Newtons 3 Laws — JEE Main Physics MCQs with Solutions
Free JEE Main Physics Newtons 3 Laws MCQs with step-by-step solutions (40 questions). Part of Laws of Motion. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Newtons 3 Laws · medium · numerical
A gun of mass $2$ kg fires a bullet of mass $0.02$ kg at $100$ m/s. The recoil velocity of the gun is
A. $1$ m/s ✓ Correct
B. $2$ m/s
C. $100$ m/s
D. $0.01$ m/s
Solution: Momentum conservation: $M V = m v \Rightarrow V = \frac{0.02 \times 100}{2} = 1$ m/s.
Q2 — Newtons 3 Laws · medium · numerical
A man pushes a wall with a force of $50$ N. The force exerted by the wall on the man is
A. $100$ N
B. $50$ N ✓ Correct
C. Zero
D. $25$ N
Solution: By Newton's third law, the reaction force is equal and opposite: $50$ N.
Q3 — Newtons 3 Laws · medium · numerical
A $60$ kg man jumps out of a $40$ kg boat at $2$ m/s. The boat's recoil speed is
A. $3$ m/s ✓ Correct
B. $1.33$ m/s
C. $2$ m/s
D. $5$ m/s
Solution: $60 \times 2 = 40 \times V \Rightarrow V = 3$ m/s.
Q4 — Newtons 3 Laws · medium · numerical
Blocks of $2$ kg and $3$ kg are in contact on a smooth floor, pushed by $10$ N applied to the $2$ kg block. The contact force between them is
A. $10$ N
B. $4$ N
C. $5$ N
D. $6$ N ✓ Correct
Solution: $a = \frac{10}{5} = 2$ m/s²; contact force accelerates the $3$ kg block: $3 \times 2 = 6$ N.
Q5 — Newtons 3 Laws · medium · numerical
A bullet of $0.01$ kg leaves a $5$ kg gun at $200$ m/s. The gun's recoil velocity is
A. $200$ m/s
B. $4$ m/s
C. $0.4$ m/s ✓ Correct
D. $0.04$ m/s
Solution: $V = \frac{0.01 \times 200}{5} = 0.4$ m/s.
Q6 — Newtons 3 Laws · easy · numerical
A $40$ kg boy steps off a $20$ kg trolley (on smooth ground) at $1$ m/s. The trolley's speed is
A. $0.5$ m/s
B. $2$ m/s ✓ Correct
C. $1$ m/s
D. $4$ m/s
Solution: $40 \times 1 = 20 \times V \Rightarrow V = 2$ m/s.
Q7 — Newtons 3 Laws · easy · numerical
Two skaters push apart. A $50$ kg skater moves at $2$ m/s; a $40$ kg skater moves at
A. $5$ m/s
B. $1.6$ m/s
C. $2.5$ m/s ✓ Correct
D. $2$ m/s
Solution: $50 \times 2 = 40 \times V \Rightarrow V = 2.5$ m/s.
Q8 — Newtons 3 Laws · medium · theory
A ball strikes a wall with a force of $20$ N. The force exerted by the wall on the ball is
A. $10$ N
B. Zero
C. $40$ N
D. $20$ N (opposite direction) ✓ Correct
Solution: Third law: equal and opposite reaction of $20$ N.
Q9 — Newtons 3 Laws · medium · numerical
On a smooth floor, blocks of $4$ kg and $6$ kg are in contact; $20$ N is applied to the $4$ kg block. The contact force is
A. $20$ N
B. $12$ N ✓ Correct
C. $6$ N
D. $8$ N
Solution: $a = \frac{20}{10} = 2$ m/s²; contact force on $6$ kg $= 6 \times 2 = 12$ N.
Q10 — Newtons 3 Laws · medium · numerical
A gun of mass $4$ kg fires a $0.05$ kg bullet at $80$ m/s. The recoil velocity of the gun is
A. $0.05$ m/s
B. $1$ m/s ✓ Correct
C. $80$ m/s
D. $4$ m/s
Solution: $V = \frac{0.05 \times 80}{4} = 1$ m/s.
Q11 — Newtons 3 Laws · easy · numerical
A $50$ kg man walks at $2$ m/s on a $100$ kg boat (smooth water). The boat moves at
A. $0.5$ m/s
B. $2$ m/s
C. $4$ m/s
D. $1$ m/s ✓ Correct
Solution: $50 \times 2 = 100 \times V \Rightarrow V = 1$ m/s.
Q12 — Newtons 3 Laws · medium · numerical
A rope has a tension of $30$ N pulling block A. The force block A exerts back on the rope is
A. $60$ N
B. Zero
C. $30$ N ✓ Correct
D. $15$ N
Solution: Action-reaction: block A pulls the rope with $30$ N.
Q13 — Newtons 3 Laws · medium · numerical
A cannon of mass $500$ kg fires a $5$ kg ball at $200$ m/s. The recoil speed of the cannon is
A. $0.2$ m/s
B. $2$ m/s ✓ Correct
C. $20$ m/s
D. $200$ m/s
Solution: $V = \frac{5 \times 200}{500} = 2$ m/s.
Q14 — Newtons 3 Laws · easy · numerical
A swimmer pushes water backward with $60$ N. The force propelling the swimmer forward is
A. $30$ N
B. $60$ N ✓ Correct
C. Zero
D. $120$ N
Solution: Third law: the water pushes the swimmer forward with $60$ N.
Q15 — Newtons 3 Laws · medium · numerical
Blocks of $1$ kg and $2$ kg touch on a smooth floor; $6$ N is applied to the $1$ kg block. The contact force is
A. $4$ N ✓ Correct
B. $3$ N
C. $6$ N
D. $2$ N
Solution: $a = \frac{6}{3} = 2$ m/s²; force on $2$ kg block $= 2 \times 2 = 4$ N.
