The third law of motion states that for every action, there is an equal and opposite reaction. This means that whenever an object exerts a force on another thing, the second object will exert an equal but opposite force back on the first object. In other words, forces always occur in pairs and are of equal strength but opposite in direction.
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Daily life Examples
- Jumping off a diving board: The force of your push on the board is the action, and the reaction is the board pushing back on you, propelling you into the air.
- Rowing a boat: Your oars push against the water, and the water pushes back on the oars, propelling the boat forward.
- Walking: Your foot exerts a force on the ground, and the ground exerts an equal and opposite force back on your foot, allowing you to move forward.
- Bouncing a ball: The action is the ball hitting the ground, and the reaction is the ground pushing back on the ball, causing it to bounce back up.
- Launching a rocket: The action is the force of the rocket’s engines pushing against the ground, and the reaction is the ground pushing back on the rocket, propelling it upwards.
Equations Related to the Third Law of Motion
| Equation | Description |
| F1 = -F2 | If object 1 exerts a force (F1) on object 2, then object 2 will exert an equal and opposite force (-F2) on object 1. |
| F = ma | The force (F) on an object is equal to its mass (m) times its acceleration (a). This means that if an object exerts a force on another object, the second object will experience an acceleration that is directly proportional to the force and inversely proportional to its mass. |
Third Law of Motion in Simple Words
The third law of motion states that every action has an equal and opposite reaction. When one object exerts a force on another object, the second object always exerts a force back on the first object that is equal in strength but opposite in direction. This applies to all kinds of situations and helps us understand how forces work in the world around us.
So, imagine you are pushing a heavy box across the floor. The force of your push on the box is the action, and the reaction is the force of the box pushing back on you with the same amount of force. This is why it takes effort to move the box – you are not only pushing the box, but the box is also pushing back on you.
Third Law of Motion and Momentum
| Third Law of Motion and Momentum | Description | Examples |
| Third Law | Every action has an equal and opposite reaction. When one object exerts a force on another object, the second object will always exert an equal and opposite force back on the first object. | When you push a book across a table, the book exerts an equal and opposite force back on you. |
| Momentum | Momentum is a property of moving objects that describes how difficult it is to stop them. It is calculated by multiplying an object’s mass by its velocity. The greater an object’s momentum, the harder it is to change its motion. | A truck has more momentum than a bicycle because it has a larger mass and is moving at a higher velocity. |
| Relationship | When we consider the third law of motion and momentum together, we can see that the force of one object on another will always result in a transfer of momentum from one object to the other. Mathematically, we can express this as: F1,2 = -F2,1 = Δp/Δt where: F1,2 is the force exerted by object 1 on object 2; F2,1 is the force exerted by object 2 on object 1; Δp/Δt is the rate of change of momentum. | When a baseball bat hits a baseball, the force of the bat on the ball results in a transfer of momentum from the bat to the ball. |
| Examples | When a moving billiard ball collides with a stationary ball, the moving ball will exert a force on the stationary ball, and the stationary ball will exert an equal and opposite force back on the moving ball. As a result, the momentum of the moving ball will be transferred to the stationary ball, causing it to move in the direction of the force. | When a rocket launches, the force of the exhaust gases pushing out of the bottom of the rocket results in a transfer of momentum from the rocket to the gases, propelling the rocket forward. |
Summary
- The third law of motion states that for every action, there is an equal and opposite reaction.
- This means that when one object exerts a force on another object, the second object will exert an equal and opposite force on the first object.
- The forces involved are always equal in magnitude and opposite in direction.
- The third law applies to all objects and all forces, including gravitational, electrical, and magnetic forces.
- The third law plays a crucial role in the motion of objects, as it governs the interactions between them.
- Understanding the third law is important in many areas of science and technology, such as engineering, physics, and astronomy.
- Examples of the third law in action include the launching of rockets, the recoil of a gun, and the bouncing of a ball.
Related Links
Inertia and the Laws of Motion
Newton’s Second Law of Motion
Inertia in Action: Real-Life Examples of Newton’s First Law
SI Unit of Work| Definition, Formula, and Examples
Numerical Problems
Problem: Two objects of different masses, A and B, are in contact with each other. A exerts a force of 20 N on B. What force does B exert on A?
Solution: According to the third law of motion, the force that B exerts on A will be equal in magnitude but opposite in direction to the force that A exerts on B. Therefore, the force that B exerts on A will also be 20 N, but in the opposite direction.
