WEBVTT

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Have you ever noticed that it's harder 
to start pedaling your bicycle

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than it is to ride at a constant speed?

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Or wondered what causes your bicycle to move?

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Or thought about why it goes forward 
instead of backwards or sideways?

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Perhaps not, and you wouldn't be alone.

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It wasn't until the 17th century

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that Isaac Newton described 
the fundamental laws of motion

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and we understood the answer 
to these three questions.

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What Newton recognized was that 
things tend to keep on doing

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what they are already doing. 
So when your bicycle is stopped,

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it stays stopped, and when it is going,

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it stays going.

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Objects in motion tend to stay in motion

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and objects at rest tend to stay at rest.

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That's Newton's First Law.

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Physicists call it the Law of Inertia, 
which is a fancy way of saying

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that moving objects don't spontaneously 
speed up, slow down, or change direction.

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It is this inertia that you must overcome 
to get your bicycle moving.

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Now you know that you have to overcome 
inertia to get your bicycle moving,

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but what is it that allows you to overcome it?

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Well, the answer is explained by Newton's Second Law.

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In mathematical terms, Newton's Second Law says

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that force is the product of mass times acceleration.

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To cause an object to accelerate, or speed up,

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a force must be applied.

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The more force you apply,

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the quicker you accelerate. 
And the more mass your bicycle has,

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and the more mass you have too,

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the more force you have to use 
to accelerate at the same rate.

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This is why it would be really difficult 
to pedal a 10,000 pound bicycle.

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And it is this force, which is applied 
by your legs pushing down on the pedals,

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that allows you to overcome Newton's Law of Inertia.

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The harder you push down on the pedals, 
the bigger the force

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and the quicker you accelerate.

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Now on to the final question:

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When you do get your bike moving,

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why does it go forward?

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According to Newton's Third Law, 
for every action,

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there is an equal and opposite reaction.

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To understand this, think about what 
happens when you drop a bouncy ball.

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As the bouncy ball hits the floor,

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it causes a downward force on the floor.

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This is the action.

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The floor reacts by pushing 
on the ball with the same force,

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but in the opposite direction, upward,

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causing it to bounce back up to you.

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Together, the floor and the ball form what's called

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the action/reaction pair. 
When it comes to your bicycle,

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it is a little more complicated. 
As your bicycle wheels spin

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clockwise, the parts of each tire 
touching the ground

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push backwards against the Earth:

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the actions. The ground pushes 
forward with the same force

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against each of your tires: the reactions.

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Since you have two bicycle tires, 
each one forms an action/reaction pair

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with the ground. And since 
the Earth is really, really, really big

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compared to your bicycle, it barely moves

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from the force caused by your bicycle 
tires pushing backwards,

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but you are propelled forward.