# Force problem

1. Feb 27, 2006

### tsoya

im studying for a test, unfortunately i seem to be having difficulty with the simple problems, so i decided to try some practice problems:
1. two forces, each of 100N, act on an object having a mass of 40 kg. What is the magnitude of the net acceleration of the object if one of the forces is directed upward and the other is to the left?
the answer it says is 3.5 m/s^2. i know net external force=mass x acceleration, so i tried figuring out the net external force and I think that's where I get confused. because force is mass times acceleration? ug i dont know. can someone help please?
*******
oh thank you so much!! i got it now, square root of (100^2 + 100^2) then divide by 40. may i ask another one?

a 250-N weight is suspended in equilibrium by two cables. one cable applies a horizontal force to the right. the other cable applies a force upward to the left at an angle of 45 deg. to the negative x-axis. what is the tension in the second cable? answer: 350 N
-how do you incorporate the first cable into figuring out for the second? i've tried solving this a couple of days ago, and still can't. any type of hint would be helpful!

Last edited: Feb 27, 2006
2. Feb 27, 2006

### Staff: Mentor

First find the net force by adding the two force vectors to get their resultant. (Remember to use vector addition.) Then use Newton's 2nd law to find the acceleration.

3. Feb 27, 2006

### tsoya

i posted another problem in the first post if anybody is willing to offer any help =)

4. Feb 27, 2006

### Staff: Mentor

Hint: Consider the vertical components of the forces. Since the object is in equilibrium, the net force on it must be zero. (Note that the first cable doesn't contribute to the vertical forces.)

5. Feb 27, 2006

### tsoya

Last edited by a moderator: Apr 22, 2017
6. Feb 27, 2006

### Hootenanny

Staff Emeritus
Ok, if we just consider the vertical components we can effectively ignore the horizontal cable as Doc Al said. Now, what force(s) are acting to pull the weight down and how big is this force?

7. Feb 27, 2006

### tsoya

gravity is pulling it down (9.8, so total force is 250 N?)ok the answer is given to be 350, is that right?

Last edited: Feb 27, 2006
8. Feb 27, 2006

### Hootenanny

Staff Emeritus
Nearly right, what is the equation of force (one of Newton's laws)?

9. Feb 27, 2006

### tsoya

force= mass x acceleration

10. Feb 27, 2006

### Hootenanny

Staff Emeritus
Yes, so what is the total force acting down?

11. Feb 27, 2006

### tsoya

25.5 kg x 9.8 m/s^2= 250 N

12. Feb 27, 2006

### Hootenanny

Staff Emeritus
Yes, so as the system is in equilibrium that force must be balanced by an upward force. However, this upward force can only act through the angled cable. Draw a free body diagram, showing the weight, the downward force, the required upward force acting straight up and the angled wire.

13. Feb 27, 2006

### tsoya

oh is this correct?:
250/(cos45)
i get a bit higher than 350 though, 353.55

14. Feb 27, 2006

### Hootenanny

Staff Emeritus
Yes, that is the answer which I got.

15. Feb 27, 2006

### tsoya

thank you very much for all of your help =)

16. Feb 27, 2006

### Hootenanny

Staff Emeritus
No problem. For these types of problems I suggest drawing a diagram as it makes things alot simpler.