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← 05-45 More on Motionlessness

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

  1. If your brain is getting a little fried, stick with me for a little bit longer.
  2. We've got one more thing to talk about with motionlessness,
  3. which is actually surprisingly fascinating.
  4. I want to point out one thing from before.
  5. An object that is motionless or at rest, well, from a force point of view,
  6. it's in the exact same situation as an object moving at constant speed.
  7. From here on out, whenever I talk about an object that's at rest,
  8. we could also be talking about an object with a constant speed.
  9. It's just a little easier to visualize an object that's not moving, and the key to motionlessness is
  10. realizing that when an object is at rest, its acceleration is 0.
  11. F=ma tells us that 0 acceleration means 0 total force.
  12. 0 total force doesn't necessarily mean that there's no force acting on an object at all.
  13. All it means is that any upwards forces must be balanced by corresponding downwards forces.
  14. Likewise, leftwards forces must be balanced by rightwards forces.
  15. Let's take a look at Aayush and Brent who are each pushing on this cart.
  16. This cart has mass m and we'll say that the mass is 5 kg and right now the cart is exactly at rest.
  17. They're both pushing very hard but the cart is at rest.
  18. I should mention that most of these questions we do,
  19. we're going to ignore something called friction.
  20. Friction is the force you feel between your hands when you rub them together.
  21. It's a real force and it's something we can talk about,
  22. but to make matters simpler, we'll often ignore it.
  23. So this is a frictionless world for now--so let's go ahead and take
  24. a look at all the forces acting on this cart.
  25. I'm going to draw a force diagram.
  26. Well, the first force I'm going to draw is mg, and in this case,
  27. mg is going to equal this 5 mg*g and we know g is 10 ms² which gives me a total of 50 N
  28. which gives me a total of 50 N and Newtons are the unit of force.
  29. The ground must also be pushing upwards with a normal force.
  30. Aayush is doing his best and he's pushing to the right, we'll label that Fa for Aayush
  31. and Brent is also doing his best pushing back to the left and.we'll label that Fb.
  32. Noticed how I've drawn these arrows--you know Aayush is pushing from the left
  33. to the right--I don't draw an arrow like this.
  34. For me, it's much easier to see what's going on
  35. if the arrow is coming from the object and moving out.
  36. So we know that the cart is at rest and I've given you a couple of the forces.
  37. I told you that mg is 50 N and I've told you that Aayush is pushing with a force of 100 N.
  38. Can you use this data as well as what we learned up here to fill in these two answers.