Determine the location of her center of mass

[ual8658]

Homework Statement

To determine the location of her center of mass, a physics student lies on a plank of weight 100.0 N and length 2.5 m supported by two scales 2.50 m apart. The left scale reads 420.0 N, and the right scale reads 210.0 N. (Note: Don’t neglect the weight of the plank when you do this problem.) a. Find the student’s weight. [4] b. Find the distance of her center of mass from the left scale (x=0). [4]

Homework Equations

T = rF
W_girl = 420 + 210

The Attempt at a Solution

I get her weight as 630 N. But when I go to solve for the distance using center of mass, when I place the point of rotation at the left support I get her center of mass as .635 m from the left side, but using the right support, I get her center of mass as 1.47 m away from the right side or 1.03 m from the left side. What am I doing wrong to get two different center of masses.

 

[mattbeatlefreak]

Homework Statement

To determine the location of her center of mass, a physics student lies on a plank of weight 100.0 N and length 2.5 m supported by two scales 2.50 m apart. The left scale reads 420.0 N, and the right scale reads 210.0 N. (Note: Don’t neglect the weight of the plank when you do this problem.) a. Find the student’s weight. [4] b. Find the distance of her center of mass from the left scale (x=0). [4]

Homework Equations

T = rF
W_girl = 420 + 210

The Attempt at a Solution

I get her weight as 630 N. But when I go to solve for the distance using center of mass, when I place the point of rotation at the left support I get her center of mass as .635 m from the left side, but using the right support, I get her center of mass as 1.47 m away from the right side or 1.03 m from the left side. What am I doing wrong to get two different center of masses.

The girl's weight is not 630 N. Recall the problem says: "don't neglect the weight of the plank when you do this problem".

 

[jbriggs444]

Homework Statement

To determine the location of her center of mass, a physics student lies on a plank of weight 100.0 N and length 2.5 m supported by two scales 2.50 m apart. The left scale reads 420.0 N, and the right scale reads 210.0 N. (Note: Don’t neglect the weight of the plank when you do this problem.) a. Find the student’s weight. [4] b. Find the distance of her center of mass from the left scale (x=0). [4]

Homework Equations

T = rF
W_girl = 420 + 210

The Attempt at a Solution

I get her weight as 630 N.

How does the weight of the plank figure into that calculation?

But when I go to solve for the distance using center of mass, when I place the point of rotation at the left support I get her center of mass as .635 m from the left side, but using the right support, I get her center of mass as 1.47 m away from the right side or 1.03 m from the left side. What am I doing wrong to get two different center of masses.

How did you calculate either number?

Edit: Too late.

 

[ual8658]

The girl's weight is not 630 N. Recall the problem says: "don't neglect the weight of the plank when you do this problem".

How does the weight of the plank figure into that calculation?How did you calculate either number?

Edit: Too late.

So I placed the point of rotation on either scale. The plank is 100 N with its cm 1.25 m from either side. I then used the other scale's reading and multipled that by 2.5 m, and subtracted the plank's torque contribution before dividing the whole thing by 630.

 

[mattbeatlefreak]

So I placed the point of rotation on either scale. The plank is 100 N with its cm 1.25 m from either side. I then used the other scale's reading and multipled that by 2.5 m, and subtracted the plank's torque contribution before dividing the whole thing by 630.

Why divide by 630? You have done the calculation for the torque from the plank. Again, the girl does not weigh 630 N.

 

[ual8658]

Why divide by 630? You have done the calculation for the torque from the plank. Again, the girl does not weigh 630 N.

But isn't there a torque from the plank's cm, and then an added torque of the girl's cm?

EDIT: I now understand why you say the girl does not weigh 630 N. I now get the correct values. Thanks!