Common Misconceptions when using Newton’s 3rd Law of Motion

Newton’s 3^rd Law (N3L) presents the greatest challenge to students unlike the first two laws of motion because what it claims is contrary to many daily experiences and hence counter-intuitive. Newton's Laws of motion allows us to break down the problem systematically by first looking at the forces exerted on bodies, compute the resultant force and predict its motion.

Challenges to learners include:

·          N3L only work for specific situations, e.g., system at rest

In using N3L, we are looking only at the forces between two bodies at the point/surface of direct contact (except for the fundamental forces which are action-at-distance[1]). If the two bodies are in contact, the force on each body due to the other body must be equal and opposite. There is no need to look beyond the contact point when using N3L.

·      Object being pushed experience a larger force exerted by the imparter (e.g., a person) while the force exerted on the imparter by the object should “logically” be less.

This is a compromised version of N3L. Students recognized and know that they need to apply N3L but preconceptions compel students to come up with alternate N3L to accommodate daily experiences. The single most difficult misconception to rectify in students since it is contrary to daily experiences.

·           Using N3L to describe the forces acting on the same body

Forces like gravitational force (or weight) and normal force exerted by the table are taken to be the pair of forces in accordance to N3L because they are “equal and opposite”.

·           Dealing with massless string

In basic mechanics, the sole purpose of the string is to connect various bodies so that they behave as a single system yet they are not in contact. N3L is not applicable to the forces between two bodies even though they satisfy “equal and opposite forces on two different bodies” condition because they are not in contact. N3L describe the pair forces between each end of the string with the body. For the string, the forces on both ends must cause it to be taut and have equal magnitude regardless of the state of motion of the system. This is a consequence of Newton’s 2^nd Law

     Resultant force on string = mass x acceleration

=>                        T_left – T_right =     0    x        a

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[1] Actually most of the forces we are discussing in Mechanics are action-at-a-distance electromagnetic interactions. We don’t observe this because we perceive only the macroscopic form of the interaction which is the contact between objects.