Q16 — Newtons 3 Laws · easy · numerical
Two people of $60$ kg and $90$ kg push apart on ice. The $60$ kg person moves at $3$ m/s; the $90$ kg person moves at
A. $3$ m/s
B. $1$ m/s
C. $4.5$ m/s
D. $2$ m/s ✓ Correct
Solution: $60 \times 3 = 90 \times V \Rightarrow V = 2$ m/s.
Q17 — Newtons 3 Laws · medium · numerical
A bullet of $0.04$ kg is fired at $50$ m/s. The magnitude of the gun's recoil momentum is
A. $1.25$ kg·m/s
B. $2$ kg·m/s ✓ Correct
C. $0.8$ kg·m/s
D. $50$ kg·m/s
Solution: Recoil momentum equals bullet momentum: $0.04 \times 50 = 2$ kg·m/s.
Q18 — Newtons 3 Laws · medium · numerical
Blocks of $3$ kg and $2$ kg touch on a smooth floor; $15$ N pushes the $3$ kg block. The contact force is
A. $15$ N
B. $6$ N ✓ Correct
C. $9$ N
D. $5$ N
Solution: $a = \frac{15}{5} = 3$ m/s²; force on $2$ kg $= 2 \times 3 = 6$ N.
Q19 — Newtons 3 Laws · medium · numerical
A gun of $10$ kg recoils at $0.5$ m/s after firing a $0.05$ kg bullet. The bullet's speed is
A. $100$ m/s ✓ Correct
B. $10$ m/s
C. $50$ m/s
D. $200$ m/s
Solution: $0.05 v = 10 \times 0.5 \Rightarrow v = 100$ m/s.
Q20 — Newtons 3 Laws · medium · numerical
A rocket ejects gas at the rate of $2$ kg/s with a speed of $500$ m/s. The thrust on the rocket is
A. $1000$ N ✓ Correct
B. $502$ N
C. $2000$ N
D. $250$ N
Solution: Thrust $= v \frac{dm}{dt} = 500 \times 2 = 1000$ N.
Q21 — Newtons 3 Laws · medium · numerical
A gun of $6$ kg fires a $0.02$ kg bullet at $300$ m/s. The recoil speed of the gun is
A. $1$ m/s ✓ Correct
B. $6$ m/s
C. $0.1$ m/s
D. $300$ m/s
Solution: $V = \frac{0.02 \times 300}{6} = 1$ m/s.
Q22 — Newtons 3 Laws · medium · numerical
On a smooth floor, $20$ N pushes a $6$ kg block that is in contact with a $4$ kg block. The contact force on the $4$ kg block is
A. $8$ N ✓ Correct
B. $12$ N
C. $20$ N
D. $4$ N
Solution: $a = \frac{20}{10} = 2$ m/s²; force on $4$ kg $= 4 \times 2 = 8$ N.
Q23 — Newtons 3 Laws · easy · numerical
Two carts push apart. A $20$ kg cart moves at $4$ m/s; a $40$ kg cart moves at
A. $2$ m/s ✓ Correct
B. $4$ m/s
C. $8$ m/s
D. $1$ m/s
Solution: $20 \times 4 = 40 \times V \Rightarrow V = 2$ m/s.
Q24 — Newtons 3 Laws · medium · numerical
An $80$ kg astronaut throws a $2$ kg tool at $8$ m/s. The astronaut's recoil speed is
A. $8$ m/s
B. $2$ m/s
C. $0.2$ m/s ✓ Correct
D. $0.02$ m/s
Solution: $V = \frac{2 \times 8}{80} = 0.2$ m/s.
Q25 — Newtons 3 Laws · easy · theory
A horse pulls a cart with a force of $200$ N. The force the cart exerts on the horse is
A. $100$ N
B. Zero
C. $200$ N backward ✓ Correct
D. $400$ N
Solution: Third law: equal and opposite, $200$ N on the horse.
Q26 — Newtons 3 Laws · medium · theory
A book of weight $20$ N rests on a table. The normal force the table exerts on the book is
A. $10$ N
B. $20$ N upward ✓ Correct
C. Zero
D. $40$ N
Solution: The table's reaction balances the weight: $20$ N up.
Q27 — Newtons 3 Laws · medium · numerical
A rocket ejects $5$ kg/s of gas at $200$ m/s. The thrust developed is
A. $40$ N
B. $100$ N
C. $1000$ N ✓ Correct
D. $205$ N
Solution: Thrust $= 200 \times 5 = 1000$ N.
Q28 — Newtons 3 Laws · medium · numerical
A bullet has momentum $3$ kg·m/s. A $6$ kg gun firing it recoils at
A. $18$ m/s
B. $0.5$ m/s ✓ Correct
C. $2$ m/s
D. $3$ m/s
Solution: Recoil momentum $= 3$ kg·m/s; $V = \frac{3}{6} = 0.5$ m/s.
Q29 — Newtons 3 Laws · medium · numerical
On a smooth floor, $10$ N pushes a $4$ kg block in contact with a $1$ kg block. The contact force on the $1$ kg block is
A. $10$ N
B. $5$ N
C. $2$ N ✓ Correct
D. $8$ N
Solution: $a = \frac{10}{5} = 2$ m/s²; force on $1$ kg $= 1 \times 2 = 2$ N.
Q30 — Newtons 3 Laws · easy · numerical
A $40$ kg girl jumps forward at $2$ m/s off a $10$ kg cart (smooth ground). The cart moves backward at
A. $0.5$ m/s
B. $4$ m/s
C. $2$ m/s
D. $8$ m/s ✓ Correct
Solution: $40 \times 2 = 10 \times V \Rightarrow V = 8$ m/s.