Newton's Third Law of Motion; Action and Reaction

Isaac Newtons Third Law consists of the fact that when an action occurs, an equal and opposite reaction occurs as well. For instance, when the action of air rushing out of a balloon takes place, the reaction is that the balloon is forced away. (See figure 1) Newtons third law is also called the law of reciprocal forces. This indicates that there is never a case of undirectional force. One example of this that is used to make people think is "What two forces push against each other to make a car move?" Many people think the answer has something to do with the engine, but the only interaction of two opposing forces, is between the tires and the ground. How fast the car goes is a question of
Figure 1
acceleration. But specifically how the car moves is a simple question of force. Since the formula for calculating force is F=ma. (Force is equal to the mass of an object multiplied by the amount of acceleration applied to the object) This too is true for the Air and balloon example, the force of the air moving out of the balloon IS, indeed equal to the force of the balloon being pushed away from the air. But how fast the baloon is pushed away from the air is determined by the speed (accelaration) of the air being pushed out.


  • Interaction- Mutual action between one thing and another
  • Action Force- The Origional force (ex: Action, the hammer pounds the nail)
  • Reaction Force- The other force (ex: Reaction, the nail halts the hammer)
  • Force- A powerfull effect or influence.
  • Mass- How much matter an object contains
  • Acceleration- Speed with a direction
  • Momentum- Accumulated force from acceleration.

Section 1

Forces and interactions go hand in hand. One of the first things that had to be realized about a force, which in the beginning was thought of as a simple push or pull, was that they come in pairs and that they are always equal. For example, If you where to cause the common interaction of, a hammer pounding a nail into a piece of wood. The obvious action is that the nail is forced into the wood. But what exactly causes the hammer from continuing right on through? THE FORCE OF THE NAIL! The nail has an equal amount of force pushing back on the hammer that stops it from smacking it all the way down into the piece of wood. In general, the terms push and pull are strictly used for living being that could physically do the action. But the same could be said for a wall with a rope attached, with the other end of the rope attached to a truck's trailer hitch. If the wall were to for some reason fall backwards away from the truck. It would be plausible that the wall physically invoked the action of pulling.[1] One of the hardest parts about recognizing Newton's Third Law of Motion, is deciding which force is the "action" force, and which is the "reaction" force. An example of this would be the case of the falling boulder. To decide which force is which, there is a relatively simple "equation."
Action: Object A exerts a force on object B.
Reaction: Object B exerts a force on object A.
If it is concluded that Object A is the boulder and that the force placed upon it is that the earth's gravitational field is pulling on it, (causing it to fall) then the earth must be object B.
It is thusly concluded that the Action is the earth exerting force on the boulder, then the reaction is that the boulder is exerting force on the earth.

Section 2
Figure 2

Mass plays an enormous role in Newton's Third Law. As stated in the previous section, If a boulder were to fall towards the earth, you could rightly say that the earth is falling upwards toward the boulder as well. But the fact that the Earth's mass is so much greater than that of the boulder that the movement of our planet towards the boulder is so infinitely small that you don't notice it. Another example is if you were walking down a flight of stairs, each step actually comes up a miniscule amount to meet you. A more obvious show of this display is when a cannon fires a cannon ball, (see figure 2) why does the cannon ball shoot out so far with a large amount of acceleration; when the cannon itself is only "kicked back" about a foot or so?

In physics sometimes you will be comparing two objects to figure out their contrasting forces, mass's, and accelerations. The force of the two objects is always equal and opposite, so it is always represented by "F". You would use a large "M" for the object that has more mass, and a lower case "m" for the object that has less mass. The same rule applies for the acceleration of the objects. (ie: "A" is the object that gains the most acceleration, while "a" is the object with less acceleration.) In the example of the cannon and the cannon ball the formula comparison would look like this.

Cannon: F/M=a while Cannonball: F/m=A

As you can see, the reason the cannonball moves so much further than the cannon is because the masses of the two objects differ so greatly that the resulting acceleration is also a differential variable.(3)

Section 3

Considering wheels and/or non-friction inducing environments.
Imagine yourself in the situation where a friend is sitting in a cart, and asks you to pull them by the rope that they are holding. If you remember that because of Isaac Newton's third law of motion, equal and opposite forces ( you pulling on the cart, which in turn too would be pulling on you) cancel out. Thus implying that you should not theoretically be able to make the cart move. But it is a well known fact that Newton's third law of motion is true, and does indeed apply to every situation that contains any two forces. But it is also a well known fact that you can pull a cart by a rope. How is this possible? Well if you think of the different forces at work, it is actually quite simple. The main problem in the logic of this proposed theory is that the typical interaction between forces, happens between only two forces. But in this scenario, there are six total forces to compare. To make it simpler there are three pairs of interactions. Once you figure out what pushes/pulls on what, the questioned theory is easier to understand. It is not just a question of you and the cart, you have to take into account that the ground, as well as friction are also in play.

So, once it is decided that you pull on the cart, and the cart pulls on you and that that interaction is canceled, move on to the next interaction. Between you and the ground, typically, that interaction would end with you moving in the direction of your choice, but because the mass of the cart is so much more than yours, you wouldn't move anywhere. ( force cancled?) let's move on. friction.


1. The Physics Classroom
2. Newtons law of motion- Wikipedia the free encyclopedia
3. Nassa Quest> Space team online
4. Newton's 3rd Law