## Learning lessons relearned! Or

## Should weight be measured in Newtons?

A small group (n=3) of middle school kids recently asked me to work with them on some physics topics. Why they asked me a physician who has not read physics since high school is a mystery. I think it had something to do with them knowing my interest in education.

I am a sucker for this type of stuff and thus I agreed. I did not even ask what topic they were struggling with; after all how difficult could middle school physics be? The fact that they asked me while we were at a charity fund raiser probably meant I was under the influence (of good thoughts!).

Turns out they wanted to know more about simple machines, solving problems regarding mechanical advantages of levers and inclined planes and pulleys. I had to do a crash course on the topic quickly finding some resources on the web that helped me get up to speed. The detailed description of the session follows but if you want to cut to the chase, here are the take home points. These were brought home to me very strikingly during the 2 X 90 minute sessions I spent with them over the next 2 days. Looking back these are the essence of constructivism and it was great to see these validated at a personal level.

Take home points:

- Spend more time on the fundamentals even if the learner appears to have gotten them. Don't assume they know these even if it should have been learned a while back. If nothing else, it helps to revise.
- The learners needs to know why they are learning something before they learn it
- Connect to practical real life examples
- You have to know what they already know and don't know
- Words are constructs and carry a much deeper meaning for each person. Having a learner rephrase something in their own words will tell you how much they understand
- Since peers are more likely to be at the same level they innately understand the perspectives and understanding of their co-learners.
- If you sense that one of them had an AHA moment, let them explain it to the others in their words. The thrill of a learner's AHA moment will give you goosebumps. Savor it and leverage it.
- When only one person in a group does not appear to get it, it is not because they are less intelligent but often they are thinking about it differently or even at a deeper level. This conflict between their prior knowledge and what is being discussed is a great opportunity to leverage further discussion to clarify the fundamentals.
- Learning occurs best when the learner is motivated to learn.

As I started my crash course on simple machines, I found myself struggling to remember the definitions and units of force and work and power and the relationship between them. So I quickly brushed up on some basics:

- Force (Newtons) = Mass (kg) X acceleration (m/sec/sec)
- Work (Joules) = Force (Newtons) X distance (metres)
- Power (Watts) = Work (Joules)/Time (sec)

As I reviewed this material, all the memories of the joy of learning physics came rushing back. I was ready to work with the students. I hoped I could get them to love physics as much as I used to. I decided to make sure they were well founded in their concepts before we got into any calculations. Our conversations went something like this (there might be some inaccuracies here but read this in the spirit in which this conversation took place):

S1: It makes work easy

Me: What does that mean?

S2: It lets you do more work than you put in

S3: But that's not possible, you cannot generate more work than you input!

S1: You mean you cannot create energy right? That the principle of conservation of energy

Me: What is the

S2: Well their units are the same

S3: You put in energy to do work right?

S1: It is force X distance

Me: So how does that apply to a lever?

S2: In an ideal case with no friction, the work put in = work put out. So force put in X length of effort arm = force put out X length of resistance arm

S3: So depending on the length of the 2 arms, the lever changes the amount of force.

S1: So the amount of work stays the same, the lever can make things easier by making you put in less force to do work!

Me: Great job! Now lets take another question.

S1: That your weight is 98 kg?

S2: punches S1 lightly

S3: I am not sure what you mean.

Me: What would happen if you used the same scale on Moon?

S1: Oh I see what you mean. 98 kg is your weight on Earth. It would be less on Moon

S2: Yeah, its got less gravity of course.

Me: What about in space?

S3: Well the scale would read zero I think? Since you are beyond Earth's gravitational force.

S1: That's 9.8

Me: 9.8 what?

S2: 9.8 Newtons.... No... g is 9.8 m/sec^2

Me: So what would that be...

S3: that 98/9.8 = 10

Me: 10 what?

S1: 98 kg / 9.8 m/sec^2 = 10 kg/m/sec^2

Me: so what is the difference between mass and weight?

S2: Oh I get it, what you weigh on Earth is your weight and the amount of matter you contain as measured in space is your mass.

Me: OK, so what is the unit of mass?

S3: Kg of course.

Me: So you just told me in space a 98 kg person would "weigh" 10 kg/m/sec^2

S1: This is confusing. Seems like the units are all messed up.

Me: Seems like it doesn't it? What happens when you stand on a scale on Earth?

S2: Your weight pushes it down.

S3: The gravity acts on your mass and pulls it down.

S1: You are exerting a downwards force on the scale

Me: How much force?

S2: Is that your weight? So weight is a force? Then why is measured in kg?

S3: Yeah the unit of force is Newton!

Me: What is force according to Newton?

S1: Mass X acceleration

S2: Newton is kg X m^2

Me: So how does that convert to your weight?

