torque stuff

[pizza1994]

---

Members don't see this ad.

A uniformly dense plank of length L supports a 1 kg mass on its left end and a 3 kg mass on its right end. How far from the left is the system’s center of mass?

a) L/4
B) L/3
C) 2L/3
D) 3L/4

answer is 2L/3

source is TPR!

how did they get this? I get 3L/4 using center of mass equation

 

[DrknoSDN]

You are solving for a massless plank of length L. If the plank is massless then the answer is 3L/4.
You can verify it with the calculator here: http://hyperphysics.phy-astr.gsu.edu/hbase/cmms.html

The question is mentioning a uniformly dense plank. Did somewhere they mention the mass of the plank? The reason being, if a plank is uniformly dense its mass will act at it's physical center changing the center of mass of the system.

 

[pizza1994]

You are solving for a massless plank of length L. If the plank is massless then the answer is 3L/4.
You can verify it with the calculator here: http://hyperphysics.phy-astr.gsu.edu/hbase/cmms.html

The question is mentioning a uniformly dense plank. Did somewhere they mention the mass of the plank? The reason being, if a plank is uniformly dense its mass will act at it's physical center changing the center of mass of the system.

Yeah so when you say that "will act at it's physical center"....I got lost. I agree that 3L/4 is the value for a maseless plank and I also agree that the plank has a mass. But I dont get why the value of 3L/4 gets shifted towards the center......

 

[DrknoSDN]

If the plank is uniformly dense, then the center of mass of the plank is it's... physical center. (center of plank = center of mass for the plank)

So if the plank has a mass it will shift the center of mass of the system towards itself relative to if the system was using a massless plank.
If the plank is heavy enough the masses on the end become irrelevant and the center of mass is essentially the center of the plank. (Grain of sand on one end and 3 grains of sand on the other end, etc)

 

[pizza1994]

If the plank is uniformly dense, then the center of mass of the plank is it's... physical center. (center of plank = center of mass for the plank)

So if the plank has a mass it will shift the center of mass of the system towards itself relative to if the system was using a massless plank.
If the plank is heavy enough the masses on the end become irrelevant and the center of mass is essentially the center of the plank. (Grain of sand on one end and 3 grains of sand on the other end, etc)

THANK YOU! 🙂

 

[DrknoSDN]

NP, and if the answer is 2L/3 then the plank must have a mass of 2kg assuming it is in equilibrium (motionless and not rotating)

Assume L equals 1 meters you can solve and get actual numbers.
Holding the left of the plank stationary you have (2kg*0.5L) + (3*L) = 40 Newton clockwise rotation.
CCW rotation torque location is given as (2/3)L with a total system mass of (1kg+3kg+2kg=6kg); (2/3)*6kg=40 Newtons CCW

X solves for mass of plank using balanced torque rotation.
Left side is upward torque at 2L/3 and right side is 3kg mass and mass of plank.