Physics

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  • Hi! I'm Matt. I'm going to tell you the story of how I learned physics.
  • One day, I decided to visit my friend who lives in an apartment building downtown.
  • I was going up in the elevator when I started to think " Isn't it so weird that I'm going up even though I'm not moving? I wonder how much force it takes to move me..."
  • So when I got to my friend's house, I asked her about it!
  • Hey Sarah! You're good at physics right? When I'm going up in an elevator, how much force do you think it takes to move me up?
  • Well actually, you can calculate that! All you need to do is use Newtons laws!
  • Newton? What's a Newton?
  • Newton was an old scientist who was sitting under a tree one day when an apple fell on his head. That's how he discovered gravity. He also made a bunch of physics laws, and had a unit of force named after him.
  • Cool! But... how does that help us solve the elevator problem?
  • Well, two of Newton's laws are Fg=mg and Fnet=ma, and those formulas will help us calculate the force of the elevator on you
  • Let me explain. An elevator by itself needs force to move itself up. A person, by themselves does not move, but they have a force of gravity pushing them down.
  • So overall, the elevator has to exert a much larger force, because it not only needs the force to move up, but it also needs a force to overcome gravity.
  • I see.
  • So to find that force, you need to add the original force of the elevator, and a force equal and opposite to gravity, and that'll give you the answer.
  • So first lets calculate the force of gravity using Fg=mg. Fg represents the force of gravity, m represents mass, and g represents the value of gravitational acceleration, which is -9.8m/s^2. How much do you weigh?
  • I weigh 100kg, I think.
  • So using the formula, it would be Fg= (100kg)(-9.8m/s^2), which gives you -980 newtons. So if the elevator needs to overcome gravity, then it would be the same amount of force, but in the opposite direction. So it would be 980N.
  • Yeah! So Fnet means the net force, which in this case is the force of the elevator moving upwards, and a means acceleration. In order to solve this problem, we need to know how fast the elevator accelerates... where could we find that...
  • Cool! So I guess now we calculate the force of the elevator moving up, using Fnet=ma, right?
  • Over here on the elevator! It's 5m/s^2. Which means that the net force is 5m/s^2*100kg, which is 500N!
  • This elevator accelerates at 5m/s^2
  • So that means that the force the elevator is exerting on me is... 500N+980N... which is.... 1480N! Wow! I just did physics!
  • Yeah! You did!
  • Yes! So in that case, since there's no force to move it, the only force it's exerting is overcoming gravity to prevent you from falling. So it would be exerting 980N!
  • Hm... so what about when the elevator is standing still? Is it exerting any force on me then?
  • Well, the elevator isn't moving at all, and it's not preventing gravity from acting on you since you're falling...so... it's not exerting any force at all!
  • What about... if the elevator cord breaks and I'm freefalling in the elevator?!
  • This one will surely stump her...
  • Wow! Cool! Well thanks for helping me!
  • Any time! I hope you learned something about physics today!
  • The End
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