An excellent example to explain the action and reaction law, also known as Newton's Third Law of Motion, is when someone pushes a box, the box applies back to the hand of the person a force with the same magnitude and opposite direction.
Understanding Newton's Third Law of Motion
Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that whenever one object exerts a force on a second object, the second object simultaneously exerts a force equal in magnitude and opposite in direction on the first object. These forces always come in pairs and are known as action-reaction pairs.
The Pushing a Box Example Explained
Let's break down the scenario of pushing a box to clearly illustrate this fundamental principle:
- The Action: When you push a box, your hand exerts a force on the box in the direction you are pushing (e.g., forward). This is the "action" force.
- The Reaction: Simultaneously, the box exerts an equal and opposite force back on your hand. This force is directed backward, opposite to the direction you are pushing. This is the "reaction" force.
Even though you might not consciously feel the box pushing back with the same intensity, especially if the box is light, that force is always present. If you push harder, the box pushes back harder. The sensation of the box pushing back becomes more apparent when you push against a heavy, stationary object, or if your hand starts to hurt from pushing too hard.
Key Characteristics of Action-Reaction Pairs
It's crucial to understand several aspects of these force pairs:
- Act on Different Objects: The action force acts on one object, while the reaction force acts on the other object involved in the interaction. In our example, your hand pushes the box (force on the box), and the box pushes your hand (force on your hand).
- Equal in Magnitude, Opposite in Direction: The strength (magnitude) of the two forces is always the same, but they point in precisely opposite directions.
- Simultaneous Occurrence: Action and reaction forces happen at the exact same moment; one does not precede the other.
- Do Not Cancel Out: Because they act on different objects, action and reaction forces do not cancel each other out. If they acted on the same object, the net force would be zero, and no motion would occur.
Other Practical Examples of Action and Reaction
The action and reaction law is evident in countless everyday phenomena:
- Walking: When you walk, your foot pushes backward on the ground (action). The ground, in turn, pushes forward on your foot (reaction), propelling you forward.
- Rocket Propulsion: A rocket expels hot gases downward (action). The expelled gases push the rocket upward (reaction), causing it to accelerate. For more details on this, you can explore resources on rocket science.
- Bird Flight: A bird's wings push air downward and backward (action). The air pushes the wings upward and forward (reaction), allowing the bird to fly.
- Swimming: A swimmer pushes water backward with their hands and feet (action). The water pushes the swimmer forward (reaction).
- Recoil of a Gun: When a bullet is fired, the gun exerts a force on the bullet, pushing it forward (action). The bullet simultaneously exerts an equal and opposite force on the gun, causing it to kick backward (reaction).
Summarizing Action-Reaction Pairs
Here’s a table summarizing common action-reaction scenarios:
| Action Force (on Object 2) | Reaction Force (on Object 1) | Outcome/Effect |
|---|---|---|
| Foot pushes ground backward | Ground pushes foot forward | Person walks forward |
| Rocket pushes gases downward | Gases push rocket upward | Rocket accelerates upward |
| Wing pushes air downward & backward | Air pushes wing upward & forward | Bird flies |
| Hand/Foot pushes water backward | Water pushes hand/foot forward | Swimmer moves forward |
| Gun pushes bullet forward | Bullet pushes gun backward | Bullet accelerates, gun recoils |
| Person's hand pushes box forward | Box pushes person's hand backward | Box moves, hand feels pressure |
Understanding Newton's Third Law is fundamental to comprehending how forces work in the physical world and is critical in fields from engineering to sports.
