Newton’s Third Law of Motion – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain, with an example of walking, how Newton’s Third Law helps us move forward. Why do the action and reaction forces not cancel each other?
Answer:
When we walk, we push the ground backward with our foot — this is the action. The ground pushes our foot forward with an equal and opposite force — this is the reaction.
These two forces are equal in magnitude and opposite in direction, and they occur simultaneously as Newton’s Third Law states.
They do not cancel because they act on different objects: the backward force acts on the ground, while the forward force acts on you.
The forward reaction accelerates you (overcoming inertia and friction), while the ground receives the backward push (transfers force to Earth).
Thus, walking depends on the interaction between you and the ground, and the separate objects ensure forces do not nullify each other on a single body.
Q2. Describe how a rocket uses Newton’s Third Law to launch. Include the role of expelled gases in producing thrust.
Answer:
A rocket engine expels hot gases downward at high speed; this expulsion is the action force.
By Newton’s Third Law, the expelled gases exert an equal and opposite force on the rocket — this is the reaction, called thrust, which pushes the rocket upwards.
The rocket’s rise is a result of momentum conservation: gases gain downward momentum, the rocket gains upward momentum so total momentum is conserved.
The amount of thrust depends on the mass flow rate of gases and their exhaust velocity.
Even in space, where there is no air, rockets work because they push mass (gases) away; a surrounding medium is not required.
Thus, rocket motion is a clear example of action-reaction and momentum exchange producing useful propulsion.
Q3. Using the example of swimming, explain how Newton’s Third Law enables forward movement in water.
Answer:
When a swimmer pushes water backward with hands and feet, this is the action force applied to the water.
The water responds by pushing the swimmer forward with an equal and opposite reaction force.
These forces act on different objects — the action on water, the reaction on the swimmer — so they do not cancel on the swimmer.
The swimmer’s forward acceleration depends on the force magnitude, the duration of the stroke, and the swimmer’s mass.
Technique matters: pushing a larger volume of water or pushing it faster produces a greater reaction thrust, improving propulsion.
Thus, swimming is practical use of Newton’s Third Law and momentum transfer between swimmer and water.
Q4. Explain the recoil of a gun when it is fired. How does Newton’s Third Law and conservation of momentum describe this effect?
Answer:
When a bullet is fired, the explosion pushes the bullet forward — this is the action force on the bullet.
The gun experiences an equal and opposite force backward — this is the recoil or reaction.
Initially the system (gun + bullet) has zero momentum; after firing the forward momentum of the bullet and backward momentum of the gun are equal and opposite, so total momentum remains zero.
The recoil velocity of the gun is small because the gun’s mass is much larger than the bullet’s mass (momentum p = mv).
Newton’s Third Law guarantees the forces are equal; conservation of momentum gives the relation between their velocities.
Proper handling and recoil mitigation systems reduce the felt recoil for safety and accuracy.
Q5. Two ice-skaters push each other and move apart. Explain why the forces they feel are equal and opposite but their accelerations differ.
Answer:
When skaters push off, each applies a force on the other; these are action–reaction pairs and are equal in magnitude and opposite in direction.
The forces act on different skaters, so they don’t cancel; each skater experiences a net force on their own body.
According to Newton’s Second Law (F = ma), the acceleration of each skater is a = F/m. Since the same force F acts on both, the skater with smaller mass gets larger acceleration, and the heavier skater gets smaller acceleration.
Thus, equal forces produce different accelerations because of the difference in masses.
Momentum conservation also applies: the product of mass and velocity for one skater equals and opposes that of the other, keeping total momentum constant.
High Complexity (Analytical & Scenario-Based)
Q6. Two ice-skaters, A (50 kg) and B (70 kg), initially at rest, push off each other. If skater A moves backwards at 2 m/s, calculate the velocity of skater B. Explain how this illustrates Newton’s Third Law and conservation of momentum.
