Newton’s Third Law

What’s the Big Idea‽

N3L. Newton’s Third Law. Though we’re quite familiar with the idea that “For every action, there is an equal, but opposite reaction”, do we really know what it means? Time to investigate, to determine what Newton’s Third Law looks in a real, physical setting!

Procedure

For this lab, we used a Vernier LabQuest, a dynamics track, two (relatively) frictionless carts, two force sensors, and three different kinds of connectors. First we placed the track flat on the table and used the iPad to measure how level it was, adjusted its height as needed. We placed two dynamics carts on the track, each having a force sensor (one labelled Sensor A and the other Sensor B) on top of it, but facing towards each other. Next, we calibrated the force sensors as necessary and zeroed their values when they were experiencing minimal force. For one of the force sensors, we reversed the sign recorded values to account for the difference in direction.

In total, we performed 12 trials plus an extra trial just for the heck of it. The first four trials investigated two different methods of moving the carts and two different types of connectors. During the next four trials, we repeated these same steps but with an additional mass of 500g on the cart with Sensor A on it. The propreantepenultimate, preantepenultimate, antepenultimate, and penultimate trials primarily dealt with pushing the sensors towards each other in lieu of using a connector (bumpers were placed where the connector would have been). And finally, the very last trial had no major goal and used a spring as the connector between the carts. 

Above shows various setups for different trials. Counterclockwise from the top right: an additional mass on the cart; the carts attached by string; the Vernier LabQuest; the carts attached by a rubber band. Here’s a general overview of each trial:

Trial	Connector	Method	Additional Mass (Sensor A)
1	string	pull on Sensor A and Sensor B	0g
2	string	pull on Sensor A	0g
3	rubber band	pull on Sensor A and Sensor B	0g
4	rubber band	pull on Sensor A	0g
5	string	pull on Sensor A and Sensor B	500g
6	string	pull on Sensor A	500g
7	rubber band	pull on Sensor A and Sensor B	500g
8	rubber band	pull on Sensor A	500g
9	none	push Sensor A and Sensor B towards each other	0g
10	none	push Sensor A towards Sensor B	0g
11	none	push Sensor A and Sensor B towards each other	500g
12	none	push Sensor A towards Sensor B	500g
13	spring	none in particular	0g

The following images show the data for each trial sequentially.

Wow, that’s a lot of data. The thing is, it all boils down to the same conclusion!

Takeaways

The consistency among the thirteen trials, even the last one, is remarkable. To reiterate Newton’s Third Law, “For every action, there is an equal, but opposite reaction”. That shows itself quite clearly here—the data collected by the force sensors is is the same for each, just in the opposite direction! It didn’t matter if we changed the cart that was being pushed or pulled, the type of connector we used, or even the mass of one of the carts. Isn’t physics fantastic?