How Do You Calculate Torque and Rotational Inertia in Physics Problems?

  • Thread starter draconicspectre
  • Start date
In summary: What is the property of matter that resists changes in rotational motion? The answer to that question is the rotational inertia, which is given the variable I. Once you figure out what I represents, you just need to plug in the values given in the problem and solve for I.In summary, the conversation discusses several problems related to rotational motion and the use of formulas to find solutions. The main topics include calculating torque and rotational inertia, as well as converting units from rev/min to radians per second. The formula P=\tau\omega is suggested for finding torque, while the formula i=mr^2 is initially suggested for rotational inertia but later corrected to be
  • #1
draconicspectre
10
0
need some help with some physics homework, would really appreciate explanations for these problems and not just answers :D, neways here they are!

An automobile crankshaft transfers energy from the engine to the axle at the rate of 38.0 kW when rotating at a speed of 2900 rev/min. What torque does the crankshaft deliver?

If a 38.0 N·m torque on a wheel causes angular acceleration 22.9 rad/s2, what is the wheel's rotational inertia?

A disk, initially rotating at 150 rad/s, is slowed down with a constant angular acceleration of magnitude 3.4 rad/s2. (a) How much time does the disk take to stop? (b) Through what angle (rad) does the disk rotate during that time?

Thank you ahead of time! :D
 
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  • #2
draconicspectre said:
An automobile crankshaft transfers energy from the engine to the axle at the rate of 38.0 kW when rotating at a speed of 2900 rev/min. What torque does the crankshaft deliver?

I'm sure you've seen the expression P=FV, where P is power, F is force, and V is velocity. Can you think of an analogous situation for rotational motion?

If a 38.0 N·m torque on a wheel causes angular acceleration 22.9 rad/s2, what is the wheel's rotational inertia?

Going back to linear mechanics again, I'm sure you've seen f=ma What's the analogous situation for rotational motion?

A disk, initially rotating at 150 rad/s, is slowed down with a constant angular acceleration of magnitude 3.4 rad/s2. (a) How much time does the disk take to stop? (b) Through what angle (rad) does the disk rotate during that time?

Can you think of a formula that, given initial velocity and deceleration, let's you calculate how long it takes for something moving in a straight line to stop? Try converting all of that into rotational terms. Then use a distance formula that, given initial velocity, time, and acceleration, let's you find distance, and try finding an analogous rotational equation
 
  • #3
I'm sorry to be a hassle, but that didnt really help me at all :\ , can someone show me some formulas to help me solve these questions ?
 
  • #4
draconicspectre said:
I'm sorry to be a hassle, but that didnt really help me at all :\ , can someone show me some formulas to help me solve these questions ?
You must have met some formulae during your studies of rotational motion. Perhaps you could hazard a guess as to which would be appropriate here?
 
  • #5
i have some formulas, but I am not really sure how/what to use. Would u like me to start listing formulas that i have?
Once again, I'm sorry for any hassle. TY for help
 
  • #6
draconicspectre said:
i have some formulas, but I am not really sure how/what to use. Would u like me to start listing formulas that i have?
Once again, I'm sorry for any hassle. TY for help
It is no hassle at all, Officeshredder gave you some pretty good hints with his post. Okay let's break the first one down.
An automobile crankshaft transfers energy from the engine to the axle at the rate of 38.0 kW when rotating at a speed of 2900 rev/min. What torque does the crankshaft deliver?
Now, as Officeshredder said, the expression P=Fv, is valid for linear motion. Now, could you take a guess at what the rotational version would be? Or have a glance at your notes for something similar.
 
  • #7
KE = 1/2 Iw^2
Would this one be suitable for our current problem?
 
  • #8
draconicspectre said:
KE = 1/2 Iw^2
Would this one be suitable for our current problem?
I was thinking more of [itex]P=\tau\omega[/itex].
 
  • #9
Just realized i posted my problems backwards from the list of my HW, so i think that formula actually helps the last problem i listed.

Our angular velocity is 2900 rev / min.
Our power is 38 kW
so (not quite sure how to make that symbol) = 38/2900 would be our solution?
 
  • #10
draconicspectre said:
Our angular velocity is 2900 rev / min.
Our power is 38 kW
so (not quite sure how to make that symbol) = 38/2900 would be our solution?
We're almost there, but you must consider your units. Note that power is in kilowatts and our angular velocity is in rev/min, you must convert your values into SI units before doing the calculation; otherwise, you get yourself in all sorts of a mess. The SI units of power are watts and the SI units of angular velocity are radians per second.
 
  • #11
38000w/
130(3.14)/sec
 
  • #12
draconicspectre said:
38000w/
130(3.14)/sec
Your power conversion is correct, but you may wish to check your velocity conversion.
 
  • #13
2900 /60 = to seconds
48.33 *2(pie) = 96.67(pie)

not sure how i got 130, but how about this
 
  • #14
draconicspectre said:
2900 /60 = to seconds
48.33 *2(pie) = 96.67(pie)

not sure how i got 130, but how about this
Looks a lot better :smile:
 
  • #15
would i use the same formula for the second problem?

p=(38Nm) (22.9rad/s^2)
 
  • #16
draconicspectre said:
would i use the same formula for the second problem?

p=(38Nm) (22.9rad/s^2)
No, you need to find the moment of inertia (I). The formula you need is analogous to the linear equation F=ma.
 
  • #17
i=mr^2 ?
 
  • #18
draconicspectre said:
i=mr^2 ?
Not quite. Think about F=ma, m is the property of matter which gives it inertia...
 

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