# Homework Help: Force problems- help pls

1. Oct 1, 2004

### debbie18

Force problems- help pls!!

I need help with these problems:
1. A 3.9 kg object is subjected to two forces, F1= (2.8N)i + (-4N)j and F2= (3.7N)i + (-10.4N)j. The object is at rest at the origin at time t= 0.
a) What is the magnitude of the object's acceleration? Got That one.
b) What is the magnitude of the velocity at t= 3.8 s? Got that one too.
c) What is the magnitude of the object's position at t= 3.8 s? No idea how to do this one.

2. A 1.5 kg particle starts from rest and moves a distance of 2.3 m in 2.9 s under the action of a single, constant force. Find the magnitude of the force.

Please!! Can someone help me asap!!

2. Oct 1, 2004

### vsage

Magnitude = sqrt(i^2 + j^2) (components not just the unit vectors). Integrate the velocity function to find the position function and take the magnitude of the position vector at t = 3.8s. Shouldn't be too hard given this info.

3. Oct 1, 2004

### debbie18

I am sorry, I still don't get it . What do u mean by integrate the velocity function to get position function

4. Oct 1, 2004

### vsage

How did you derive the velocity function? It should be a similar process going from velocity function to position function as acceleration to velocity function (integration)

5. Oct 4, 2004

### debbie18

I got the velocity like this:
v-v0 = at = (4.05) (3.8) = 15.39 m/s. which was correct.
I tried to figure out the magnitude of object's position like this:
v = change in x / change in t
15.39 = change in x / 3.8
change in x = 58.48 m. which was the wrong answer.
What am I doing wrong?