Outline of Lesson

The following text is the lesson outline that I handed out to students when I taught two lessons in London Secondary.  If you are planning to give a lesson over this subject, this is a format you may wish to follow.  I strongly suggest that you try to encourage the students to participate.  

The lesson included a student handout (white text). Throughout the lesson, I tried to engage my audience by stating and asking certain questions, seen in blue text, and getting them to 'fill in the blanks'.  Red text represents information that students filled into their handouts (copied from my copy off the projector screen).  

Lesson Plan:

We will be recalling Newton’s three laws of motion and particularly focusing on Newton’s third law.  Many students, including myself, have trouble understanding, grasping and believing the concepts of Newton’s third law.  This law is very important, and should be accepted with ease.  We experience it in everyday life.  Every time we interact with our surroundings we experience this third law.  In fact, all moving objects, and stationary objects, experience this law.

Newton’s third law will be demonstrated by several physical collisions between two carts.

Who is this Newton guy, anyway?!

Sir Isaac Newton was born on Christmas Day in Lincolnshire, England 1642 [the same year of Galileo’s death] and died in London, England 1727.  Many people associate Sir Isaac Newton for his discovery of gravity, supposedly as he observed an apple drop from a tree. 

Though, Newton is also responsible for laying down the fundamental laws of the physical universe.  These are the principles that describe how and why things work the way they do.  These principles are known as Newton’s Laws of Motion.

Newton laid out the principles of his laws of motion by building on and merging together the work of his great predecessors- such as Galileo Galilei, Copernicus, and Kepler- all by a very young age [early twenties].  He never published anything until years later; only when he was threatened that someone else might publish similar work.

Sir Isaac Newton, the first scientist to have been knighted, was very intelligent, short tempered, little bit of a loner, but a great physicist, an a true genius in mathematics [he discovered the fundamentals for physical optics, theory of gravitation, calculus], but also very modest, and once said “If I have seen further than other men, it is because I stood on the shoulders of giants.”

Newton’s Laws of Motion:

Newton’s first law: The Law of Inertia  Does anyone know what Newton's First Law is?


An object in motion tends to stay in motion, and an object at rest tends to stay at rest, unless the object is acted upon by an outside force.

Newton’s second law: The Law of Acceleration Can anyone state Newton's Second Law?


The acceleration with which an object moves is directly proportional to the magnitude of the force applied to the object and inversely proportional to the mass of the object.

          In terms of equations:    a=F/m    or   F=ma    where a = acceleration, F = force and m = mass

Newton’s third law: The Law of Interaction (The Action-Reaction Law) Who knows Newton's Third Law?


  For every action there is an equal and opposite reaction.

Further analysis of the Action-Reaction Law:

The third law stated is correct, but it sometimes can be a little confusing.  To actually know what the law means we must understand the meaning of the words “action…reaction”, “opposite” and “equal”

“Action…reaction” means that forces always occur in pairs.  Single, isolated forces never happen! The two forces involved are called the ‘action force’ and the ‘reaction force’.  These given names are confusing for two reasons:

1)     Either force in an interaction can be the ‘action’ force or the ‘reaction’ force.

2)     Unfortunately we associate ‘action’ and ‘reaction’ with ‘first an action, then a reaction’.

This is NOT what occurs in the third law.  The action force and the reaction force exist at the SAME time.

“Equal” means two things:

1)     Both forces are exactly the same size.  They are equal in magnitude.

2)     Both forces exist exactly at the same time.  They are equal in time.

“Opposite” means that the two forces always act in opposite directions à exactly 180 degrees apart.

Does anyone have any questions about the three laws?

Now, keeping these laws in mind, I'm going to physically show you Newton's Third Law through a series of collisions.

[Set up of demonstrations, procedures and what the different components actually do should be explained here and/or before each demonstration]


Students were asked to predict the F vs T graphs for all the following types of collisions.  After the prediction, some students were asked to draw their prediction on the board; and then were asked why they think the graph will look like that.  The demonstrations were conducted, and discussed.

Colliding the two carts (the carts not sticking together)

Carts of equal masses (roughly 0.6 kg)

            Prediction of F vs t graph                                      Actual F vs t graph




 Why does the graph look like a 'bell' curve?  Click for Answer

Carts of different masses (0.5 kg added to moving cart)

            Prediction of F vs t graph                                      Actual F vs t graph




Carts of different masses (0.5 kg added to stationary cart)

            Prediction of F vs t graph                                      Actual F vs t graph




 Why do the graphs of these different massed carts have the same force as the graph of the equal massed cart? Click for Answer

Pushing the two carts (the carts sticking together)

At constant velocity

            Prediction of F vs t graph                                      Actual F vs t graph





At constant acceleration

            Prediction of F vs t graph                                      Actual F vs t graph




 Why doesn't the force come back to zero?  Click for Answer


Through these collisions and their displayed Force versus Time graphs we can agree with all of Newton’s Laws of Motion.  Now that we have a good grasp and understanding these laws, specially the third law, we can tackle conceptual questions that are worded tricky, but in fact, are not tricky at all.  For example:

If forces are equal and opposite why don’t they cancel each other out?

If the forces are equal and opposite how do two different objects obtain different accelerations in the same interaction?

When a small bug is splattered across a fast moving windshield what experiences more force- the bug or the windshield?

Why does the force have a greater effect on the bug?

How can we modify our collision experiment to test the bug-car situation?

Answers can be obtained in Questions and Answers

Hand in Questions:

1)     Why must the forces in an interaction be equal and opposite (i.e. what would happen if they are not equal and opposite)?


2)     Can a space ship accelerate in space (in a vacuum)?  If no, why not? If yes, how and why?


3)     A 1000 kg truck at a red light soon starts in its way as soon as the light turns green.  The driver doesn’t notice that the next red light is creeping up on him quite fast…because he was distracted by a cell call.  Unfortunately he smashed into a little 400 kg Austin Mini stopped at the red light at a velocity of 25 m/s with a force of 5000 N.  How far will the Mini travel in 4 seconds after the collision? Assume the cars do not stick together.

  4)  Mike while riding a unicycle was able to balance himself still for a few seconds.  Draw a free body diagram of all the forces        acting on mike and the unicycle when he was still. (Hint: center of masses are involved)

Answers can be obtained in Questions and Answers

If you wish use this lesson plan, click here for a version without the added  comments