Answer:
Initially total momentum is zero because they are at rest. After pushing, momenta must still add to zero: mA·vA + mB·vB = 0.
Given mA = 50 kg, vA = −2 m/s (negative sign indicates backward), mB = 70 kg, solve for vB: 50(−2) + 70·vB = 0 ⇒ −100 + 70·vB = 0 ⇒ vB = 100/70 = 1.4286 m/s forward.
This shows skater B moves forward at about 1.43 m/s.
Newton’s Third Law: the push forces on each skater are equal and opposite. They act on different bodies, producing opposite momenta.
Conservation of momentum links these momenta; the larger mass (B) ends up with smaller velocity than the lighter skater A, consistent with F = ma and the equal force impulse applied during the push.
Q7. Analyze why a rocket produces thrust in space (vacuum) using Newton’s Third Law, and discuss whether the presence of air affects thrust magnitude.
Answer:
A rocket produces thrust by expelling mass (exhaust gases) backward at high speed — that is the action. The rocket receives an equal and opposite reaction force forward, producing acceleration.
In vacuum, there is no surrounding air, but this does not prevent thrust because the mechanism depends on momentum exchange between rocket and expelled gases, not on pushing against air.
In atmosphere, external pressure can slightly alter the effective thrust (nozzle design matters), but the fundamental thrust arises from mass flow rate and exhaust velocity.
Thus, Newton’s Third Law explains propulsion both in air and vacuum; the efficiency and net thrust may vary with ambient pressure but the basic action–reaction remains the same.
Q8. A book rests on a table. Identify action–reaction pairs related to the book, Earth, and table. Explain common misconceptions about forces canceling in this situation.
Answer:
Relevant pairs: (1) Earth pulls book downward by gravity; the reaction is book pulls Earth upward (equal and opposite). (2) Book pushes table downward with its weight; the table exerts an upward normal force on the book — these two are an action–reaction pair.
Common misconception: People think the book’s weight and the normal force on the book are action–reaction. They are equal and opposite but act on the same object (book) so they do not form an action–reaction pair; however they do balance as forces on the same object producing no net acceleration.
Real action–reaction pairs always act on different objects (book–Earth, book–table), which clarifies why forces don’t simply cancel across objects and why the table supports the book.
Q9. A person stands on a weighing scale in an elevator. When the elevator accelerates upward, the scale shows a larger reading. Using Newton’s Third Law and force analysis, explain why the reading increases.
Answer:
Let the person have mass m. When the elevator accelerates upward with acceleration a, the person must accelerate upward too.
Forces on the person: upward normal force (N) from the scale and downward weight (mg) from Earth’s gravity. Net force = N − mg = m·a. So N = m(g + a).
The scale reading equals the normal force N, so it increases when a > 0.
Newton’s Third Law: the person pushes down on the scale (action), and the scale pushes up on the person with equal and opposite force (reaction). During upward acceleration, the person must push harder on the scale, producing a larger reaction force (higher reading).
Thus, combined Newton’s laws explain the increased apparent weight during upward acceleration.
Q10. A toy balloon is inflated and released without tying the neck; it flies around as air rushes out. Explain this motion using Newton’s Third Law and conservation principles. How would changing the neck size affect the speed?
Answer:
When the balloon is released, air rushes out of its neck backward — this is the action (mass ejected rearwards).
The balloon receives an equal and opposite reaction forward, causing it to fly erratically in the opposite direction. This is a clear demonstration of Newton’s Third Law and momentum conservation: expelled air gains backward momentum, balloon gains forward momentum.
If the neck is narrower, air exits at a higher exit velocity (for the same pressure), increasing thrust and producing greater forward speed but for a shorter time of thrust; a wider neck gives lower exit speed but larger mass flow, so the detailed result depends on pressure and mass flow.
Overall, balloon motion depends on exhaust velocity, mass flow, and the balloon’s mass, illustrating action-reaction and momentum exchange in a simple experiment.
