Rocket works on the principle of:
Newton's Third Law
Understanding Rocket Propulsion and Newton's Laws
Rockets are amazing machines that work by expelling hot gas at high speed in one direction. This action creates a powerful force that pushes the rocket in the opposite direction. This principle is a direct application of one of Isaac Newton's fundamental laws of motion.
Which Newton's Law Powers Rockets?
Let's look at Newton's Laws of Motion to understand which one explains rocket propulsion:
- Newton's First Law (Law of Inertia): This law states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. While relevant to how a rocket behaves once propelled (it will continue moving unless acted upon by external forces like gravity or air resistance), it doesn't explain how the propulsion force is generated.
- Newton's Second Law (Law of Acceleration): This law states that the acceleration of an object is directly proportional to the net force acting upon it and inversely proportional to its mass ($\text{F} = \text{m}\text{a}$). This law helps calculate the rocket's acceleration once the propulsion force (thrust) is known, but it doesn't explain the origin of the thrust force itself.
- Newton's Third Law (Law of Action-Reaction): This law states that for every action, there is an equal and opposite reaction. When a rocket expels hot gas downwards (the action), the gas exerts an equal and opposite force upwards on the rocket (the reaction). This upward force is called thrust, and it is what propels the rocket forward.
- Newton's Fourth Law: There is no recognized "Newton's Fourth Law" of motion. Newton formulated three primary laws of motion.
How Newton's Third Law Explains Rocket Motion
The core mechanism of a rocket is based on expelling mass. Fuel and oxidizer burn in the combustion chamber, creating high-pressure, high-temperature gas. This gas is then expelled through a nozzle at very high velocity. According to Newton's Third Law:
- Action: The rocket pushes the expelled gas downwards.
- Reaction: The expelled gas pushes the rocket upwards with an equal magnitude of force.
This reaction force is the thrust that overcomes gravity and air resistance, allowing the rocket to accelerate into space.
Therefore, the principle on which a rocket works is Newton's Third Law of Motion.
| Law | Statement | Relevance to Rocketry |
|---|---|---|
| First Law | Inertia: Object stays at rest or constant velocity unless a force acts on it. | Explains the rocket's motion path once thrust is applied and how it moves in space without thrust. |
| Second Law | $\text{F} = \text{m}\text{a}$: Force equals mass times acceleration. | Used to calculate the rocket's acceleration based on the thrust force and its mass. |
| Third Law | Action-Reaction: For every action, there is an equal and opposite reaction. | Explains the origin of the thrust force itself – the primary principle of rocket propulsion. |
Revision Table: Key Concepts
| Concept | Brief Explanation |
|---|---|
| Newton's Laws of Motion | Three fundamental laws describing the relationship between an object, the forces acting upon it, and its motion. |
| Thrust | The reaction force that propels a rocket forward, generated by expelling mass. |
| Action-Reaction | The principle of Newton's Third Law where every force has a corresponding equal and opposite force. |
| Momentum | The product of mass and velocity. Rocketry is also deeply connected to the conservation of momentum. |
Additional Information: Conservation of Momentum
While Newton's Third Law is the direct principle explaining thrust generation, rocket motion can also be understood through the principle of conservation of momentum. The total momentum of the rocket-exhaust system remains constant (assuming no external forces). As the rocket expels mass (exhaust) with momentum in one direction, the rocket itself gains an equal amount of momentum in the opposite direction. This continuous expulsion of mass with high velocity results in a continuous change in the rocket's momentum, leading to acceleration.
Both Newton's Third Law and the conservation of momentum describe the same physical phenomenon from different perspectives, and both are fundamental to understanding rocket propulsion.
Important Questions from Laws of Motion
A uniform meter scale of mass 0.24 kg is made of steel. It is kept on two wedges, W1 and W2 , in a horizontal position. W1 is at a distance of 0.2 m from one of its ends, while W2 is at distance of 0.4 m from the other end. If the force on the scale is N1 due to W1 and N2 due to W2, then : (take g =10·0 m s-2
A rocket is launched to travel vertically upward with a constant velocity of 20 m/s. After travelling for 35 seconds, the rocket develops a snag and its fuel supply is cut off. The rocket then travels like a free body. The height achieved by it is:
According to Newton's third law of motion, mark the correct option.
1. Action and reaction act on different bodies and so they can be cancelled out.
2. The internal action and reaction forces between different parts of a body do, however, sum to zero.
If Force (F), velocity (V) and time (T) are taken as the fundamental dimensions, instead of mass, length and time, what will be dimensions of linear Momentum (P) ?
A block is tying stationary on an inclined plane of coefficient of friction μ and angle θ, if θ is slowly increased the frictional force will
