Newton's Second Law of Motion

NEET > Physics > Laws of Motion > Newton's Laws of Motion > Newton's Second Law of Motion

Unit Progress

NEET Physics — Newton's Laws of Motion

Newton's Second Law of Motion – Complete Notes, Revision, Important Questions & Downloads

Newton's Second Law states that the rate of change of linear momentum of a body is directly proportional to the external force applied, and this change always occurs in the direction of the force. For a body of constant mass, this reduces to the vector equation F = ma. NEET Physics tests this law through numerical questions calculating net force, acceleration, or mass from given quantities; questions on F = dp/dt vs F = ma (choosing the correct form when mass varies); impulse-momentum theorem applications; and the distinction between the Second Law's quantitative definition of force and the First Law's qualitative one. At least 1–2 questions per NEET exam directly or indirectly use F = ma.

⬇ Download Notes PDF View Important Questions →

Theory + NumericalsNewton's Laws Ch.4F = ma

Expected QuestionsQ

1–2

Newton's Second Law is one of the most directly tested topics in NEET mechanics. Direct numerical questions, F = ma substitution, and F = dp/dt for variable-mass systems appear regularly.

Time Required⏱

90 min

15 min for the law statement and F = dp/dt derivation; 20 min for F = ma in component form; 20 min for variable-mass case (F = dp/dt); 35 min for numerical practice (force, mass, acceleration problems).

Difficulty⚡

Medium

The law itself is straightforward. Difficulty comes from applying F = ma in vector form with multiple forces, recognising when mass varies (requiring F = dp/dt), and connecting to impulse-momentum theorem. Sign convention errors are the most common trap.

NRI USA Curriculum GapUS

Low

AP Physics 1 and C cover Newton's Second Law in depth. The NEET-specific emphasis is on the F = dp/dt formulation before F = ma, the explicit K = 1 constant derivation in SI/CGS, and the distinction between this law's quantitative force definition and the First Law's qualitative definition — all of which US students encounter in different formats.

3Subtopics

8+Practice Questions

4Free Downloads

90 minPrep Time

⬇ Get Free Downloads

NEET Weightage — Newton's Second Law

Newton's Laws of Motion (Chapter 4)

NEET Year	Questions from this Topic	Bar	Marks
2024	1	1 Q	4
2023	1	1 Q	4
2022	1	1 Q	4
2021	0	0 Q	0
2020	1	1 Q	4
2019	1	1 Q	4
6-Year Total (2019–2024)	3–5		12–20

Second Law: F = dp/dt (general form, valid even when mass varies). For constant mass: F = ma. In SI/CGS, the proportionality constant K = 1, giving the equality directly. The law is strictly valid only in inertial reference frames.
Second Law gives the QUANTITATIVE definition of force (contrast: First Law gives qualitative definition). Force is that agent which causes rate of change of momentum. SI unit of force: Newton (1 N = 1 kg·m/s²).
For variable-mass systems (rockets, conveyor belts): use F = dp/dt, NOT F = ma. Newton's Second Law does NOT say F = m(dv/dt) when mass changes — this is a common NEET trap. The general form F = d(mv)/dt = m(dv/dt) + v(dm/dt).

📊

0.8

Avg Questions / Year

🎯

Total Marks (6 yrs)

📈

Direct

Pattern

⚠️

Medium

Difficulty

How to Prepare Newton's Second Law for NEET

Master F = dp/dt before F = ma The NCERT and textbook present the Second Law in the F = dp/dt form first. This is not cosmetic — NEET questions about variable-mass systems (rockets, sand dropped on a conveyor belt) REQUIRE F = dp/dt. For constant mass, F = ma is the simplification. Never start with F = ma as the primary statement; know the general form.

Component-form practice: Fx = max, Fy = may, Fz = maz In 2D problems, the x and y equations are independent. Draw the free-body diagram, resolve all forces into components, then apply Newton's Second Law in each direction separately. The most common error: not resolving forces correctly before applying F = ma, or mixing x and y components.

