# Homework Help: Work Done

1. Oct 20, 2008

### shimizua

1. The problem statement, all variables and given/known data
What is the work done by the two forces together in moving an object a distance of d = 9 m as shown in the diagram? The magnitude of each force is 21 N.
There is a picture that comes with this
http://capa-new.colorado.edu/teacher/capalibrary/Graphics/Gtype12/prob39.gif

2. Relevant equations
So i know that W==Fnet*d but i am unsure of how to do it with two angles. i also know that work can be found by W=Fcos(theta)d and i have tried both and neither seem to work.

I also have a second question that is similar
With brakes fully applied, a 1430 kg car deccelerates from a speed of 83.0 km/hr. What is the work done by the braking force in bringing the car to a stop?

So the way i tried to find this was by doing W=mad and that came up as wrong.

and a second part to that question is
What is the change in kinetic energy of the car

I know that KEI+PEI=KEF+PEF
so would i just do 1.2mvsquared to find the KEF?

Last edited: Oct 20, 2008
2. Oct 20, 2008

### mgb_phys

The forces simply add.
Calculate each force in the direction of 'd' ( this is equvialent to the length of F in that direction) then add them for the total force.

3. Oct 20, 2008

### shimizua

so for the first question i should do
(21cos(45)*9)+(21cos(45)*9)?

4. Oct 20, 2008

### shimizua

oh nm about the second question i figured out what was wrong. i forgot to make it in m/s and kept it in km/hr. my bad

5. Oct 20, 2008

### shimizua

please can someone help me!

6. Oct 20, 2008

### mgb_phys

Isn't the change in KE just the initial ke of the car, since it has no ke at the end.

7. Oct 20, 2008

### shimizua

i figured out that second part already, it was the first part that was giving me trouble. the part about what is the work done by the two forces together. i mean would it be 0J since it is technically a 90deg angle?

8. Oct 21, 2008

### shimizua

ok so i was told to do W=Fdcos(theta) but what i am not sure of is do i basically multiply by 2 cause there are two angles or do i just do the one?

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