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**1. Homework Statement**

A sled weighing 100 N is pulled horizontally across a frozen lake such that the coefficient of kinetic friction between the sled and the snow is 0.1. Grilka is riding the sled and she weighs 195 N. If the coefficient of static friction between Grilka and the sled is 0.7 and the acceleration is increasing by t^3/(1 S + t) m/s^4, at what time will the horizontal force applied to the sled cause her to begin to slide off?

## Homework Equations

F_f = u*F_N

F = ma

## The Attempt at a Solution

First we find the kinetic force of friction on the sled:

F_k = (.1)(100 N + 195 N) = 29.5 N

We can also find the force necessary to begin Grilka sliding backwards:

F_s = (.7)(195 N) = 136.5 N

The system is accelerating in the positive x-direction and is begin affected by the force pulling it in this direction and the opposing force of kinetic friction, so to represent the system we have:

F_x - f_k = ma

or

F_x = [(100 N + 195 N)/9.8 m/s^2] * a + 29.5 N

If we solve for a we can find the acceleration of the system:

a = (F_x - 29.5 N)/30.10kg

Then, given that we know what F_s is, we can figure out what F_x must be to equal (and overcome) F_x:

F_s = ma

136.5 N = (195 N/ 9.8 m/s^2)[(F_x - 29.5 N)/30.10kg]

Solving for F_x we get:

F_x = 235.96 N

This is the horizontal force that must be applied to the system to cause Grilka to begin sliding on the sled.

At this point I'm not sure how to proceed. Do we simply plug in the F_x equal to Grilka's mass and the given acceleration, then solve for t? Like this?

235.96 N = (195 N/ 9.8 m/s^2)[t^3/(1 S + t) m/s^4]

t = 3.86s

The problem is asking for a time that a force is reached given a certain acceleration, but don't we have to know the initial velocity at which the sled is being pulled horizontally across the snow?

Also I should ask whether it's right to assume that the "S" in the given acceleration is supposed to mean second?

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