Second Law defines force quantitatively NEET asks: 'Newton's Second Law gives definition of ___: Force (quantitative).' This is the counterpart to the First Law's qualitative definition. Memorise: First Law → force defined qualitatively (what it does). Second Law → force defined quantitatively (F = ma, measurable in Newtons).

Solve 10 standard force problems Practise: mass on a surface with friction, Atwood machine (two masses over pulley), block on inclined plane, rocket thrust, stacked blocks. Each type tests a different aspect of the Second Law. Time yourself — these should take under 2 min each in NEET.

Study Materials — Newton's Second Law

PDF · Cheat Sheet · MCQ Set · PYQ

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Full Notes

Second Law statement and derivation of F = ma from F = dp/dt. Constant-mass and variable-mass cases. Component form equations. Quantitative definition of force. Units: Newton, Dyne. Connection to First Law. Validity conditions.

1 Law + derivation5 pagesConceptual + Numerical

Download Notes

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Formula Sheet

F = dp/dt (general); F = ma (constant mass); F = ma with component form Fx=max, Fy=may; 1 N = 1 kg·m/s²; 1 dyne = 1 g·cm/s²; 1 N = 10⁵ dyne; 1 kg-f = 9.8 N.

6 key formulas1 pageReady for exam

Download Sheet

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MCQ Practice

20 questions: direct F=ma numerical, F=dp/dt for variable mass, unit conversions, assertion-reason on Second Law and variable mass, multi-force problems, and inclined-plane applications.

20 MCQsNumerical + ConceptualSolved

Download MCQs

📒

PYQ

Year-tagged NEET questions on Newton's Second Law — F = ma numericals, force unit questions, variable-mass problems, and Second Law as quantitative force definition.

12+ year-tagged Qs2015–2024Step-by-step solutions

Download PYQs

Subtopics in Newton's Second Law

2-Column Table

Column A	Column B
Inertia of rest↗	Inertia of motion↗
Inertia of direction↗

Rapid Revision — Newton's Second Law

Concept → Trap → Example

1) Law Statement and F = dp/dt

Core

The rate of change of linear momentum of a body is directly proportional to the external force applied on the body, and this change takes place always in the direction of the applied force. For constant mass: F = ma. General (including variable mass): F = dp/dt.

Proportionality constant K = 1 in both SI and CGS systems. This is why the proportionality becomes an equality: F = dp/dt without any extra constant.
The Second Law is strictly valid only in inertial reference frames. In non-inertial frames, pseudo-forces must be added.
NEET direct question: 'Which of the following is the most general form of Newton's Second Law?' Answer: F = dp/dt (not F = ma, which is only for constant mass).

Example (NEET-style)A 2 kg body accelerates from 3 m/s to 7 m/s in 2 s. Net force = rate of momentum change = m(v-u)/t = 2 × (7-3)/2 = 4 N. Direction: same as direction of acceleration (from 3 m/s to 7 m/s). If slowing down, force is in the direction of deceleration (opposite to velocity).

2) F = ma for Constant Mass

NEET-Key

For a body of constant mass m moving with velocity v, momentum p = mv. Differentiating: F = d(mv)/dt = m(dv/dt) = ma. This simplification requires constant mass — do NOT apply F = ma to rockets or conveyor belts where mass changes with time.

F = ma is a vector equation: force and acceleration point in the same direction. If force has components (Fx, Fy, Fz), then Fx = max, Fy = may, Fz = maz independently.
For multiple forces on a body: F is the NET (resultant) force — vector sum of all external forces. F_net = ma.
NEET trap: 'F = ma formula is valid only if force is changing the state of rest or motion and the mass of the body is constant and finite.' — both conditions must hold.

