Rotational Dynamics Intro With Formula & Examples (AP Physics)
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- čas přidán 13. 09. 2024
- Early in the year we learn that Newton's 2nd Law states that F=ma. Now in rotation we can look at that same relationship using Torque, Angular Acceleration and the Rotational Variable for mass which is called the Moment of Inertia.
After looking at this relationship we will work through two examples of how to use the moment of inertia and the formula its in.
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110/10 would physics again
Great video! One question: at 14:28 you say that the only force contributing to net torque in the system is the tension force. Why is that? Why doesn't the force from gravity affect net torque? Thanks
Yea, I also don't quite understand that either.
Awesome question Jake. Gravity DOES act on the pulley but the weight of the entire pulley acts at it center of mass. This is ALSO the center of rotation or the pivot point. If we tried to solve for torque there we would see the “r” is equal to zero. Therefore the torque from the force of gravity (weight) is equal to zero. Kind of like if you tried to open a door by pushing the hinge.
So to clarify. Gravity does In fact supply a force to the pulley. It does so at the pivot point so that point does not apply a torque.
Math And Physics Tutor Got it thanks so much!
Thank you so much
You're most welcome
Hi, wondering why the net force of the whole system was used in the same equation that only the mass of the bucket was used in. Shouldnt it be 4.5kg, as in the 1.5kg bucket plus the 3kg pulley?
Great question and I see your confusion Matt. The mass of the pulley does not move linearly like the bucket. Because it is rotating it’s mass isn’t directly related to its acceleration. It’s mass effects it’s moment of inertia. So where the mass of the bucket is directly related to linear acceleration, the mass of a spinning disk is related by looking at “I” and for a disk that is 1/2mr^2. Hope that helps. If it doesn’t, where are you still confused.
In linear dynamics we had to overcome the other boxes mass to accelerate. In rotational dynamics we need to overcome the disks “I” (moment of inertia) not it’s mass. And I and m are not 1:1
Hey there, is this the difficulty of the problems in the test ,or are they harder?
Difficulty is relative. I also dont make the exam. Your guess is as good as mine
they dont ask problems like these on the test