# Newton’s Third Law Misconception: “not equal when accelerating”

#### Abstract

Students memorize Newton’s Third Law but their answers to questions show that they misunderstand it. In this demonstration we seek to dispel the misconception that Newton’s Third Law is somehow briefly suspended when motion is initiated.

#### Teacher commentary

In the diagram below, Blue “overcomes” Green and the pair accelerates to the right. Nearly 100% of our incoming first year physics students are completely convinced that the force Blue applies on Green is briefly larger than the force Green applies on Blue, and that’s what starts the motion. The students think that after Blue and Green are moving at constant velocity the forces are again equal.

This is absolutely incorrect. Newton’s Third Law says that for every action there is an equal and opposite reaction on the other object. Nothing in there about “except when accelerating”. Newton’s third law applies regardless of whether the two objects pushing on each other are stationary, moving at constant velocity, doing backflips, or spinning round and round.

The short video clip below shows one way to demonstrate this to students using force meters.

So if the forces are still equal and opposite, why do Blue and Green accelerate to the right? This question contains a misconception. The forces never balanced in the first place.

The answer is this: Blue pushes harder the ground to the left with greater force than green pushes the ground to the right. So the ground pushes Blue to the right harder than it pushes green to the left.

The forces on each separate person are illustrated below with numbers just made up, but assuming the masses are the same:

The blue and green vectors form an action-reaction pair.

Internal and External Forces. One can carefully use the concept of internal and external forces but this can easily reinforce the misconceptions. We can say that the two forces in the action-reaction pair Green pushing Blue and Blue pushing Green are internal forces if the system is the pair of students. Internal forces cannot accelerate the centre of mass of the system. But students are inclined to think this means the forces cancel.

Video:

Year 10 Forces

PHYS 2.4

#### Instructions

This demonstration can be enhanced using a pair of force meters as shown below. These are instructions for Vernier force meters and interfaces but the experiment should work with Pasco force meters as well.

1) Connect the force meters together. Here we have used older style Vernier force meters with rectangular extensions as shown in the photos. We have manufactured convenient plastic joiners, but actually Blu-Tack works almost as well.

 The force meters joined together The joiner Or use BluTack/

2) Connect the force meters to the interface (LabPro, LabQUEST, etc) and connect the interface to the computer.

3) It is very convenient to set up the sensors so that one force is negative and the other positive. This is easy to do in LabPro software but you can download a .cmbl file that is already set up below. Just put it on your desktop and double-click to open it. LoggerPro will launch ready to run with the settings tailored for this experiment.

#### Safety

Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.

#### Related Resources

Newton’s Third Law: Misconception 1

#### uaCredits

This teaching resource was developed with support from