Introduction
Sir Isaac Newton’s Three Laws of Motion are the foundation of classical mechanics. While the First Law describes inertia and the tendency of objects to maintain their state, and the Second Law explains the relationship between force, mass, and acceleration, the Third Law gives us one of the most intuitive and famous principles in physics:
“For every action, there is an equal and opposite reaction.”
This simple yet powerful law governs almost all interactions we see around us. Whether you’re walking on the ground, flying a kite, rowing a boat, or even launching a rocket, Newton’s Third Law is at work.
In this article, we’ll dive deep into Newton’s Third Law of Motion, understand its meaning, explore everyday life examples, solve problems, discuss misconceptions, and appreciate its importance in science and technology.
Understanding Newton’s Third Law
Statement
Newton’s Third Law states:
If body A exerts a force on body B, then body B exerts an equal and opposite force on body A.
In symbolic form: FAB=−FBAF_{AB} = -F_{BA}FAB=−FBA
What It Means
- Forces always come in pairs.
You can’t have a single, isolated force. Whenever one object applies a force on another, the second object simultaneously applies a force of equal magnitude but in the opposite direction. - Action and reaction act on different bodies.
- If you push a wall, the wall pushes back on you.
- If Earth pulls you down with gravity, you pull Earth upward with the same force (though the Earth’s mass is so large that its motion is negligible).
- Equal does not mean identical motion.
- Forces are equal in size and opposite in direction.
- But the resulting accelerations may differ because different objects may have different masses.
Everyday Life Examples of Newton’s Third Law
Let’s explore some common and relatable situations where this law appears:
1. Walking or Running
When you walk, your foot pushes backward on the ground (action). In response, the ground pushes you forward with equal and opposite force (reaction). Without this reaction, you couldn’t move forward.
2. Rowing a Boat
The oar pushes water backward (action), and the water pushes the boat forward (reaction).
3. Swimming
A swimmer pushes water backward with their hands and legs (action). The water pushes the swimmer forward (reaction).
4. Jumping off a Boat or Raft
When you jump forward from a boat, you exert a force on the boat in the opposite direction. As a result, the boat moves backward.
5. Rocket Propulsion
A rocket expels hot gases downward at high speed (action). The gases push the rocket upward with equal force (reaction). This is one of the most powerful applications of Newton’s Third Law.
6. Gun Recoil
When a bullet is fired, the gun pushes the bullet forward (action). In turn, the bullet pushes the gun backward (reaction), which we feel as recoil.
7. Bird Flight
Birds push air downward with their wings (action). The air pushes the bird upward and forward (reaction), allowing it to fly.
8. Walking on Sand or Mud
If the surface is soft, the ground cannot push back strongly, which is why it feels difficult to walk on sand compared to a hard floor.
9. Balloon Rocket Experiment
When the air rushes out of a balloon in one direction (action), the balloon moves in the opposite direction (reaction).
10. Car Tires on Road
Tires push backward on the road (action). The road pushes the car forward (reaction), enabling acceleration.
Scientific Importance of Newton’s Third Law
- Foundation of Mechanics
It explains how objects interact with each other. No force exists in isolation. - Engineering and Technology
- Rockets, airplanes, helicopters all rely on action–reaction.
- Everyday vehicles like cars depend on tire–road interactions.
- Machines like jet engines and turbines work based on this principle.
- Explains Natural Phenomena
- Flight of birds and insects
- Swimming and rowing
- Even planetary motion involves gravitational action–reaction pairs.
Solved Problems on Newton’s Third Law
Problem 1: Gun Recoil
A bullet of mass 0.02 kg is fired from a gun of mass 2 kg. The bullet leaves the gun with a velocity of 300 m/s. What is the recoil velocity of the gun?
Solution:
By law of action–reaction, momentum is conserved.
Initial momentum = 0
Final momentum = 0 mbvb+mgvg=0m_b v_b + m_g v_g = 0mbvb+mgvg=0 (0.02)(300)+(2)(vg)=0(0.02)(300) + (2)(v_g) = 0(0.02)(300)+(2)(vg)=0 6+2vg=0 ⟹ vg=−3 m/s6 + 2v_g = 0 \implies v_g = -3 \, m/s6+2vg=0⟹vg=−3m/s
✅ The gun recoils backward at 3 m/s.
Problem 2: Rocket Thrust
A rocket of mass 1000 kg expels gases at a rate of 10 kg/s with velocity 500 m/s relative to the rocket. What is the thrust on the rocket?
Solution:
Thrust = Rate of change of momentum of gases F=m˙⋅vF = \dot{m} \cdot vF=m˙⋅v F=(10)(500)=5000 NF = (10)(500) = 5000 \, NF=(10)(500)=5000N
✅ The rocket experiences an upward thrust of 5000 N.
Problem 3: Jumping off a Boat
A man of mass 60 kg jumps off a stationary boat of mass 240 kg with a velocity of 5 m/s relative to the ground. What is the velocity of the boat?
Solution:
Initial momentum = 0
Final momentum = 0 mmvm+mbvb=0m_m v_m + m_b v_b = 0mmvm+mbvb=0 60(5)+240(vb)=060(5) + 240(v_b) = 060(5)+240(vb)=0 300+240vb=0 ⟹ vb=−1.25 m/s300 + 240 v_b = 0 \implies v_b = -1.25 \, m/s300+240vb=0⟹vb=−1.25m/s
✅ The boat moves backward with velocity 1.25 m/s.
Misconceptions About Newton’s Third Law
- “Action causes reaction later.”
Wrong! Action and reaction occur simultaneously, not one after the other. - “Action and reaction cancel each other.”
False. They act on different bodies, so they don’t cancel. For example, when you push a wall, the wall pushes back on you. Forces exist in pairs, but they don’t neutralize each other. - “Only moving objects obey this law.”
No. Even stationary objects exert forces. A book resting on a table pushes downward due to weight, and the table pushes upward with an equal reaction.
Leave a Reply