# Homework Help: Object subjected to constant force, determine the force

1. Jun 30, 2008

### student 1

1. The problem statement, all variables and given/known data
An object of Mass 3 in the x-y plane is subjected to a constant force F (in addition to its weight). The object starts from rest at x=0 y=0 and t=0. At t=2 the object's displacement is 2i+4j. Determine F.

2. Relevant equations

3. The attempt at a solution Where do I begin? I know you have the position it moved so would I use work. Or do I need to find the acceleration and try and Find F that way???

2. Jun 30, 2008

### neutrino

Re: Force?

If you know the general solution to Newton's second law of motion, then you would just need to plug-in the numbers to find the answer. The key here is to note that the force is constant.

3. Jun 30, 2008

### student 1

Re: Force?

So would I use an Integral to figure it out since it is a constant force? or would I use F=MA.

4. Jun 30, 2008

### neutrino

Re: Force?

Essentially, that's what you will do, but the solution to that is something you are probably already familiar with. It's an equation of motion.

5. Jun 30, 2008

### student 1

Re: Force?

Equation of motion is confusing me? Would I say it traveled the magnitude of the disposition vector or that it only traveled 2 m in x direction?

6. Jun 30, 2008

### neutrino

Re: Force?

Find the equation that relates displacement, initial velocity and time lapsed and use it to find the acceleration, in both the x and y directions. And you must be knowing how acceleration relates to force.

7. Jun 30, 2008

### student 1

Re: Force?

ok so you use Rf=Ri+Vi(t)+1/2(a)T^2 in both direction and since it started at rest you have no initial velocity. so u basically multiply the I and J by 2 to get your acceleration and the F would be your mass times acceleration. Right?

8. Jun 30, 2008

### neutrino

Re: Force?

That's right. That equation would look better (accurate, actually) if you substitute tf - ti for t.

Huh?

Right.

9. Jun 30, 2008

### student 1

Re: Force?

Alright, I messed up algebraically what I'm saying is since the rest of the equation is o+o+1/2A(Tf-Ti)^2, So you would have 2-0 for the T square that would give you 4. 1/2(A)4 would give you 2a so you would divide that and find each acceleration. Will your final answer be in unit vector form for the force???