S3: I get it, its the gravity acting on your mass

S1: Yeah it is 10 kg X 9.8 m/sec^2 = 98 kg m/sec^2

S2: Which is 98 Newtons

S3: So weight is a force? Why do we measure it in kg?

Me: Yup so weight should be measured in Newtons not kg, I think we use kg because its become ingrained in our system.

S1: Got it "

On that very profound note we took a break while S2 and S3 gave S1 very strange looks!

We covered many other questions.

Why are screws and wedges like an inclined plane?

How does the concept of distance apply to inclined planes and to pulleys?

How does a movable pulley generate mechanical advantage when a fixed one does not?

The conversations went on for a long time. Their eyes were shining, there were numerous "AHA" moments. They were excited. They were teaching each other. I had the most fun I had in a long time.

The only problem...

They want me to do physics sessions with them throughout the summer vacation!

I wonder where all the retired folks go... this would be an amazing win -win situation. They could work with school kids to help ignite their passion for the sciences or other subjects while keeping their own brains and hearts young.

**Me: What does a machine do?**S1: It makes work easy

Me: What does that mean?

S2: It lets you do more work than you put in

S3: But that's not possible, you cannot generate more work than you input!

S1: You mean you cannot create energy right? That the principle of conservation of energy

Me: What is the

**relationship between work and energy**? Are they the same thing?S2: Well their units are the same

S3: You put in energy to do work right?

We went of on a tangent here:

Work= force X distance = mass X acceleration X distance = Kg X m/sec/sec X m = kg m^2/sec^2

Kinetic energy= 1/2 mv^2 = 1/2kg (m/sec)^2 = 1/2 kgm^2/sec^2

Potential energy= mgh = kg X m/sec^2 X m = kg m^2/sec^2

So they got the idea that theMe: So what does a machine do? Seems you decided it cannot do more work than you put it. What is work?units of work and energy are the same.

S1: It is force X distance

Me: So how does that apply to a lever?

S2: In an ideal case with no friction, the work put in = work put out. So force put in X length of effort arm = force put out X length of resistance arm

S3: So depending on the length of the 2 arms, the lever changes the amount of force.

S1: So the amount of work stays the same, the lever can make things easier by making you put in less force to do work!

Me: Great job! Now lets take another question.

**What does it mean when you get on your bathroom scale and it reads 98 kg?**S1: That your weight is 98 kg?

S2: punches S1 lightly

S3: I am not sure what you mean.

Me: What would happen if you used the same scale on Moon?

S1: Oh I see what you mean. 98 kg is your weight on Earth. It would be less on Moon

S2: Yeah, its got less gravity of course.

Me: What about in space?

S3: Well the scale would read zero I think? Since you are beyond Earth's gravitational force.

S1: That's 9.8

Me: 9.8 what?

S2: 9.8 Newtons.... No... g is 9.8 m/sec^2

Me: So what would that be...

S3: that 98/9.8 = 10

Me: 10 what?

S1: 98 kg / 9.8 m/sec^2 = 10 kg/m/sec^2

Me: so what is the difference between mass and weight?

S2: Oh I get it, what you weigh on Earth is your weight and the amount of matter you contain as measured in space is your mass.

Me: OK, so what is the unit of mass?

S3: Kg of course.

Me: So you just told me in space a 98 kg person would "weigh" 10 kg/m/sec^2

S1: This is confusing. Seems like the units are all messed up.

Me: Seems like it doesn't it? What happens when you stand on a scale on Earth?

S2: Your weight pushes it down.

S3: The gravity acts on your mass and pulls it down.

S1: You are exerting a downwards force on the scale

Me: How much force?

S2: Is that your weight? So weight is a force? Then why is measured in kg?

S3: Yeah the unit of force is Newton!

Me: What is force according to Newton?

S1: Mass X acceleration

S2: Newton is kg X m^2

Me: So how does that convert to your weight?

S3: I get it, its the gravity acting on your mass

S1: Yeah it is 10 kg X 9.8 m/sec^2 = 98 kg m/sec^2

S2: Which is 98 Newtons

S3: So weight is a force? Why do we measure it in kg?

Me: Yup so weight should be measured in Newtons not kg, I think we use kg because its become ingrained in our system.

S1: Got it "

**The heavier you are, the more you weigh!**"On that very profound note we took a break while S2 and S3 gave S1 very strange looks!

We covered many other questions.

Why are screws and wedges like an inclined plane?

How does the concept of distance apply to inclined planes and to pulleys?

How does a movable pulley generate mechanical advantage when a fixed one does not?

The conversations went on for a long time. Their eyes were shining, there were numerous "AHA" moments. They were excited. They were teaching each other. I had the most fun I had in a long time.

The only problem...

They want me to do physics sessions with them throughout the summer vacation!

I wonder where all the retired folks go... this would be an amazing win -win situation. They could work with school kids to help ignite their passion for the sciences or other subjects while keeping their own brains and hearts young.

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