Example (NEET-style)Net force on a 5 kg block: Two forces act: 30 N rightward and 10 N leftward. Net force = 20 N rightward. Acceleration = F/m = 20/5 = 4 m/s² rightward. Check: momentum changes at rate dp/dt = 5 × 4 = 20 N — consistent. If a third force of 20 N leftward also acts: net = 0, so a = 0, body remains in its state (First Law).

3) Units of Force and Quantitative Definition

Core

Newton (SI): force that gives 1 kg body an acceleration of 1 m/s². So 1 N = 1 kg·m/s². Dyne (CGS): force that gives 1 g body an acceleration of 1 cm/s². So 1 dyne = 1 g·cm/s². Conversion: 1 N = 10⁵ dyne. The Second Law provides the quantitative definition of force.

1 kg-force (gravitational) = 9.8 N. 1 gram-force = 980 dyne. These are gravitational (not absolute) units — force that produces standard gravity acceleration.
From the Second Law, force has dimension [MLT⁻²]. This is tested in NEET: 'Dimension of force is ___'.
NEET: 'Newton's Second Law gives the ___ definition of force.' Answer: Quantitative (contrast: First Law → qualitative). This distinction appears in assertion-reason questions.

Example (NEET-style)Converting gravitational to absolute force: A weight of 5 kg-f = 5 × 9.8 = 49 N. Why? Because 1 kg-f = force needed to give 1 kg body acceleration of 9.8 m/s² (standard gravity). So 5 kg-f acts on a 5 kg body → a = 9.8 m/s² (free fall). Check: F = ma → 49 = 5 × 9.8 ✓.

US Curriculum Gaps — Newton's Second Law

Topics in this section are tested in NEET but organised differently in standard US physics courses.

F = dp/dt as Primary Form (AP Physics C: Mechanics Gap)

AP Physics 1 introduces Newton's Second Law as F = ma and later motivates impulse-momentum theorem. AP Physics C introduces F = dp/dt more rigorously but does not emphasise it as the 'primary form' that reduces to F = ma only for constant mass. NEET textbooks present F = dp/dt first, then derive F = ma as a special case. NEET questions directly ask: 'The most general form of Newton's Second Law is ___' — answer: F = dp/dt, NOT F = ma.

NEET direct question: 'F = dp/dt is the general form; F = ma is valid only for constant mass' — tested in MCQ and assertion-reason
Variable-mass problems (rockets): must use F_net = v(dm/dt) or thrust formula, not F = ma with changing mass
US AP courses often teach F = ma first; NEET emphasis is reversed — know dp/dt is foundational

Force Unit Conversions: Newton, Dyne, kg-force (AP Physics 1 Gap)

AP Physics 1 uses SI units exclusively, rarely requiring dyne-Newton-kilogram-force conversions. NEET regularly tests conversion between absolute units (Newton, dyne) and gravitational units (kg-force, gram-force), directly derived from the Second Law definition. A student who only knows SI force units will miss 'Convert 5 N to dyne' or 'Express 2 kg-f in Newton' questions.

1 N = 10⁵ dyne (both absolute units; derivable from 1 kg = 10³ g, 1 m = 10² cm)
1 kg-f = 9.8 N; 1 g-f = 980 dyne (gravitational units, not tested in US AP courses)
Dimension of force: [MLT⁻²] — NEET tests this directly; AP tests it differently in problem contexts

NEET-Style Practice Questions — Newton's Second Law

4 Questions

1The most general form of Newton's Second Law of Motion is:General Form

F = ma

F = dp/dt

F = mv

F = m(dv/dt) only for constant mass systems

F = dp/dt is Newton's Second Law in its most general form, valid even when mass changes with time (variable-mass systems such as rockets). F = ma is derived from F = dp/dt only when mass is constant: d(mv)/dt = m(dv/dt) = ma. For a rocket where mass decreases as fuel burns, F ≠ ma (this would give a wrong answer). The general form handles both cases.

