3 Act Physics – Momentum

Basic Principles of the 3 Acts are shown here

Momentum – Demolition balls

You may want to start with the shortest question you can. In this case;

Should demolition balls be bouncy or not?

How do ‘dead blow’ hammers work ?

dead blow illustration

Stilleto Tools suggest that their titanium hammers have a 97% efficiency compared to 70% for a steel hammer. Could this be true and how would you test it?

Act 1:  Demolition Ball 

Act 2: The explore

What questions does this make you think of?

Which one would be the most effective at knocking the glass over?

Which is the most efficient at transferring energy?

What could happen if ….? (what matters?)

Students could investigate

  • Different material balls
  • Different material ‘buildings’ most demolitions would be brick built
  • Materials to dampen the ball
  • The effect of the height
  • Finding the best compromise between effectiveness and bounce (practically we don’t want our demolition ball to bounce too far or we will struggle to control it and may demolish many things we don’t want to!


Two things we may want to consider are the law of conservation of momentum (The total momentum of the system is conserved)  and the law of conservation of energy (energy is neither lost, nor destroyed)

There would be  little change in the kinetic energy of the bouncy ball as it rebounds with a similar speed to the impact speed, however there is a large change in the velocity as although the magnitude of the velocity doesn’t change much, the direction has reversed. So there will be a large change in the momentum of the ball.

The plasticine ball is likely to stop on impact and so there will be a large change in the kinetic energy of the ball, but how efficient is this transfer? There will be less change in the momentum than for the bouncy ball as there is no rebound.

So what will happen?

Act 3: The reveal

The bouncy ball is clearly more effective at knocking the glass over.


If we ignore the energy aspects and simply look at the momentum it is very clear

Momentum (kgm/s) = Mass (kg) x velocity (m/s)

Total momentum before collision = total momentum after collision

Both balls have the same initial momentum before the collision. For the bouncy ball this momentum becomes negative after the collision and zero for the plasticene ball. Hence the glass must have more momentum after the bouncy ball hits it than the plasticene one

There are many other aspects that could be investigated

The efficiency of hammers

Why ‘dead blow’ hammers do less damage to soft surfaces

What real demolition balls are like

3 Act Physics – First attempts

These are the first drafts of applying Dan Meyer’s 3 act maths here  into physics having had some previous ideas here and a premise of using intuition here – After all isn’t maths just physics with the toys removed?

Below are act 1s . These are the ‘hooks’ to generate questions that would lead into explorations of act 2. The reveals , act 3 are all on my youtube channel but please dont look until you have really thought the problems through. These will probably be also put on Dan Meyer’s 101QS when/if  I get permission here

Bouncy demolition ball

A great way to break a cup

How to blow a candle out

Suspended ball mystery

Comments, suggestions and contributions as always welcome. This is a non commercial resource, so please keep it that way.

There will be many more to follow

So you do understand Physics – You just thought you didn’t

Could we try a new approach to learning physics?

What if we start with the premise that our students intuitively understand lots already. Having done a project with Arsenal on the physics of football with the Institute of Physics, I was struck that the players were applied physicists, they knew the laws of physics so well they could make the ball do pretty much everything they wanted. What they couldn’t do was communicate how they did it in scientific language. I could get the ball to pretty much do nothing that I wanted , but I could explain what they were doing (to a point)

If we used this paradigm that for many topics we can start with this understanding then add the knowledge and language to allow them to communicate it. We might then transform some people’s perception of physics from mysterious and difficult into something that makes sense to them.

What might this look like in a lesson? `these are early ideas so please feel free to comment and improve and develop.

Centre of Mass Lesson

I didnt have access to technology so instead of using socrative I used post-it notes. Asking the students to write what they knew about centre of mass and putting it on a window that displayed their confidence. High , Medium or Low confidence. I was interested in those with high competence, but low confidence, these are often girls, but far more important are the low competence and high confidence students, usually boys, these I am going to have to destroy their misconceptions before I can teach them.

