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.
2 only
Understanding Newton's Third Law of Motion and Forces
Newton's third law of motion is a fundamental principle in physics that describes the interaction between two objects. It states that for every action, there is an equal and opposite reaction.
Let's analyze the given statements based on Newton's third law.
Analysis of Statement 1: Action and Reaction Forces
Statement 1 says: "Action and reaction act on different bodies and so they can be cancelled out."
According to Newton's third law, when one body exerts a force on a second body (action), the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body (reaction). The key point is that these forces act on different bodies.
For example, if you push on a wall, you exert a force on the wall (action). The wall simultaneously exerts an equal and opposite force back on you (reaction). The action force is on the wall, and the reaction force is on you.
Because the action and reaction forces act on different bodies, they cannot cancel each other out to determine the motion of a single body. Forces can only cancel each other out if they act on the same body. The motion of a body is determined by the net external force acting on it.
Therefore, statement 1 is incorrect because action and reaction forces, although equal and opposite, act on different bodies and thus cannot cancel each other out in terms of determining the motion of one of the bodies.
Analysis of Statement 2: Internal Forces
Statement 2 says: "The internal action and reaction forces between different parts of a body do, however, sum to zero."
Consider a system consisting of multiple parts, like a complex object made of many particles, or even just the parts within a single rigid body. Forces between different parts of this system are called internal forces.
According to Newton's third law, if one part of the system exerts a force on another part, the second part exerts an equal and opposite force back on the first part. These pairs of forces are internal to the system.
When we consider the net internal force acting on the entire system, all these action-reaction pairs cancel each other out because they are equal in magnitude and opposite in direction, regardless of which parts they act on, as long as both parts are within the system.
For example, the gravitational force between your hand and your arm is an internal force within your body. Your hand pulls on your arm, and your arm pulls equally and oppositely on your hand. Within the system of your body, these forces sum to zero.
Therefore, statement 2 is correct. The sum of all internal forces within a system (or a single body considered as a system) is always zero due to Newton's third law action-reaction pairs.
Conclusion
Based on the analysis:
- Statement 1 is incorrect because action and reaction act on different bodies and do not cancel each other in determining the motion of a single body.
- Statement 2 is correct because internal action and reaction forces within a body (or system) sum up to zero.
Thus, only statement 2 is correct according to Newton's third law of motion.
Revision Table: Newton's Third Law Concepts
| Concept | Description | Applicability |
|---|---|---|
| Action-Reaction Pair | Forces that are equal in magnitude, opposite in direction, and act simultaneously. Represented as $\vec{F}_{AB} = -\vec{F}_{BA}$. | Always act on two different bodies. |
| External Force | A force exerted on a system by something outside the system. | Determines the motion of the center of mass of the system. |
| Internal Force | A force exerted by one part of a system on another part within the same system. | Acts within the system. Do not affect the motion of the center of mass of the system (sum to zero for the entire system). |
Additional Information on Action-Reaction and Internal Forces
It is crucial to distinguish between action-reaction pairs and forces that are in equilibrium. Forces in equilibrium act on the same body and sum to zero, causing no change in its state of motion. Action-reaction forces, on the other hand, always act on different bodies.
Internal forces are important for understanding the deformation or structure of a body, but they do not cause the body as a whole to accelerate or change its overall momentum. Only external forces can change the momentum of a system.
Consider a book resting on a table. The Earth pulls the book down (gravity, external force on the book). The table pushes the book up (normal force, external force on the book). These two forces are equal and opposite, causing the book to be in vertical equilibrium. However, the action-reaction pair for the gravitational force on the book is the book pulling the Earth up. The action-reaction pair for the normal force on the book is the book pushing down on the table.
Understanding the distinction between forces acting on a single body (which determine its motion) and forces acting between two interacting bodies (action-reaction pairs) is key to correctly applying Newton's laws.
Important Questions from Laws of Motion
Which of the following is correct?
I. The mass of an object is a measure of its inertia
II. In an isolated system the total momentum remains conserved
A 20.0 kg block is placed on a smooth horizontal table. A horizontal force of 12.0 Newton is applied to the block. The position of the block at 5.0 sec is
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A ball is dropped from a window of 19.62 meters high. How long will it take to reach the ground?
A subway train starts from rest at a station and accelerates at a rate of 3 ms-2 for 10s. It then runs at a constant speed for 20s and decelerates at 5 ms-2 unit it stops at the next station. The distance between the two stations is:
