# Homework Help: Finding a net force

1. Dec 5, 2009

### dois

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

the questions says,
"Person A who's mass is 40kg, is being pulled across the ground by a horse at a speed of 4 m/s [E]. All of a sudden the horse accelerates for a period of 5 seconds. During this time person A experiences a displacement of 40 m [E]. What net force was experienced by person A?"

2. Relevant equations

3. The attempt at a solution

I have no idea how to even begin this question. I'm not sure what equations i even need to use.

2. Dec 5, 2009

### rl.bhat

Hi dois, welcome to PF.
In the problem the initial velocity, duration pof acceleration and the displacement is given. Can find find a kinematic equation from the text book which relates these quantities with acceleration?

3. Dec 5, 2009

4. Dec 5, 2009

### dois

ok so first I would need to find the acceleration so I can sub that answer into F= m*a
so i would use
acceleration= v/t
?

5. Dec 5, 2009

### jamesweston0

The problem gives you the initial speed (Vi), time (t), and displacement (d). If you take a look at the kinematic equations, you'll find that you can solve one of them for acceleration. The kinematic equation you're looking for is this one.

d = (vi)(t) + (1/2)(a)(t)^2

Go from there.

6. Dec 6, 2009

### dois

Ok so if we sub all of our know variables into the equation
d = (vi)(t) + (1/2)(a)(t)^2
then we would get:
40= 4x5 +1/2 (?) x5^2
40=20 +25/2 x
20=25/2 x
20=12 1/2 x
x= 1.6

but as soon as i figure out the acceleration where do I go from there to find out the net force?

Last edited: Dec 6, 2009
7. Dec 6, 2009

### AEM

Okay, your algebra looks good, so a = 1.6 m/sec squared.

Remember I said this was a two step question. Go back and read my first post. That indicates what you do next.

8. Dec 6, 2009

### dois

So then we would sub a into F=ma
so (40kg) (1.6 m/s)
and then our answer would be Fnet= 64 kg m/s [E]

9. Dec 6, 2009

### AEM

That should do it, except that the units you give are wrong. The seconds should be squared. That combination of units has its own name: Newtons.

10. Dec 6, 2009

### dois

alright, thank you!!

11. Dec 6, 2009

### AEM

You are quite welcome.