2Newton's Second Law of Motion gives the _______ definition of force.Definition of Force

Qualitative

Quantitative

Dimensional

Operational

Newton's First Law gives the qualitative definition of force (force is what changes the state of rest or motion — tells WHAT force does). Newton's Second Law gives the quantitative definition of force: F = dp/dt. This allows force to be measured numerically — 1 Newton accelerates 1 kg at 1 m/s². 'Dimensional' is not a type of definition in this context. First Law = qualitative; Second Law = quantitative — a standard NEET contrast.

3A body of mass 5 kg is acted upon by two forces: 20 N eastward and 10 N westward. The acceleration of the body is:Numerical (F = ma)

2 m/s² eastward

6 m/s² eastward

2 m/s² westward

4 m/s² eastward

Net force (vector sum): 20 N east + 10 N west = (20 - 10) N east = 10 N east. Acceleration = F_net / m = 10 / 5 = 2 m/s² east. Direction of acceleration = direction of net force (by Newton's Second Law: F = ma, both F and a are vectors in the same direction). Check: p changes at rate ma = 5 × 2 = 10 N·s/s = 10 N ✓.

4For F = ma to be valid, which conditions must hold?Validity

Mass must be constant, and the frame must be inertial

Mass must be zero, and speed must be high

Force must be in the perpendicular direction to velocity

Body must be at rest initially

The textbook explicitly states: 'F = ma formula is valid only if force is changing the state of rest or motion and the mass of the body is constant and finite.' Additionally, Newton's laws (including the Second Law) are valid only in inertial reference frames. In a non-inertial frame (accelerating frame), pseudo-forces must be added. If mass varies (rocket), F = dp/dt = m(dv/dt) + v(dm/dt) — the second term is non-zero.

Practice Problems — Newton's Second Law

Click "Reveal Answer" after attempting

1A 3 kg block is initially moving at 6 m/s. A net force of 9 N acts opposite to its direction of motion. After how long will it stop, and what is the impulse delivered?

t = 2 s; impulse = 18 N·s

t = 2 s; impulse = 27 N·s

t = 3 s; impulse = 18 N·s

t = 1 s; impulse = 9 N·s

👁 Reveal Answer

Deceleration: a = F/m = 9/3 = 3 m/s² (opposing motion). Time to stop: v = u + at → 0 = 6 - 3t → t = 2 s. Impulse = F × t = 9 × 2 = 18 N·s. OR: Impulse = Δp = m(v - u) = 3 × (0 - 6) = -18 N·s (magnitude 18 N·s). Both methods confirm 18 N·s. The negative sign indicates impulse opposes initial velocity direction.

2Two blocks of masses 4 kg and 6 kg are connected by a string and pulled by a 20 N force on a frictionless surface. What is the tension in the string between them?

8 N

12 N

10 N

16 N

👁 Reveal Answer

Total mass = 4 + 6 = 10 kg. Common acceleration a = F/m_total = 20/10 = 2 m/s². The 20 N force accelerates both. For the 6 kg block (being pulled from behind through the string): Tension T = m₂ × a = 6 × 2 = 12 N. Check: net force on 4 kg block = 20 - 12 = 8 N. Acceleration of 4 kg = 8/4 = 2 m/s² ✓ (same as the system). Tension = 12 N.

3A rocket of mass 1000 kg ejects exhaust gases at 500 m/s. If the fuel is consumed at 2 kg/s, what is the thrust force on the rocket?

1000 N

500 N

1500 N

250 N

👁 Reveal Answer

Thrust = v_exhaust × (dm/dt) = 500 × 2 = 1000 N. This uses F = v × (dm/dt) from the variable-mass form of Newton's Second Law — NOT F = ma (mass is changing). The thrust is the reaction force from exhaust gases leaving the rocket. Net acceleration of rocket = thrust / instantaneous mass = 1000 / 1000 = 1 m/s² (neglecting gravity). The dm/dt carries a negative sign conventionally (mass leaving), so thrust is positive upward.

