What is a Force?

I have loved this unit with the 8th graders.  There's something concrete about this idea that the kids can really hold on to.

To start out this unit, I first introduced the fact that

a force has both magnitude and direction.

I started out by holding a book up in front of me, then quickly letting go and catching it again lower down.

Question:  Why does it fall?
Answer: Gravity.
Question: Gravity is doing what to the book?
Answer: pulling it down.

Okay, so now we know that there is a force of gravity with some magnitude in the direction towards the ground.

Now the book is placed on a table.

Question: Is there still gravity on this book?

I got some weird looks with that.   . . . . . No?  . . .  Yes? . . . . I don't really know, but I think so, but it's not moving down. . . .?????

When the kids say "no," my eyes get real big and I say, "What?? There is no gravity anymore??"  And the kids say "okay, yeah, there is."

Question:  Then why is the book not falling down?
Answer: Because the table is in the way.

Oh!

Introduce this image on the smart board:

So we know there's gravity pulling it down. (Draw downward arrow).  But the book isn't falling through the table, so the table must be pushing up on the book. (Draw upward arrow).

HuUh??!?

Yes.  The table is pushing up on the book.  At this point you take the book back into your hands.

Question: Am I holding the book up with a force?
Answer: Yes.
Question: Then doesn't it make sense that the table also has to hold the book up with a force?
Answer: I guess so.

Then you do a whole bunch of other examples demonstrating the forces and which direction they are going. Examples can be cars, balloons, something falling (this was done to introduce the idea of air resistance), other things sitting motionless.

Now you introduce the idea that you can combine forces to find a net force.

SCENARIO 1

Here's a wonderful little picture analogy.  I chose a girl in the class to be the first person.  Sammy* (*names has been changed) is driving along the road when suddenly she runs out of gas.  She gets out and sees a gas station at the end of the road so tries to push her car.  But the car is too heavy for her to push and it won't budge.  She calls her friend (motion to Sammy for her to give her desired friend's name), Brittany*.  Brittany comes and tries to help, but they are both still not strong enough to push the car.  They then call a third friend (motion again for them to choose a third comrade) who comes and they all push with all their might.  They're finally able to get the car rolling.

Truth:

Students are more likely to listen when you use their name for an analogy.

Kids can easily see that because there are three people there is more force and the car will move.

Do lots of examples like this where you practice adding the forces, stressing that the forces are all going the same direction so you can add them.  Your answer must have both magnitude and direction.  I don't want "24N".  I want "24N ->"

SCENARIO 2

Tug-of-War.  Who doesn't love a good tug-of-war?

Team A is pulling with a combined force of 20N <-- to the left.  Team B is pulling with a combined force of 30N --> to the right.  

Question: Are the teams pulling in the same direction, or in the opposite direction?

Answer: Opposite direction.

Question: So do you think we're going to add their forces or subtract their forces?

Answer: subtract.

The net force is 10N --> to the right. 

Question: Why is the rope going to the right?

 .. . . pause while students try to figure this out . . 

timid student answer: because the team on the right is stronger?

YAY!  (remember that this is SPED, so kids need time to process answers). 

More tug-of-war examples with net forces to the right, to the left, and a net force of 0.

More examples like this on the board where both I model and the kids practice as a group:

Remember, the answer is 4N -->, not just 4N.

Independent Practice

Disclaimer, this is not my worksheet.  I found this fabulous worksheet online that I thought was just perfect for what we were doing.  I don't remember what website I got it off of, but it was one of those great teaching resource websites with free worksheets.  

To get worksheet, click here.

I printed the first two pages double sided for this day.  I printed pages 3-4 for another day to keep up practice.