Showing Revision 1 created 07/09/2012 by Amara Bot.

1. Title:
   05-45 More on Motionlessness
2. Description:

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   If your brain is getting a little fried, stick with me for a little bit longer.
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   We've got one more thing to talk about with motionlessness,
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   which is actually surprisingly fascinating.
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   I want to point out one thing from before.
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   An object that is motionless or at rest, well, from a force point of view,
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   it's in the exact same situation as an object moving at constant speed.
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   From here on out, whenever I talk about an object that's at rest,
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    we could also be talking about an object with a constant speed.
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    It's just a little easier to visualize an object that's not moving, and the key to motionlessness is
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    realizing that when an object is at rest, its acceleration is 0.
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    F=ma tells us that 0 acceleration means 0 total force.
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    0 total force doesn't necessarily mean that there's no force acting on an object at all.
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    All it means is that any upwards forces must be balanced by corresponding downwards forces.
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    Likewise, leftwards forces must be balanced by rightwards forces.
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    Let's take a look at Aayush and Brent who are each pushing on this cart.
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    This cart has mass m and we'll say that the mass is 5 kg and right now the cart is exactly at rest.
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    They're both pushing very hard but the cart is at rest.
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    I should mention that most of these questions we do,
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    we're going to ignore something called friction.
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    Friction is the force you feel between your hands when you rub them together.
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    It's a real force and it's something we can talk about,
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    but to make matters simpler, we'll often ignore it.
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    So this is a frictionless world for now--so let's go ahead and take
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    a look at all the forces acting on this cart.
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    I'm going to draw a force diagram.
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    Well, the first force I'm going to draw is mg, and in this case,
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    mg is going to equal this 5 mg*g and we know g is 10 ms² which gives me a total of 50 N
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    which gives me a total of 50 N and Newtons are the unit of force.
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    The ground must also be pushing upwards with a normal force.
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    Aayush is doing his best and he's pushing to the right, we'll label that Fa for Aayush
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    and Brent is also doing his best pushing back to the left and.we'll label that Fb.
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    Noticed how I've drawn these arrows--you know Aayush is pushing from the left
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    to the right--I don't draw an arrow like this.
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    For me, it's much easier to see what's going on
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    if the arrow is coming from the object and moving out.
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    So we know that the cart is at rest and I've given you a couple of the forces.
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    I told you that mg is 50 N and I've told you that Aayush is pushing with a force of 100 N.
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    Can you use this data as well as what we learned up here to fill in these two answers.