Confusing regarding Kinematic Equations

[dolpho]

"A 93kg water skier floating in a lake is pulled from rest to a speed of 13 m/s in a distance of 20m. What is the net force exerted on the skier, assuming his velocity is constant.

So I know I have to find the force I'm going to need acceleration, and to find it I'll need to use a kinematic equation. So this is where the confusion comes in.

The correct way to do it is v^2= U^2 + 2ad, and isolate the a. The answer to this would be 4.22 m/s^2. Then we can find the force by plugging it in, f=(4.22)(93)= 392 Newtons

But why couldn't I use this kinematic equation and solve for a?

x=x₀ + v₀t + 1/2(a)t^2)
20=0+0+1/2(a)(16)
20=8a
20/8 = a

Sorry if this seems like a dumb question but I think all the different letters and their meanings are getting a little confusing.

Or this one, acceleration = delta V / Delta T, couldn't you just do 13-0 / 4-0? so its 13/4?

 

[SHISHKABOB]

where'd you get the four seconds from?

 

[dolpho]

where'd you get the four seconds from?

Mmm, I think I accidentally put 4 seconds because I was reading something else. V= D/T, = T=D/V which is .65 seconds. But even then I couldn't solve for the equation in the other kinematic equation. Am I totally plugging in the wrong numbers into the x= Vsub0T + Xsub0 + 1/2(a)(t^2)?

 

[SHISHKABOB]

well, v = d/t only if the velocity is constant, and in this case the velocity is not constant, the guy is under a constant acceleration

so basically, you want to take what information you know, and pick the equation that works best with that stuff

the x = x0 + v0t + (1/2)at^2 equation doesn't work for this given set of values because we don't have the time