On a snowy mountain . . . on its very pointy top . . . on a sled that teeters and totters . . . you sit and stare straight down the steep hill.  You are balanced--for now.  At the bottom of the mountain is a short plain of ice.  Then there is another mountain.  Do you think you could sled down this mountain fast enough to speed across the ice and then up the other mountain?  What would you need to be able to do that?  Well, first, you would just need to lean forward . . .


RUSH!  Many forces are acting on you when you are balanced.  Sitting on this hill, you're being pulled down by gravity, but the snow pushes back and stops you from moving.  When you lean forward your sled is no longer sitting on the mountain.  Now you're pulled down the hill by gravity more than the snow can push back.  The forces on you are unbalanced, which means there is more push in one direction than another.  The pull down the hill is stronger than the push of the snow trying to stop you.  Take that, snow!


You are going so fast that you speed down the mountain in no time at all.  Now you are sledding across the ice at the bottom.  Because the land is flat, you are no longer getting faster.  The speed you have from going down the mountain moves you ahead, but the ice rubs against your sled and tries to stop you.  Right now, your speed is pushing about the same strength as the ice rubbing against the bottom of your sled, trying to stop you.  You are at an equilibrium, a point where the forces acting on you all add up to zero.  There is no slowing down and there is no speeding up.  For now you can enjoy the snow in your face and keep moving at the same speed.

All things being equal, maybe I'll just stay right here for now.


Not much changes as you head toward the other mountain.  It seems like things should be changing because you are moving.  But all the speed from the mountain and drag from the snow are all balanced right now.  Constant velocity means you are not going any faster and you are not going any slower because the forces acting on you are balanced.  You just keep moving.  If you want to get up the mountain this is going to have to change.  What will you do?

Look mom no hands!


Once you start going up the mountain, you start to slow down.  Uh-oh.  This is not good.  You had hoped to get to the top of this other mountain, but you are slowing down a lot!  Acceleration is a change in speed or direction.  This happened when you went faster and faster down the first mountain.  It also happens as you go slower and slower up this mountain.  Going up a hill or down a hill are not the only things that change acceleration, though.  You could also turn on the sled's rockets.  Oh, did I forget to tell you your sled has these?

Now where did you say the brake was on this thing?


VROOM!  You might think it's a bad thing when an object is unbalanced.  But it's the only way things can move more faster or slower!  When things are balanced all of the forces acting on them are at an equilibrium.  When they are moving that means they're at a constant velocity, not getting any faster or any slower.  But sledding down a mountain or up a mountain (or turning on rockets) will change the forces, making you accelerate.  You have made it to the top of the other mountain!  On a snowy mountain . . . on its very pointy top . . . on a sled that teeters and totters . . . you sit and stare straight down the steep hill.  Want to go again?


References:

Physics 4 Kids.  "Newton's Law of Motion"  Physics 4 Kids, 2010.  <http://www.physics4kids.com/files/motion_laws.html>