4A body is moving with constant velocity on a frictionless surface. A student says 'A net force must be acting to maintain this velocity.' Using Newton's Second Law, evaluate this claim.

Correct — force is needed to sustain motion

Incorrect — constant velocity means zero acceleration, so F_net = 0

Partially correct — force keeps changing direction

Cannot be determined without knowing the mass

👁 Reveal Answer

Incorrect. By Newton's Second Law: F_net = ma. If v = constant → a = dv/dt = 0 → F_net = m × 0 = 0. Zero net force. This claim is the classic Aristotelian error — Aristotle thought a continuous force is needed for constant motion. Newton (and Galileo before him) showed this is wrong: constant motion requires ZERO net force. This is also Newton's First Law restated through the Second Law.

Physics — Newton's Laws of Motion Revision Checklist

Check off chapters as you revise

Use this section for quick chapter tracking before mocks, part tests, and final NEET revision.

F = dp/dt (General Form): Write the Second Law in integral form and differential form. Derive F = ma as the special case for constant mass. Clearly state the two conditions for F = ma: (1) constant mass, (2) inertial frame. Solve one variable-mass problem (conveyor belt or simple rocket) using F = dp/dt.

F = ma in Component Form: Draw a free-body diagram. Resolve all forces. Apply Fx = max and Fy = may separately. Practice with at least two problems: (1) horizontal surface with friction, (2) inclined plane. Confirm that a points in the direction of F_net, not velocity.

Quantitative Definition of Force: Memorise: 'Second Law gives quantitative definition of force.' Write the definition: 1 Newton is the force that gives 1 kg body an acceleration of 1 m/s². Write conversion: 1 N = 10⁵ dyne. Solve a unit-conversion question: 'Express 50 N in dyne' → 50 × 10⁵ = 5 × 10⁶ dyne.

Dimension of Force: Derive [F] = [M][L][T⁻²] = [MLT⁻²] from F = ma. Confirm this matches Newton (kg·m/s²) and dyne (g·cm/s²). Solve one dimensional analysis question involving force.

FAQ — Newton's Second Law

Notes · Downloads · Revision · Important Questions

What is the exact statement of Newton's Second Law?

The rate of change of linear momentum of a body is directly proportional to the external force applied on the body, and this change takes place always in the direction of the applied force. Mathematically: F ∝ dp/dt. In SI/CGS, the proportionality constant K = 1, giving F = dp/dt exactly.

When should I use F = dp/dt instead of F = ma?

Use F = dp/dt (the general form) when: (a) the mass of the body is changing with time (rocket propulsion, sand falling on a conveyor belt, snowball rolling); (b) the question asks about the 'general form' of the Second Law. Use F = ma when mass is constant. F = ma is derived from F = dp/dt by assuming mass is constant: d(mv)/dt = m(dv/dt) = ma.

What is the 'quantitative definition of force' from Newton's Second Law?

Newton's Second Law defines force quantitatively as the rate of change of momentum: F = dp/dt. This allows force to be measured numerically. 1 Newton is defined as the force that produces an acceleration of 1 m/s² in a 1 kg mass (from F = ma with m=1 kg, a=1 m/s²). This is the quantitative definition — it gives a number to force. Contrast: First Law gives qualitative definition (force is what changes state of motion).

What are the units and dimensions of force?

SI absolute unit: Newton (N). 1 N = 1 kg·m/s². CGS absolute unit: Dyne. 1 dyne = 1 g·cm/s². Gravitational unit: kilogram-force (kg-f). 1 kg-f = 9.8 N. Conversion between absolute units: 1 N = 10⁵ dyne. Dimension: [MLT⁻²], derived from F = ma: [M][LT⁻²].

Is F = ma valid for all situations?

No. F = ma requires: (1) constant mass, and (2) inertial reference frame. For variable-mass systems (rockets, conveyor belts), use F = dp/dt. For non-inertial frames (accelerating bus, lift), pseudo-forces must be added before applying Newton's Second Law. At speeds approaching c (speed of light), relativistic mass effects make F = ma invalid — relativistic mechanics is required, though this is beyond NEET scope.