It appeared most had no idea what centre of mass meant and confidence in what they had written was low.

I started with the matchbox trick . See the video below and immediately beat five of them. I could make my matchbox stand up and theirs all tipped over.  I also did a ridiculous suck and wobbled my finger slowly which was enough to fox them.

Can you work out what I did?


I then gave them an empty matchbox per table and they had to figure out my winning strategy.

Next was the juice carton dilemma. You have a full, empty and half full carton. What order will they fall over in if pushed until they topple?


“Which would fall over last? (at the greatest angle) Left hand up for full, right hand for empty and both for half.” Few got the right answer

“Find a person who disagrees with you and try and convince them you are right. You have two minutes before I ask again.” A slight increase in numbers with the right answer

“Lets see if you are right…(Lines up cartons) … Actually lets leave it “ Complaints from class, “we want to see it now!”

Its the Zeigarnik Effect in action – Keep them thinking for longer

“I’m now going to beat you in another competition…(Draws circle on the classroom floor) …”A simple competition to try and push each other out of the circle. Have we a volunteer?”  Biggest lad steps forward  “Now I am going to step outside and give you a minute to train your champion. How should he stand? What strategy should he use?”

Goes out of room.

Comes back after a minute and stand eyeball to eyeball with Mohammed – Mohammed couldn’t look me in the eye!

“So how have you told your champion to stand?”  Mohammed crouches low, with a wide stance.

“Hang on a minute, you told me you didn’t know anything about centre of mass, but you do understand it as well as stability.”

This is the different paradigm, the knowledge is shared with them at the point where they need it. Most people intuitively get physics, but are poor at communicating their understanding in the standard way that scientists need

Then we have a discussion about centre of mass, what it is, how to find it (balance point) and stability , wide base etc At this point it makes sense to them. They intuitively know it. I am just modelling a scientific way of describing what they already know. To give them the language they need in order to communicate their understanding effectively

I then tell them I’m not happy with their choice of champion and choose a small girl (have checked she is ok with this first) Outrage from class

“Whats your problem with this? “ Lots of discussion about size – size or mass? “So you understand how forces work too. For any given Force that I can exert there will be a bigger change in the motion of Anna than there will Mohammed” – Leads into a discussion of forces and Newton’s second law.

Back to the cartons

“Which one now? Hands up – Now persuade others you are right “ – now around 90%

“So let’s see….. actually lets leave it for next time.” One girl shouted out

“What! you are going to leave us in suspense? “

“I certainly am!”

“You are the most annoying teacher alive!”

“Thank you, I’ll take that as a compliment!!”

Other fun stuff to do with Centre of Mass

But who has a lower centre of mass . Men or women? and how can you find out without any equipment?

How can you balance on a single point on the side of a glass two forks and a toothpick ?

How is it possible to jump over a bar, but never have your centre of mass go over it? (fosbury flop)

Three Act Science – Collaboration?

Before we start educating them, it makes sense to ask the question what makes a great scientist?


The ability to ask astute questions ?

Being able to consider innovative and divergent strategies for getting the answers?

Having the capability to create valid experiments and spotting flaws in techniques and patterns/ anomalies in results?

Have a sufficient depth of knowledge to connect concepts to reach the Extended Abstract level of SOLO Taxonomy?

To be able to evaluate and extend understanding with more investigations?

How well does our education system facilitate the development of great scientists?

Often very poorly. All too often we teach from the bottom up; tell stories where the answers are already known. In many lessons no conflict is created, nor curiosity excited; no creativity is allowed and students are limited by the outcome.

In short, often students are taught merely to decode exam questions and performance takes the place of real learning.

Teachers complain of students lacking:

  • Initiative
  • Independence
  • Resilience
  • Thinking skills
  • Communication skills

This is not entirely our students fault.  Our education system is not designed to encourage creativity and free thinking, but geared towards conformity and mass production. Complaining that our students lack these skills is the equivalent of moaning that cars don’t fly.

