Newton's Laws lesson plan dream

As I've said I don't normally recount dreams, but I've been having really vivid ones recently and on Saturday night out popped a complete lesson plan for getting children to think about how Newton's Laws work.

In the dream I took a class out into the playground where I'd set up a trolley and chalked some lines next to it. The trolley was purposefully heavy and I demonstrated this by getting one of the pupils to try pushing it. I then recruited several and stated that I was going to time them as to how long it would take for them to push the trolley entirely over the line. I also stated that they weren't to overstrain themselves.

They pushed and it took 10 seconds. I then moved the trolley back to its start position and pointed out another line ten times the length of the trolley away. I then asked if it took 10 seconds to push the trolley the original distance how long would it take to push it to the next line?



The obvious answer came back of 100 seconds. The same pupils pushed the trolley and I noted the time it took to complete the original distance and then the next one. Once they'd reached it they had trouble stopping the trolley and I had to run after it and curve it around to allow it to easily slow down.

The time they took for the original distance was roughly the same 10 seconds, but the longer distance took less than the 100 seconds. Why?

I led them back to the classroom and parked the trolley outside the window so it wouldn't 'disappear' then wrote up Newton's first law - A body remains at rest or constant speed (yes I know it should be velocity) unless acted upon by an external force.

I put a ball on my desk and stated it represented the trolley. I pointed out that it would do nothing and just sit there unless I did something. I then flicked it with my finger and it moved some distance then stopped.

"So why did it move?"
"Because you pushed it?"
"So why did it stop?"
Silence

I then asked a pupil to come forward and gave her two pieces of paper and had her hold the two together in a prayer position. Then rub the two sheets around each other. I then swapped out the two sheets of paper for two pieces of sandpaper and asked her to do the same.

"Why was it more difficult the second time?"
"Because they're rougher than the paper"
"That's called friction. So think about what the ball is doing and what it's touching. What's slowing it down?"
"The desk"

[At this point I digressed into air resistance too by asking them to imagine a dog with its head hanging out a moving car's window compared to one sitting on the pavement as the car drove past, but pointed out that in this case it's negligible]

"So the ball isn't moving, I push it and then the desk slows it back down to a halt. So if you'd stop pushing the trolley what would happen?"
"It would keep going until the ground slowed it down"
"So that's what happens when I push something and then stop pushing it. What happens if I keep pushing it?"
"It'd keep going"

Newton's Second Law goes up on the board the famous F=ma. I explain that Force is them pushing it, mass is the weight of the trolley and acceleration is the rate at which its speed increases.

"So if it took 10 seconds to push it [distance] why didn't it take 100 seconds to push it [10x distance]?"
Silence
"What makes the difference in how long it takes for the trolley to travel a distance?"
"Its speed"
"Is there anything in that Law that has anything to do with speed?"
Silence
"If I push the trolley so it has an acceleration of 1m/s/s then after one second it's travelling at 1m/s. If there's nothing slowing it down then after two seconds it still has an acceleration of 1m/s/s but is now travelling at 2m/s."
I draw a graph. "So what happens if I keep pushing it?"
"It keeps accelerating"
"So after one second its acceleration is 1m/s/s and its speed is 1m/s. If I keep pushing the same way after two seconds its acceleration will have increased to 2m/s/s. So what would its speed be?"
"2m/s?"
"If it was already travelling at 1m/s and it increases by 2m/s...?"
"3m/s?"
"I keep pushing so now its acceleration is 3m/s/s it's speed is...?"
"6m/s?"
"Correct. So after one second it travelled 1m, after two seconds 3m and after three seconds 9m. So as you kept pushing the trolley you maintained its acceleration which meant its speed kept increasing which is why it didn't take as long to cover the distance as you thought it might."

I add Newton's Third Law - A force applied to an object results in an equal but opposite force.

"That means when you push something it pushes back"
Laughter
"It seems wrong, when I pushed this ball I didn't suddenly move backwards did I? So how can this be right? Let's take you. You're sitting on that chair, but what's the chair doing to you?"
More laughter
"Well from this law we say that the chair is pushing you back, but you don't feel that do you? Well imagine if the chair didn't push back what would happen when you sat down on it?"

I then leant against a wall "So I push against the wall and it pushed back at me. If it didn't I'd fall straight through it"

I then got a pupil to jump up and down.

"So you're pushing against the ground and the ground pushes back which is why you go into the air"

I then put my head on top of the pupil's head and ask them to jump again with little success.

"So why didn't you go into the air this time?"
"Because you had your hand on top of my head"
"Okay I was stopping you going upwards, but all the forces were still there, you were still pushing against the ground. So why didn't the ground go down? If you couldn't go up then the ground must have gone down"
Puzzled noises.
"Is there anything keeping the ground from moving down. Why doesn't the entire Earth gone down when you jump? Okay end of lesson I want you to think about that and these laws, why don't I move backwards when I push this ball and think about what you could have done to the trolley to make it easier to push."

Then I woke up and thought what the hell was that about?