Can an object have zero net force but non-zero velocity?

Yes. F_net = 0 means a = 0, so velocity is constant (not necessarily zero). A book sliding on a frictionless surface at 5 m/s has zero net force (no friction, gravity balanced by normal) — it moves at 5 m/s indefinitely. This is actually a statement of Newton's First Law, which is a special case of the Second Law (F = ma = m × 0 = 0).

How does the Second Law relate to the First Law?

Newton's First Law is actually a special case of the Second Law: if F_net = 0, then a = 0, so velocity is constant. Thus F = ma (Second Law) contains F = 0 → a = 0 (First Law) as a special case. However, the First Law has independent physical significance: it defines inertial reference frames — frames in which Newton's laws hold. The Second Law is valid ONLY because inertial frames exist, and their existence is asserted by the First Law.

What does 'the change takes place in the direction of the applied force' mean?

It means the acceleration (and hence the change in momentum) is always parallel to the net force — they point in the same direction. This is important because velocity may point in a different direction from force (e.g., a projectile at the top of its arc has horizontal velocity but vertical downward force). The force and acceleration always share the same direction; velocity does not have to.

How is F = ma applied in component form?

Since force and acceleration are vectors, F = ma holds component-wise: Fx = max (horizontal), Fy = may (vertical), Fz = maz (depth). This means the x-equation and y-equation are completely independent. You can have Fy = 0 (ay = 0, constant vertical velocity) simultaneously with Fx ≠ 0 (horizontal acceleration). This is exactly the physics of projectile motion: horizontal F = 0 → constant horizontal velocity; vertical F = mg → constant downward acceleration.

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Newton's Laws of Motion

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Newton's Second Law of Motion

NEET > Physics > Laws of Motion > Newton's Laws of Motion > Newton's Second Law of Motion

Unit Progress

NEET Physics — Newton's Laws of Motion

Newton's Second Law of Motion – Complete Notes, Revision, Important Questions & Downloads

⬇ Download Notes PDF View Important Questions →

Theory + NumericalsNewton's Laws Ch.4F = ma

Expected QuestionsQ

1–2

Newton's Second Law is one of the most directly tested topics in NEET mechanics. Direct numerical questions, F = ma substitution, and F = dp/dt for variable-mass systems appear regularly.

Time Required⏱

90 min

Difficulty⚡

Medium

NRI USA Curriculum GapUS

Low

3Subtopics

8+Practice Questions

4Free Downloads

90 minPrep Time

⬇ Get Free Downloads

NEET Weightage — Newton's Second Law

Newton's Laws of Motion (Chapter 4)

NEET Year	Questions from this Topic	Bar	Marks
2024	1	1 Q	4
2023	1	1 Q	4
2022	1	1 Q	4
2021	0	0 Q	0
2020	1	1 Q	4
2019	1	1 Q	4
6-Year Total (2019–2024)	3–5		12–20

📊

0.8

Avg Questions / Year

🎯

Total Marks (6 yrs)

📈

Direct

Pattern

⚠️

Medium

Difficulty

How to Prepare Newton's Second Law for NEET

Study Materials — Newton's Second Law

PDF · Cheat Sheet · MCQ Set · PYQ

📘

Full Notes

1 Law + derivation5 pagesConceptual + Numerical

Download Notes

📗

Formula Sheet

F = dp/dt (general); F = ma (constant mass); F = ma with component form Fx=max, Fy=may; 1 N = 1 kg·m/s²; 1 dyne = 1 g·cm/s²; 1 N = 10⁵ dyne; 1 kg-f = 9.8 N.

6 key formulas1 pageReady for exam

Download Sheet

📙

MCQ Practice

20 questions: direct F=ma numerical, F=dp/dt for variable mass, unit conversions, assertion-reason on Second Law and variable mass, multi-force problems, and inclined-plane applications.

