Unit Blog Reflection

Unit Blog Reflection

This unit I learned a lot about things that spin around, such as their angular momentum, which is the quotient of the rotational inertia and the rotational velocity.  Rotational inertia is the “laziness” of a rotating object, or it’s resistance to being spun faster or slower.  If something has a lower rotational inertia, but the same energy as something with equal mass, it will spin faster even if they both started with the same rotational velocity.  Rotational velocity, meanwhile, is how fast something is spinning, and is directly influenced by rotational inertia.  The angular momentum will be the same regardless of whether the object is spinning quickly with a low rotational inertia, or if the object is spinning slowly with a high rotational inertia.  I also learned about the different kinds of velocities when an abject is spinning.  Rotational velocity is one of them, but there is also tangential velocity, and they are very different.  For example, if two gears, one small and with 10 teeth, and the other one large and with 20 teeth, are interlocked and spinning together at a rate in which the larger one spins once per second, they will have the same tangential velocities, because they are both spinning at a rate of 20 teeth per second, even though that is two rotations for one of them and one rotation for the other.  However, because one of them has to spin twice as fast to have the same tangential velocity, it therefore has a higher rotational velocity.  We also learned about torque, specifically the effect torque has on your center of gravity and the amount of force you can apply to an object.  

Meter Stick Blog

     The meter stick, when it is balanced on the table, does not have a torque because each side of the meter stick has a torque, but in opposite directions (clockwise vs counterclockwise), so they cancel out.  The center of gravity of the meter stick changes when you add the 100 gram weight, in order to ensure that both sides (when balanced) have the same torque.
     To figure out how much the meter stick weighs, we (me and Manuel) measured where the new center of gravity was with the weight, and it was 24.4 cm from the edge of the meter stick.  The center of gravity for the meter stick was 50.6 cm.  We had to find the torque of one side of the meter stick, so we used the one with the weight on it, where we knew both the force downward (9.8 times .1) and the lever arm (.24).  Using those numbers, we got the torque as .239 N.  We then plugged that number into the other side, with the equation .239=(.506-.244)(Force), with Force being the weight of the meter stick.
     With this equation set up, all we had to do was solve it, then convert the answer from .912 Newtons, to grams, which gave us the answer of 92.85 grams.  The actual weight was 92.1 grams.
   

(Labelled drawing in progress)

Torque Resource

The video is from a series that contains tutorials on various different subjects, namely Khan Academy and this one is on the subject of torque.  It is really just a video that will teach you about torque through use of a sort of digital blackboard to illustrate the lessons that the narrator is giving.

Rotational Inertia resource

This video shows a spin from an ice skater that, when she tucks in her arms and legs, reaches up to over 300 rpm.  I thought this was a very good video to show off rotational inertia, as it is very clear that she is able to reach those speeds only after tucking in her arms and legs.