# Homework Help: How to calculate distance when there's a changing force?D:

1. Oct 17, 2012

### monikraw

How to calculate distance when there's a changing force???D:

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

A 3.50kg box is moving to the right with speed 10.0m/s on a horizontal, frictionless surface. At t = 0 a horizontal force is applied to the box. The force is directed to the left and has magnitude F(t) = (6.00N/s^2)t^2

What distance does the box move from its position at before its speed is reduced to zero?

If the force continues to be applied, what is the speed of the box at 4.00 ?

2. Relevant equations

x = x0 + v0xt + .5axt^2
F = ma
vx = v0 + axt
I might be missing several

3. The attempt at a solution

I don't even know where to start for this problem.
I drew a free body diagram and it didn't help me.
What I calculated :
Force downward is 3.5kg x 9.8 m/s^2
or the weight
I don't know how to calculate the forces to the left and right.

For the second question I plugged in 4 to the F(t) equation and got 96
so I used 96 = ma SO
96 = 3.5 * a
a = 27.4
and I plugged 27.4 into vx = v0x + axt
so vx = 10m/s + 27.4(4)
and got 119.7 and this is not correct.

Last edited: Oct 17, 2012
2. Oct 17, 2012

### monikraw

Re: How to calculate distance when there's a changing force???D:

I tried googling the problem and someone else had a similar problem. They got 6.00 for acceleration and I'm not too sure as to how the person got it, but I used it and got 19.5 for the first question.

I don't know how to calculate the new velocity after 4 seconds though.
I used

vx = v0 + axt
vx = 10m/s + 6.00(4)
vx = 34...incorrect.

3. Oct 17, 2012

### rcgldr

Re: How to calculate distance when there's a changing force???D:

You'll need to use calculus. You start here:

F(t) = (6.00N/s^2)t^2 = (6.00 kg m / s^2) t^2
m = 3.50kg
a(t) = (6.00 / 3.50) (m/s^2) t^2
v(t) = ???
x(t) = ???