20 MCQsNumerical + ConceptualSolved

Download MCQs

📒

PYQ

Year-tagged NEET questions on Newton's Second Law — F = ma numericals, force unit questions, variable-mass problems, and Second Law as quantitative force definition.

12+ year-tagged Qs2015–2024Step-by-step solutions

Download PYQs

Subtopics in Newton's Second Law

2-Column Table

Column A	Column B
Inertia of rest↗	Inertia of motion↗
Inertia of direction↗

Rapid Revision — Newton's Second Law

Concept → Trap → Example

1) Law Statement and F = dp/dt

Core

Proportionality constant K = 1 in both SI and CGS systems. This is why the proportionality becomes an equality: F = dp/dt without any extra constant.
The Second Law is strictly valid only in inertial reference frames. In non-inertial frames, pseudo-forces must be added.
NEET direct question: 'Which of the following is the most general form of Newton's Second Law?' Answer: F = dp/dt (not F = ma, which is only for constant mass).

2) F = ma for Constant Mass

NEET-Key

F = ma is a vector equation: force and acceleration point in the same direction. If force has components (Fx, Fy, Fz), then Fx = max, Fy = may, Fz = maz independently.
For multiple forces on a body: F is the NET (resultant) force — vector sum of all external forces. F_net = ma.
NEET trap: 'F = ma formula is valid only if force is changing the state of rest or motion and the mass of the body is constant and finite.' — both conditions must hold.

3) Units of Force and Quantitative Definition

Core

1 kg-force (gravitational) = 9.8 N. 1 gram-force = 980 dyne. These are gravitational (not absolute) units — force that produces standard gravity acceleration.
From the Second Law, force has dimension [MLT⁻²]. This is tested in NEET: 'Dimension of force is ___'.
NEET: 'Newton's Second Law gives the ___ definition of force.' Answer: Quantitative (contrast: First Law → qualitative). This distinction appears in assertion-reason questions.

US Curriculum Gaps — Newton's Second Law

Topics in this section are tested in NEET but organised differently in standard US physics courses.

F = dp/dt as Primary Form (AP Physics C: Mechanics Gap)

NEET direct question: 'F = dp/dt is the general form; F = ma is valid only for constant mass' — tested in MCQ and assertion-reason
Variable-mass problems (rockets): must use F_net = v(dm/dt) or thrust formula, not F = ma with changing mass
US AP courses often teach F = ma first; NEET emphasis is reversed — know dp/dt is foundational

Force Unit Conversions: Newton, Dyne, kg-force (AP Physics 1 Gap)

1 N = 10⁵ dyne (both absolute units; derivable from 1 kg = 10³ g, 1 m = 10² cm)
1 kg-f = 9.8 N; 1 g-f = 980 dyne (gravitational units, not tested in US AP courses)
Dimension of force: [MLT⁻²] — NEET tests this directly; AP tests it differently in problem contexts

NEET-Style Practice Questions — Newton's Second Law

4 Questions

1The most general form of Newton's Second Law of Motion is:General Form

F = ma

F = dp/dt

F = mv

F = m(dv/dt) only for constant mass systems

2Newton's Second Law of Motion gives the _______ definition of force.Definition of Force

Qualitative

Quantitative

Dimensional

Operational

3A body of mass 5 kg is acted upon by two forces: 20 N eastward and 10 N westward. The acceleration of the body is:Numerical (F = ma)

2 m/s² eastward

6 m/s² eastward

2 m/s² westward

4 m/s² eastward

4For F = ma to be valid, which conditions must hold?Validity

Mass must be constant, and the frame must be inertial

Mass must be zero, and speed must be high

Force must be in the perpendicular direction to velocity

Body must be at rest initially

Practice Problems — Newton's Second Law

Click "Reveal Answer" after attempting

1A 3 kg block is initially moving at 6 m/s. A net force of 9 N acts opposite to its direction of motion. After how long will it stop, and what is the impulse delivered?

t = 2 s; impulse = 18 N·s

t = 2 s; impulse = 27 N·s

t = 3 s; impulse = 18 N·s

t = 1 s; impulse = 9 N·s

👁 Reveal Answer

2Two blocks of masses 4 kg and 6 kg are connected by a string and pulled by a 20 N force on a frictionless surface. What is the tension in the string between them?

