Newton's Third Law - UNSOLVED PRACTICE SET
Chapter: Laws of Motion | Topic: Newtons Third Law
NEWTON'S THIRD LAW - UNSOLVED PRACTICE SET
Topic: Newtons Third Law
Multiple Choice Questions
Q1. Newton's third law of motion states that:
- A body at rest remains at rest
- Force equals mass times acceleration
- To every action, there is an equal and opposite reaction
- Momentum is conserved in an isolated system
Q2. Action and reaction forces:
- Act on the same body
- Act on different bodies
- Cancel each other out
- Are always in the same direction
Q3. When you jump off a boat onto the shore, the boat moves backward. This is an example of:
- Newton's first law
- Newton's second law
- Newton's third law
- Conservation of energy
Q4. A book lying on a table exerts a force on the table. The table exerts an equal force on the book. Why doesn't the book accelerate?
- The forces cancel out because they act on different bodies
- The forces are not really equal
- The table is too heavy
- Gravity prevents acceleration
Q5. A horse pulls a cart. The cart pulls back on the horse with an equal force. How does the horse-cart system move forward?
- The horse pushes the ground backward, and the ground pushes the horse forward
- The cart's pull on the horse is less than the horse's pull on the cart
- The horse is stronger than the cart
- The forces cancel out and the system shouldn't move
Q6. When a gun is fired, it recoils. This is because:
- The bullet pushes air backward
- The explosion inside pushes the gun backward
- The gun pushes the bullet forward, and the bullet pushes the gun backward with equal force
- Gravity acts on the gun
Short Answer Questions
Q7. State Newton's third law of motion. Explain why action and reaction forces do not cancel each other out.
Q8. A man standing on a frictionless surface cannot move by himself. Explain why, using Newton's third law.
Q9. When a balloon is filled with air and released, it flies around the room. Explain this using Newton's third law.
Q10. In a school tug-of-war, Team A pulls the rope with a force of 500 N. What force does the rope exert on Team A? If Team B pulls with 450 N, what is the tension in the rope?
Q11. A swimmer pushes water backward with his hands. What pushes the swimmer forward? Identify the action-reaction pair.
Q12. A book of mass 1 kg is placed on a table. Calculate the force exerted by the book on the table and the force exerted by the table on the book. (Take g = 10 m/s²)
Long Answer Questions
Q13. State and explain Newton's third law of motion with at least four examples from daily life. Discuss why action and reaction forces, though equal and opposite, do not cancel each other. Explain the concept of action-reaction pairs with a free body diagram.
Q14. Analyse the following situations using Newton's third law:
(i) A rocket launching into space
(ii) A person walking on the ground
(iii) A bird flying in the air
(iv) A fish swimming in water
For each case, identify the action-reaction pair and explain how motion is produced.
Q15. Two blocks A (mass 3 kg) and B (mass 2 kg) are placed on a frictionless surface. A horizontal force F = 20 N is applied to block A.
(a) Draw free body diagrams for both blocks.
(b) Calculate the acceleration of the system.
(c) Calculate the force exerted by block A on block B.
(d) Calculate the force exerted by block B on block A.
(e) Verify that these two forces satisfy Newton's third law.
Application-Based Problems
Q16. A 60 kg man stands on a weighing machine inside a lift.
(a) Calculate the reading on the machine when the lift is at rest.
(b) Calculate the reading when the lift accelerates upward at 2 m/s².
(c) Calculate the reading when the lift accelerates downward at 2 m/s².
(d) Calculate the reading when the lift falls freely under gravity.
(e) Explain each case using Newton's third law and the concept of apparent weight.
Q17. Two ice skaters, A (mass 50 kg) and B (mass 70 kg), stand facing each other on a frictionless ice surface. Skater A pushes Skater B with a force of 100 N.
(a) Calculate the acceleration of each skater during the push.
(b) If the push lasts for 0.5 seconds, calculate the velocity of each skater immediately after the push.
(c) Calculate the momentum of each skater after the push.
(d) Verify that the total momentum of the system is conserved.
(e) Explain how Newton's third law ensures momentum conservation.
Q18. A jet engine on an aircraft takes in air at 100 kg/s and ejects it at 500 m/s relative to the aircraft. The aircraft is flying at 250 m/s.
(a) Calculate the change in momentum of the air per second (thrust).
(b) If the aircraft has a mass of 50,000 kg, calculate its acceleration due to this thrust alone.
(c) The aircraft also ejects burnt fuel at 2000 m/s relative to the aircraft at 2 kg/s. Calculate the additional thrust.
(d) Calculate the total acceleration of the aircraft.
(e) Explain the action-reaction pairs involved in jet propulsion.