What if I pushed you off a bridge?  That would be bad, right?  What if I tied a rope to your waist and then pushed you off a bridge?  That might be a lot worse.  You would jerk to a stop at the end of the rope's length and that would hurt a lot.  What if I tied a big rubber band around your waist and then pushed you off of a bridge?  Ah, now we're getting somewhere.  Don't worry.  I'm not going to push you . . . until you believe in the power of bouncing.


How can I make you believe that falling off a bridge can be safe?  Well, first you should know something about springs.  A spring is a strong piece of curled metal that goes back to its shape after something has squished or stretched it.  You may know them as those shiny, loopy things that go BOING in cartoons.  Springs can do great things.  But I do not think it would be very safe to put a spring at the bottom of the bridge to bounce you back up . . .

Looks like this guy's ready to spring into action.

So what makes a spring better than other things for falling off a bridge?  Why not use rope or pipes or caterpillars?  Springs are elastic.  Elasticity is what makes things return to their first shape after something has tried to change them.  Really, everything is a little elastic.  If you crush a banana in your hand, it will try to go back to its shape.  If you slap water, waves will bring it back into place.  But some things are a lot more elastic than others.  Take Gummy Worms.  Go ahead and try to stretch one out.  It will shrink back to normal size.  I would not use Gummy Worms for jumping off of bridges.  They are not that strong.  And what will happen to you if someone on the bridge gets hungry?

Bet you can't eat just one.

Elasticity is great when you want things to stick together but still be able to move apart from each other.  You can put a spring between a truck and the car it's pulling. If the truck makes a fast stop, the spring will come together and the car will softly slow down.  Springs are also used as shocks on a car's tires.  These springs crunch together when the tires ride over rocks on the road so the driver will not feel the rocks.  Then the springs spring right back to the form they once had.  Compression is when something is pushed together.  Springs squeeze together and then go back out, squeeze together and then go back out.  If a spring were a person, it would have the abs of someone who did a million crunches a day.

So that's what it looks like under a car.

Cancun, Public domain, via Wikimedia Commons

We want the opposite to happen, though.  The band tied to your waist is not going to come together and then get big again.  It's going to stretch out and then get small again.  Tension is a force that pulls back when something is stretched tight.  When you sit on a swing, this force is what stops you and the swing from falling down.  The chains pull you up and keep you of the ground.  If there was no tension on a swing, you'd fall flat on the ground.  This same thing pulls the two sides of a rubber band back together when you try to pull them apart.

Who knew there was so much tension on the playground?

Mschel, Public domain, via Wikimedia Commons

You can also use springs to hold things together but still let them move.  Think of the springs on a trampoline.  They stretch out when someone bounces and then pull right back.  This is why people do not play tug-o-war with springs.  They would just stretch out and then pull everyone back to the center.  That might cause a few heads to bonk together.


Have I talked you into it?  Springs are things that return to their spiral shape after being stretched out or crushed down.  This is because they are elastic.  Springs on a trampoline let you bounce by stretching and then collapsing again.  Or you can use them on a car tires to scrunch and then grow big again.  Ready?  I will put this elastic band around your waist.  You can jump yourself . . . Whoosh!  You jump off the bridge and fall toward the water.  The elastic band stretches and stretches and stretches until you are just a few feet above the water, and then BBBBBOOOOIIIIIIIINNNNNNNGGGGGGG!  You spring back up!  As you bounce back to the top, just make sure and high-five the guy who showed you how to fall off bridges.



References:

How Stuff Works.  "Elasticity"  Discovery, 2012.  <http://science.howstuffworks.com/dictionary/physics-terms/elasticity-info.htm>

Education.  "Hooke's Law: Spring Physics"  Education.com, 2013.  <http://www.education.com/science-fair/article/springs-pulling-harder/>