8 N

12 N

10 N

16 N

👁 Reveal Answer

3A rocket of mass 1000 kg ejects exhaust gases at 500 m/s. If the fuel is consumed at 2 kg/s, what is the thrust force on the rocket?

1000 N

500 N

1500 N

250 N

👁 Reveal Answer

4A body is moving with constant velocity on a frictionless surface. A student says 'A net force must be acting to maintain this velocity.' Using Newton's Second Law, evaluate this claim.

Correct — force is needed to sustain motion

Incorrect — constant velocity means zero acceleration, so F_net = 0

Partially correct — force keeps changing direction

Cannot be determined without knowing the mass

👁 Reveal Answer

Physics — Newton's Laws of Motion Revision Checklist

Check off chapters as you revise

Use this section for quick chapter tracking before mocks, part tests, and final NEET revision.

Dimension of Force: Derive [F] = [M][L][T⁻²] = [MLT⁻²] from F = ma. Confirm this matches Newton (kg·m/s²) and dyne (g·cm/s²). Solve one dimensional analysis question involving force.

FAQ — Newton's Second Law

Notes · Downloads · Revision · Important Questions

What is the exact statement of Newton's Second Law?

The rate of change of linear momentum of a body is directly proportional to the external force applied on the body, and this change takes place always in the direction of the applied force. Mathematically: F ∝ dp/dt. In SI/CGS, the proportionality constant K = 1, giving F = dp/dt exactly.

When should I use F = dp/dt instead of F = ma?

What is the 'quantitative definition of force' from Newton's Second Law?

What are the units and dimensions of force?

Is F = ma valid for all situations?

Can an object have zero net force but non-zero velocity?

How does the Second Law relate to the First Law?

What does 'the change takes place in the direction of the applied force' mean?

How is F = ma applied in component form?

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Subtopics

Inertia of rest

Inertia of motion

Inertia of direction

Newton's Laws of Motion

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Friction

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Newton's Second Law of Motion

Overview content

Newton's Second Law of Motion – Complete Notes, Revision, Important Questions & Downloads

1) Law Statement and F = dp/dt

2) F = ma for Constant Mass

3) Units of Force and Quantitative Definition

F = dp/dt as Primary Form (AP Physics C: Mechanics Gap)

Force Unit Conversions: Newton, Dyne, kg-force (AP Physics 1 Gap)

Physics — Newton's Laws of Motion Revision Checklist

NEET NRI Counseling & Admission eBook Download

Schedule Trial Session For NEET Prep

Inertia of rest

Inertia of motion

Inertia of direction

Subtopics

Inertia of rest

Inertia of motion

Inertia of direction

NEET > Physics > Laws of Motion Chapters

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Newton's Second Law of Motion

Overview content

Newton's Second Law of Motion – Complete Notes, Revision, Important Questions & Downloads

1) Law Statement and F = dp/dt

2) F = ma for Constant Mass

3) Units of Force and Quantitative Definition

F = dp/dt as Primary Form (AP Physics C: Mechanics Gap)

Force Unit Conversions: Newton, Dyne, kg-force (AP Physics 1 Gap)

Physics — Newton's Laws of Motion Revision Checklist

NEET NRI Counseling & Admission eBook Download

Schedule Trial Session For NEET Prep

Inertia of rest

Inertia of motion

Inertia of direction

Subtopics

Inertia of rest

Inertia of motion

Inertia of direction

NEET > Physics > Laws of Motion Chapters

Comments

Leave a comment

Comments (0)