Keep it Moving - unbalanced, equilibrium, constant velocity, acceleration
Keep it Moving
unbalanced, equilibrium, constant velocity, acceleration
Forces Unit
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.
Not much changes as you head toward the other mountain. It seems like things should be changing
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Keep it Moving - unbalanced, equilibrium, constant velocity, acceleration
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?
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?
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>
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