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If we think about this green object moving to the right,
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to the left, to the--to your left
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then there must be a force
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that's creating the motion to the left and that force
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is just big enough to overcome the resistance to motion.
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That force is equally balanced with whatever force resists motion.
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Of course the force that resists motion is going to be different
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if you're an organism that's trying to fly,
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than if you're an organism that's trying to move along the ground,
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than if you're an organism that's trying to swim.
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So, what we're going to think about today are the forces
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that resist that motion and how organisms overcome that resistance.