Final Exam Q6: Finding Problem with Torque Equation

In summary: No, you're not making it harder. You're just doing it in a different way. Instead of figuring out the equation for the triangle, you can just use the Pythagorean Theorem. Here's one way to look at it. To get across as fast as possible, just swim straight across (with respect to the water, which will drag you downstream, but who cares?). How long does it take the water to drag the swimmer the given distance downstream? Since he swims across the river in that same time, what must be his swimming speed (with respect to the water)?It takes the water 10 seconds to drag the swimmer the given distance downstream. So the
  • #106
okay so Total Mom = (5)(6) for X, For Y it is (2)(20)

so for X = 30
so for Y = 40

so the vecotr of the third one

u get an angle of 53.13
 
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  • #107
Alt+F4 said:
okay so Total Mom = (5)(6) for X, For Y it is (2)(20)

so for X = 30
so for Y = 40

so the vecotr of the third one

u get an angle of 53.13
So what's the magnitude of the total momentum (of those two pieces)?
 
  • #108
50 = MV
got it thanks
 
  • #109
http://online.physics.uiuc.edu/cgi/courses/shell/phys101/spring06/prep2a.pl?practice/exam2/sp04

Question 22

15*9.8*50 / 80 = 91.8 ~92

What exactly is the formula since i don't get why you do that, i just have it memorized Why multiply by radii?
 
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  • #110
Alt+F4 said:
http://online.physics.uiuc.edu/cgi/courses/shell/phys101/spring06/prep2a.pl?practice/exam2/sp04

Question 22

15*9.8*50 / 80 = 91.8 ~92

What exactly is the formula since i don't get why you do that, i just have it memorized Why multiply by radii?
For the cylinder to be in equilibrium, the torques must balance. Torque depends on the moment arm.
 
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  • #111
Okay i need this straighen out before exam, An elevator going downward say it is Accelerating. Does that Mean Acc is - or is it postive?

So can u just explain to me When say Gravity is negative etc..


Does - Acceleration mean that ur slowing down? Deacc
 
  • #112
Alt+F4 said:
Okay i need this straighen out before exam, An elevator going downward say it is Accelerating. Does that Mean Acc is - or is it postive?

So can u just explain to me When say Gravity is negative etc..


Does - Acceleration mean that ur slowing down? Deacc


Think about it this way. In general, g is only the magnitude of the acceleration in the y direction. Or, [tex]a_y=a_/smallfreefall=-g[/tex]
Now, this is true when you choose the positive y-direction to point vertically upward. It is [tex]a_y[/tex] that is negative, not g. Does that help?
 
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  • #113
Alt+F4 said:
Okay i need this straighen out before exam, An elevator going downward say it is Accelerating. Does that Mean Acc is - or is it postive?
Acceleration is the rate of change of velocity. It's a vector and has direction. Calling acceleration + or - is just a sign convention to indicate direction. Usually, + means up and - means down. Note that acceleration and velocity are not necessarily in the same direction. Just because the elevator is moving down, doesn't mean it's acceleration is downward. (It could be slowing down, for instance.)

So can u just explain to me When say Gravity is negative etc..
All that means is that the force of gravity (and the acceleration due to gravity) acts downward, the negative direction.

Does - Acceleration mean that ur slowing down?
Not at all. Acceleration just means that your velocity is changing. Toss a ball straight up into the air. The acceleration is always downward (negative). As the ball rises, it slows; as it falls, it speeds up. But it's always accelerating down.

"Deacceleration" is a confusing term; I would avoid it. (It sometimes is used to mean "slowing down", but it's more helpful to say that your acceleration is negative (compared to your velocity).)

Note that you can have acceleration without speeding up or slowing down. (Think circular motion.)
 

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