Problem: A rocket is launched from a stationary position on the ground. The exhaust gases that propel the rocket have a mass of 500 kg and are expelled from the rocket at a velocity of 200 m/s. If the force that the gases exert on the rocket is 50,000 N, what is the acceleration of the rocket?
Solution: According to the third law of motion, the force that the gases exert on the rocket will be equal in magnitude but opposite in direction to the force that the rocket exerts on the gases.
Therefore, the rocket experiences a force of 50,000 N in the opposite direction of the expelled gases.
Using the formula:
F = ma
where:
F is the force, m is the mass of the rocket, and a is the acceleration.
we can solve for the acceleration of the rocket:
a = F/m = 50,000 N / (mrocket + mgases) = 50,000 N / (mrocket + 500 kg).
Note that we need to add the mass of the expelled gases to the mass of the rocket since both are accelerating in opposite directions.
Multiple Choice Questions
- What does the third law of motion state?
A) For every action, there is a reaction
B) For every force, there is an equal and opposite force
C) For every motion, there is an equal and opposite motion
D) For every object, there is an equal and opposite object
Answer: B) For every force, there is an equal and opposite force
- What is an example of the third law of motion in action?
A) A person jumping off a diving board
B) A ball rolling down a hill
C) A rocket blasting off into space
D) A car accelerating from a stop sign
Answer: C) A rocket blasting off into space
- Can an object exert a force on itself?
A) Yes
B) No
Answer: B) No
- How does the third law of motion relate to the motion of objects?
A) It describes the acceleration of objects
B) It describes the force required to move an object
C) It describes the interaction between two objects
D) It does not relate to the motion of objects
Answer: C) It describes the interaction between two objects
- What is the difference between action and reaction forces?
A) Action forces are stronger than reaction forces
B) Action forces always act first, followed by reaction forces
C) Action and reaction forces are equal and opposite but act on different objects
D) Action and reaction forces are the same things
Answer: C) Action and reaction forces are equal and opposite, but act on different objects
Frequently Asked Questions
| # | Question | Answer |
| 1 | What is the third law of motion? | The third law of motion states that for every action, there is an equal and opposite reaction. In other words, when one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. |
| 2 | How does the third law of motion apply to a person sitting in a chair? | When a person sits in a chair, the person’s weight exerts a force on the chair, and the chair exerts an equal and opposite force on the person. This is an example of the third law of motion. |
| 3 | Does the third law of motion only apply to objects that are in contact? | No, the third law of motion applies to all objects, whether they are in contact or not. For example, the Earth exerts a gravitational force on the Moon, and the Moon exerts an equal and opposite gravitational force on the Earth. |
| 4 | Can an object exert a force on itself? | No, an object cannot exert a force on itself. The third law of motion requires that the action and reaction forces be exerted by different objects. |
| 5 | How does the third law of motion relate to Newton’s laws of motion? | The third law of motion is one of Newton’s three laws of motion, which describe the fundamental principles of motion and forces. The third law states that forces always occur in pairs, which is important in understanding the motion of objects. |
| 6 | What is the difference between action and reaction forces? | Action and reaction forces are equal in magnitude and opposite in direction, but they act on different objects. The action force is the force exerted by the first object on the second object, while the reaction force is the force exerted by the second object on the first object. |
| 7 | How do action and reaction forces affect the motion of objects? | Action and reaction forces have equal magnitudes and opposite directions, so they cancel each other out. However, they can still affect the motion of objects. For example, a rocket’s engines expel gas in one direction, which creates an equal and opposite reaction force that propels the rocket forward. |
| 8 | Can action and reaction forces cancel each other out completely? | Yes, action and reaction forces have equal magnitudes and opposite directions, so they can cancel each other out completely. However, they can still affect the motion of objects, as described in the previous answer. |
| 9 | How can the third law of motion be used in real-life applications? | The third law of motion can be used in many real-life applications, such as designing rockets and other propulsion systems, understanding collisions between objects, and analyzing the motion of vehicles and other machines. |
More Links
Mass vs Weight| Difference and Solved Problems
Kinematic Equations| Sample Problems and Solutions
Linear Motion or Rectilinear Motion| Daily Life Examples
Linear Acceleration
Stress in Physics| Definition and Easy Examples
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