With an education system that is exam orientated and risk averse it is easy to see why students are spoon fed and a cycle of dependency created.

How can we break the cycle and move away from this sanitised science experience?

Using Dan Meyer’s inspirational @ddmeyer  Three Act Maths  http://blog.mrmeyer.com/?p=10285 as a model, we can incorporate multimedia and digital tools to redefine the learning experience. This is one example of transformational practice

What might we do?

  • Use multimedia and digital tools
  • Let them use their intuition
  • Let the students build the problem by asking questions themselves
  • Have them share their discoveries and evaluate each others technique
  • Be less helpful

If you only do one of these do the last: as a teacher it is incredibly difficult to acheive. When delivering teacher training I often use a magic box that there is a strategy to open, which is difficult to discover. I use it to demonstrate that invoking curiosity as a starting point, is much more powerful than the ‘bottom up’ approach. We would normally teach this as ‘here is a box, here is how to open the box, then give it to them and they would be able to open it’. This completely removes the joy of discovery and makes it very boring. The interesting thing when doing this activity with teachers, is that when they learn to  open the box, most immediately feel compelled to tell other people how to do it, thus killing the learning experience. The box is only interesting when you can’t open it. No other group I have tried it with suffers this compulsion. In fact a group of bankers used it to prove their superiority and wouldn’t dream of helping others!

I would like to propose a Three Act model for improving the delivery of science and would welcome collaboration, comments and criticism in the development of this.

The Three Act structure is already used by Screenwriters who know lots about how to engage our interest:


It sounds horribly like the formulaic OFSTED three part lesson , but please bear with me. You break your lesson down intothree parts: Act One, Act Two, Act Three. Beginning, middle, end. Setup, problem solving, resolution.

Act One is the hook. The purpose of the first act is to engage the students. It must be unique, something they haven’t seen or considered before. They are then encouraged to ask questions and build the problem themselves; to make predictions and use their intuition. Most people understand far more physics than they realise – for example: they don’t fall over as they understand centre of mass. What many lack is the ability to communicate this understanding in a conventional scientific way.

An example is here: (The idea is stolen from one of the finest minds in physics I know: Dr  Lawrence Cattermole)


If you cannot see the video there are three cartons of juice: one full, one half full, one empty. The full one is pushed over until it falls.

So what questions does this invoke?

The questions can be narrowed. The highly talented Tom Harbour (a beacon for Teach First ) at a school in Leicester gets his classes to ask superb questions using his “What would happen if…?” format.

Some questions they may come up with:

Which is the most stable? (This then generates more questions – do we mean which falls at the greatest angle, or which needs the biggest force to push them over?)

In which order will they fall over when pushed, from the first to fall over to the last?

Which requires the biggest force to push it over?

How much juice will make it the most stable? (This is massively more complicated than you may think!)

Which is most likely to get knocked over – an empty , full or half full glass, if you knocked them in passing ? (real life example – that gets even more tricky as the shapes of glasses will affect this)

Act Two: Ideas are generated  towards a resolution of the problem. Engagement is increased as the key aspects of motivation are in place: Mastery, Autonomy and Purpose – See Dan Pink here

This is the stage where they can explore their understanding and knowledge can be added if needed. Allow them to make mistakes and follow wrong lines of enquiry.

They can then plan and carry out their investigation into stability to find out answers.

Digital technology used to follow their learning journey can  be transformative. Videos used to show what they predicted, what they did and what they were thinking offer great insight.

Apps such as Socrative, Showbie and Nearpod allow questions from the students to be compiled and progress to be shown. I will be developing examples of these in the future

Act Three is the reveal, the resolution to the dilemma. This ideally should be shown, if possible through the video or demonstration, rather than simply being told the answer. This builds tension and often gets the release – “Yess!!” Though for some of the activities the “right” answer is unknown and there are no limits to what students can achieve.

So what is the solution to the juice carton stability problem?

Find out for yourself!

( Contact me if you haven’t access to a carton!)

Please add comments and feel free to share and collaborate, this is not a commercial product.