# Homework Help: Average Upward Force

1. May 22, 2010

### randomss444

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

An extreme amusement park ride accelerates its riders upward from rest at 50.0 m/s in 7 seconds. Ignoring air resistance, what average upward force does the seat exert on a rider who weights 1120N?

t = 7.00s
v = 50.0 m/s
Mass = 114.285
mg = 1120N

2. Relevant equations

∑F= ma

3. The attempt at a solution

I am not sure of that v is vi or vf.

I know my first step needs to be to solve for acceleration I just do not know how to do it given the information that I have.

Thanks

2. May 22, 2010

### rock.freak667

They told you this:

So the riders go from rest to a final velocity of 50 m/s in 7 seconds. (Initial is at rest, so vi=?)

3. May 22, 2010

### randomss444

Okay so how would I solve for vi?

could I use v = x/t to find then displacement

and then use x= 1/2(vi+vf)t to solve for vi

?

If do that then the answer comes out to be vi =2 and that does not seem right

4. May 22, 2010

### jegues

In other words, intially it has zero velocity.

That being said, what's vi?

5. May 22, 2010

### randomss444

vi = initial velocity

Okay so we got
vf =50
vi=0
t =7.00s
m=114.2857143
mg=1120n

so we find acceleration by using vf=vi+at

50=0+7a
a=7.142857143

so we can find ∑E= ma

∑E = (114.2857143)(7.142857143)
∑E = 816.3265307

Is that right?
And would that be the final answer?

6. May 22, 2010

### jegues

You're missing all your units in all of your calculations, but other than that the answer looks right. (Unless I made a mistake somewhere :S)

7. May 23, 2010

### Jokerhelper

I'm not sure if that is correct. what you found is average acceleration from which you determined average net force, when instead you should be looking for is the average force exerted by the seat on the person. Are there other forces acting on the person you may have to consider?

8. May 23, 2010

### randomss444

The only one I can think of is gravity because it says to ignore air resistance.

So what should my next step be then and how do you find average force exerted by something?

9. May 23, 2010

### rock.freak667

gravity is always there, but since the person is sitting on the seat, there will something reaction wouldn't there?