
A body of mass 2 kg is moving with velocity 10 m/s along the +x direction. A force acts on it for 0.5 s, changing its velocity to 10 m/s along the +y direction. The magnitude of the force is:
Answer: B. Δv = v_f − v_i = 10ĵ − 10î. |Δv| = √(10² + 10²) = 10√2 m/s. F = m|Δv|/Δt = 2 × 10√2/0.5 = 40√2 N. The speed is unchanged but velocity direction changed by 90°, so Δv ≠ 0. NCERT Class 11 Physics, Chapter 4, page 54.
Why A is wrong: A is wrong because 40 N = m × v_f/Δt = 2 × 10/0.5, which treats the problem as if only the final velocity matters and ignores the initial velocity vector entirely. (trap: force direction deltap vs velocity)
Why C is wrong: C is wrong because 0 N assumes 'same speed means no change.' Speed is scalar; velocity is a vector. The direction changed by 90°, so |Δv| = 10√2 ≠ 0.
Why D is wrong: D is wrong because 20 = m × v = 2 × 10 is a momentum in kg·m/s, not a force; the change in momentum must be taken as a vector (magnitude 20√2 kg·m/s) and divided by Δt = 0.5 s.
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Pattern: Two blocks pushed horizontally on a frictionless surface (NEET 2024, NEET pattern: two blocks horizontal push).
Given
A horizontal force F = 30 N is applied to block A (mass m_A = 5 kg), which pushes block B (mass m_B = 3 kg) on a smooth horizontal surface. No friction.
Required
(a) Acceleration of the system.
(b) Contact force between A and B.
Concept
Newton's Second Law applied to the system as a whole gives acceleration. Then apply F = ma to block B alone to find the contact force (the only horizontal force on B).
Formula
• System: F = (m_A + m_B) × a → a = F/(m_A + m_B)• Block B: Contact force N_AB = m_B × a
Substitution
• a = 30/(5 + 3) = 30/8• N_AB = 3 × (30/8)
Calculation
• a = 3.75 m/s²• N_AB = 3 × 3.75 = 11.25 N
Note: The masses (5 kg, 3 kg) and force (30 N) are exact problem-defined values. They do not limit significant figures in the answer.
Final answer
(a) Acceleration = 3.75 m/s²
(b) Contact force between A and B = 11.25 N
Common trap
Applying the full 30 N to block B alone gives a = 30/3 = 10 m/s², which is incorrect. The full force accelerates the entire system (A + B), not block B alone. The contact force on B is always less than the applied force F (unless m_A = 0).
Similar NEET-style question
A force of 24 N pushes block P (mass 4 kg) into block Q (mass 2 kg) on a frictionless floor. Find the contact force between P and Q. [Answer: a = 24/6 = 4 m/s²; contact force = 2 × 4 = 8 N.]
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Aristotle held that an external force is required to keep a body in motion. This view is intuitively appealing but flawed. Galileo's frictionless-plane argument and Newton's first law overturn it: a body with NO net external force continues in its state of rest or uniform motion in a straight line.
-- NCERT Class 11 Physics, Ch. 4, p. 50If no external force acts on a body, the body in motion continues to move uniformly in a straight line, and the body at rest continues to be at rest. Friction had been masking this principle in everyday observation.
-- NCERT Class 11 Physics, Ch. 4, p. 51Every body continues in its state of rest, or of uniform motion in a straight line, unless compelled by an external force to change that state. This identifies inertia as the property by which a body resists change in its state of motion.
-- NCERT Class 11 Physics, Ch. 4, p. 51More in Laws of Motion: 16 exam traps and mistakes · 9 formulas · 10 question patterns from its other lessons.
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Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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