# Three forces applied to a trunk

1. Feb 13, 2008

### TS656577

1. The problem statement, all variables and given/known data

The figure shows three forces applied to a trunk that moves leftward by 2.8 m over a frictionless floor. The force magnitudes are F1 = 5.5 N, F2 = 8.7 N, and F3 = 2.6 N, and the indicated angle is θ = 60°. (a) During the displacement, what is the net work done on the trunk by the three applied forces, the gravitational force, and the normal force? (b) Is there a net transfer of energy to or from the trunk? (c) Does the kinetic energy of the trunk increase or decrease?

2. Relevant equations

W=Fd
3. The attempt at a solution
I know that Force 1 did 15.4 J of work (5.5 times 2.8) and I know that Force 2 did -12.18J of work (dotted line = 4.35 and 4.35 times 2.8= 12.18) Force 3 didnt do any work on the block. I dont know how to tie it all together
D

2. Feb 13, 2008

### Hootenanny

Staff Emeritus
There isn't sufficient information, where is the angle 'theta' measured from for example? A diagram or further explanation would be helpful.

Last edited: Feb 13, 2008
3. Feb 13, 2008

### TS656577

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4. Feb 13, 2008

### Hootenanny

Staff Emeritus
Okay, all your answers thus far are correct. For question (a) the net work done is simply the sum of all the work done by the individual forces. With respect to the work done by gravity and the normal force, you have to decide if there was any displacement that wasn't perpendicular to the direction of the normal force and the weight.

The solution to (b) follows directly from (a), positive work is energy in, negative work is energy out.

I'm sure you can figure out (c) yourself

5. Feb 13, 2008

### TS656577

The work due to normal force and gravitational force is 0. So is my answer 3.22

6. Feb 13, 2008

### Hootenanny

Staff Emeritus
3.22 Joules. Correct .

7. Feb 13, 2008

### TS656577

THANKS! and i got the last two!

8. Feb 13, 2008

### Hootenanny

Staff Emeritus
A pleasure, I hardly did any work