# Defining forces on system help

1. Feb 26, 2009

### fball558

i was reviewing for my exam and this was one of the review questions. They dont give us the answers (dont know why it is pointless studying the wrong stuff) so i was wondering if anyone knew the right answers. each system can have more than one answer and here is the problem word for word

Starting from rest, a woman lifts a barbell with a constant force F through a distance h, at which point she is still lifting, and the barbell has acquired a speed v. Let Ewoman stand for the following energy terms associated with the woman:

Ewoman = Echemical,woman + Kwoman (moving arms etc.) + Ugrav,woman+Earth + Ethermal,woman
The change in the kinetic energy of the barbell is (1/2)mv2 - 0 = (1/2)mv2.
The general statement of the energy principle is Esys = Wext. We'll consider terms on the left side of the equation (the Esys side, changes in the energy inside the system) and terms on the right side (the Wext side, energy inputs from the surroundings).

I. System: Woman + barbell + Earth
For the system consisting of the woman, the barbell, and the Earth, which of the following terms belong on the left side of the energy equation (the Esys side)?

(1/2)mv^2
+mgh
-mgh
Fh
none of these terms (left side is 0)
(1/2)mv2
-Fh
Ewoman

For the system consisting of the woman, the barbell, and the Earth, which of the following terms belong on the right side of the energy equation (the Wext side)?

2-(1/2)mv2
-mgh
none of these terms (right side is 0)
Fh
Ewoman
+mgh
(1/2)mv2
-Fh

II. System: barbell only
For the system consisting of the barbell only, which of the following terms belong on the left side of the energy equation (the Esys side)?

3-(1/2)mv2
-Fh
+mgh
(1/2)mv2
Ewoman
Fh
none of these terms (left side is 0)
-mgh

For the system consisting of the barbell only, which of the following terms belong on the right side of the energy equation (the Wext side)?

4Fh
none of these terms (right side is 0)
+mgh
Ewoman
-Fh
-mgh
(1/2)mv2
-(1/2)mv2

III. System: barbell + Earth
For the system consisting of the barbell and the Earth, which of the following terms belong on the left side of the energy equation (the Esys side)?

5Ewoman
+mgh
-(1/2)mv2
Fh
-Fh
(1/2)mv2
none of these terms (left side is 0)
-mgh

For the system consisting of the barbell and the Earth, which of the following terms belong on the right side of the energy equation (the Wext side)?

6none of these terms (right side is 0)
Fh
(1/2)mv2
+mgh
Ewoman
-Fh
-mgh
-(1/2)mv2