Week 24 (4-24-17) Newton’s Third Law of Motion

It’s Newton again.

And we’re doing our best to unpack his laws of motion. Last week we focused on Newton’s 1st law, the one that says:

Objects at rest will remain at rest and objects in motion will remain in motion in a straight line unless acted upon by an unbalanced force.

And so we started there, reviewing what we already knew to be true.

Here we are seeing if these cups will remain at rest if we apply a force to only the paper between the cups. If we can quickly pull the paper from between them, the only force on the cup (gravity) should allow the cup to stack neatly on the next.

But it’s a bit harder with cups of lighter weight.

What was difficult last week–stacking the heavy orange cups, is simpler than bending over and tying our shoes.

But those tiny cups…

Yeah. Not so much.

Newton’s third law states:

For every action there is an equal an opposite reaction.

So, say I’m leaning against the wall. With the same amount of force that I’m “pushing” against the wall, the wall is “pushing” against me. Kind of hard to see.

Which is why we’re going to use a balloon, a straw and a piece of string to SEE if we can understand our action and reaction forces a little better.

The air in our blown up balloon is going to be our ACTION force. But only when we let go of the opening holding the air in.

According to Newton’s third law, when the air pushes out of the balloon in one direction, the balloon itself should propel in an equal and opposite direction.

This is what we’re testing with our balloon rockets.

We’ve taped the body of our blown-up balloon to a straw which has been threaded through with string.

The end of the string is tied to a back of a chair.  When we let go of the opening of the balloon, the action force–the air in the balloon–should push out in one direction, and the balloon should whiz down the string in the opposite direction as it deflates–the reaction force.

Here it goes!

We can see the result of the REACTION force.

Looks like our balloon could have gone further had our string been longer.

What will happen if we tape two blown-up balloons to our straw, each opening facing a different direction?

Is this what you thought?

The action forces of both balloons in opposite directions cancel out the reaction forces. The balloon sputters in the middle of the string where it started.

Just how long can we get a balloon to fly down the string?

We need the hallway for this.

What we’re noticing is that we’re working again with several variables:

How new or old the balloon is now, how stretchy we’ve made the balloon, how tight we’re holding the string…

What has naturally happened is that once we’ve learned and understood Newton’s third law of motion, we’ve started creating our own scientific test on our balloons.

Whose will go the farthest?

And hypotheses like: I think my second balloon will go the furthest because it hasn’t been used yet. Or. I think that the balloons with less air in them will travel the furthest.

I can hear it!

The language the kids are using is language fit for a scientific experiment.

We didn’t set out to do a test, but we could have–they could have.  The understanding is there.

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