Newton's Third Law of Motion - UNSOLVED PRACTICE SET
Chapter: Force and Laws of Motion | Topic: Newtons Third Law
NEWTON'S THIRD LAW OF MOTION - UNSOLVED PRACTICE SET
Topic: Newtons Third Law
Multiple Choice Questions
Q1. Newton's Third Law states that action and reaction forces:
- Act on the same object in opposite directions
- Act on two different objects and are equal and opposite
- Act on different objects but are unequal
- Always cancel each other and produce zero motion
Q2. A rocket accelerates in space because:
- Exhaust gases push against the atmosphere
- Exhaust gases are pushed backward, and by reaction the rocket is pushed forward
- Gravity pulls the rocket forward
- The vacuum in space creates suction
Q3. When you walk, you push the ground backward. The reaction force that propels you forward comes from:
- Your muscles
- Air resistance
- The ground pushing your foot forward
- Gravity
Q4. A swimmer pushes the pool wall with her feet and pushes off. The force that propels her is:
- Her kicking muscles acting on herself
- The wall pushing her feet in the opposite direction
- Water resistance
- Buoyancy
- Q5. Which is the correct action–reaction pair when a book rests on a table?
- Weight of book (down) and normal reaction on book (up) — both on the book
- Weight of book on Earth (down) and pull of book on Earth (up)
- Friction on book and applied force on book
- Normal force on table and weight of table
Q6. A force of 80 N is the action force. The reaction force is:
- 80 N in the same direction
- 40 N in the opposite direction
- 80 N in the opposite direction
- 160 N in the opposite direction
Short Answer Questions
Q7. State Newton's Third Law of Motion. Why is it essential to say that the two forces act on different objects?
Q8. A gun recoils when fired. Explain this using Newton's Third Law. Why does the bullet move much faster than the gun even though the forces are equal?
Q9. Identify the action and reaction force pair for: (a) a person jumping off a boat to the shore, and (b) a ball hitting and rebounding from a wall.
Q10. If action and reaction are always equal and opposite, why do they not cancel each other out and prevent all motion? This is a common misconception — clear it up with a careful explanation.
Q11. How does a squid propel itself through water? Explain the Newton's Third Law principle behind biological jet propulsion.
Q12. When a firefighter holds a high-pressure hose, why is it difficult to keep steady? What strategy do firefighters use, and what law explains this?
Long Answer Questions
Q13. Explain Newton's Third Law using an ISRO rocket launch. Your answer must cover:
(a) identifying the action and reaction forces during launch,
(b) why the rocket rises despite being very heavy,
(c) why the two forces do not cancel (different objects),
(d) what happens to acceleration as fuel burns and rocket mass decreases, and
(e) how the rocket manoeuvres in space where there is no air to push against.
Q14. A student argues: 'Since action = reaction, the net force is always zero, so nothing can ever accelerate.' Identify and correct every flaw in this argument. Your answer must:
(a) state Newton's Third Law precisely,
(b) explain that the paired forces act on different bodies,
(c) show with a numerical example that each body has a non-zero net force,
(d) use the horse-and-cart example, and
(e) conclude clearly why acceleration is still possible.
Q15. Analyse Newton's Third Law in the context of everyday movement.
(a) Describe the action–reaction pair when a person walks forward.
(b) Why is walking on ice difficult?
(c) How would walking differ on the Moon?
(d) Identify the action–reaction pair when a bird flaps its wings.
(e) Give two original examples from Indian daily life where Newton's Third Law is clearly at work and identify the pairs.
Numerical / Application-Based Problems
Q16. A rifle of mass 4 kg fires a bullet of mass 0.04 kg. The bullet leaves at 320 m/s.
(a) Calculate the momentum of the bullet.
(b) Using conservation of momentum (initial momentum = 0), find the momentum of the rifle after firing.
(c) Calculate the recoil velocity of the rifle.
Q17. Two skaters start at rest facing each other. Skater P (mass 55 kg) pushes Skater Q (mass 44 kg) and P moves to the left at 3.2 m/s.
(a) State Newton's Third Law for this situation.
(b) Calculate P's momentum after the push.
(c) Using conservation of momentum, calculate Q's velocity (magnitude and direction).
Q18. A rocket in space ejects exhaust gas at 2500 m/s relative to itself at a rate of 60 kg/s.
(a) Calculate the thrust force on the rocket (Thrust = ejection rate × exhaust velocity).
(b) If the rocket's current mass is 2400 kg, calculate its acceleration.
(c) After burning half the fuel, mass drops to 1200 kg. Recalculate the acceleration.
(d) What does the increasing acceleration tell you about the motion of the rocket?