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LPH 105 W15 8.4-6

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Richard Datwyler

on 3 June 2016

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Transcript of LPH 105 W15 8.4-6

Rotational dynamics
Moment of inertial
They are forces at a distances from the axis of rotations.
They are a measure of the resistance to rotational motion.
"how much mass is how far away"
Use the table, or point masses
FBD (forces and angles)
Pick Axis
Label distances from axis to force
Positive or negative torques
Sum and solve
A 35 kg box sits at the end of a 10 m long massless lever, with the fulcrum 3 m from the end with the box.
How much force is needed to begin lifting this box, if the force is applied at the other end, straight down?
r = 3m
F = ?
F = ?
F = mg=343 N
5 m
10 m
3 N
5 N
2 N
4 N
What is the net Torque on this wheel?
If this is a uniform disk of mass 1 kg what is the angular acceleration?
If these forces remain constant for 10 seconds, how many revolutions does it make?
4 m
3 m
10 N
10 N
10 N
What is the net torque
A 2 kg solid sphere rolls down a hill with angle 20 degrees. If the coefficient of friction is known to be 0.05, what is the net torque acting on the sphere with radius 20 cm?
4 kg
25 cm
A solid disk of 3 kg and radius 25 cm has a string wrapped around it holding up a 4 kg block.

What is the tension if I hold the disk in place?
Does the T increase or decrease if I let the mass fall?
What kind of acceleration does the disk have?
Is there a net torque on the disk?
What kind of acceleration does the block have?
How are they related?
4 kg
25 cm
A solid disk of 3 kg and radius 25 cm has a string wrapped around it holding up a 4 kg block.

What is that tension?
What is the acceleration? (both)
If these are two spherical objects with the same radius, one a shell, the other solid. Let them both have the same moment of inertia. If the shell has a mass of 20 kg, what is the mass of the solid sphere?
A 20 kg child sits on the outside edge of a 2 m, 200 kg disk, what is the total moment of inertia of this system?
"I still am having trouble understanding why torque replaces net force. "
"not understanding how to set up torque with the lever arm and the perpendicular set up."
"Can we do some harder problems in class that will be more like the ones on the test so I don't feel so freaked out in the testing center?? "
"Can we go over what each of the replacements for the formula variables are again?"
"In example 8-9 what would the moment of inertia be, if the axis were instead 0.5 m to the right of the 7.0 kg mass?
- 10 N m
6.7 N m
.134 rad/s^2
1.07 rev.
33.3 kg
480 kg m^2
.184 N m
10.7 N
7.13 m/s^2
28.5 rad/s^2
" can you explain what the moment of inertia is, and how it is different than inertia"
5.0 kg, 7.0 kg, 4 m separation
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