Things are stubborn.  If they are moving, they want to keep moving.  If they are still, they want to stay still.  Think of trying to stop a rolling rock.  Now think of trying to move a rock stuck in the ground.  Both are hard to do.  Rocks are more stubborn than most things.  That's why I need you to be a rock babysitter today.  Your job will be to move rocks from where they are unwanted and stop them from rolling anywhere dangerous.  Just like little kids, rocks do not want to listen.  They are going to try to keep doing what they are doing.  Good luck.

Rocks are tricky.  They are just going to keep on doing what they are doing.  Inertia is the idea that things that are moving will keep moving and things that are still will stay still.  The more mass, or stuff, something has, the more likely they are try to keep moving or stay still.  Too bad no one needs their pebbles babysat.

Job number one!  There's a big rock at the top of this man's garden and he wants you to move it to the bottom.  When something is at rest, that means that it is not moving.  There are still forces working on it, but this rock does not want to move.  The ground pushes up and the rock pushes down.  The forces acting upon it are making sure it stays still.  Unlike kids, rocks will not listen even if you give them candy.  So what do you do?

I think I'll rest right here.

You need to get that lazy rock moving!  You can do this by pushing or pulling it.  When the force of something is balanced it will keep doing what it's already doing.  If you pull or push the rock in one direction, then you can change what it's doing and get it out of the place where it's sitting.  So you push, push, push, push, push, and . . . Yes!  That lazy rock starts rolling!  Uh-oh.

Now that's an impressive balancing act!

The rock is rolling right toward this man's prize flowers!  Luckily for you, it's not speeding up.  The dirt pushes back as the rock rolls down the hill.  This gives you lots of time to run in front of the rock and catch it, pushing the other way before it crushes the flowers.  Right now it's moving down the hill, so you'll have to push it back up toward the hill to stop it.  You'll be the force pushing in the opposite direction than the one it is moving right now.  Net force is all of the pulling or pushing on something added up.  When you push against the falling, you'll be changing its net force acting.  This will slow the rock down until it comes to a stop.  Stopping a rock hurts your hands.  But hey, this man really loves his flowers.  You better protect them.

I never promised you a rock garden.

You manage to roll the rock to a safe place in the garden.  Babysitting rocks is not easy.  Like everything else on Earth, they are stubborn, and want to stay where they are or keep moving the way they are going.  Because rocks have so much stuff to them, it takes a lot of power on your part to get a rock rolling or stop it from crashing it into something else.  Tired?  I do not blame you.  Next time, you may want to think about being a feather babysitter instead.


Georgia Perimeter College.  "Forces and Motion - Newton's Law of Motion"  Georgia Perimeter College, 2009.  <>

Physics Classroom.  "Newton's Laws of Motion Review"  Physics Classroom, 2010.  <>