![]() So, we have this formula for position, and I'm writing in a subscript one here which usually I don't write, but in this question we have three different heights to think about and so it's better to label each of them with their own subscript. That's the time between here, t one, when the hiker notices the rock, and here when the rock reaches the hiker's height. So how much time does the hiker have to move out of the way of this rock. Then the question for part B is, how much time passes between when the hiker initially notices the rock, when it's at this level here at time t one, and when the rock reaches the hiker's position which is t two. But we know that it reaches this position at t one which is 1.5 seconds. Part A is to figure out what is that height above the hiker. This rock is going to fall a little ways until it gets to this position here, which we'll call y subscript one and we don't know what that position is. Its initial height we're told is 105 meters, and we're taking this position of the hiker to be, we'll call it y subscript two, and we'll take that to be zero meters. ![]() Its acceleration will be that due to gravity of negative 9.8 meters per second squared, where we're taking down to be the negative direction and up to be positive. This rock falls off the cliff and then we presume that the initial velocity is zero. We also write down the information we're given, of course. We start this question by drawing a picture which is often a really good strategy for setting up a question in order to understand how to solve it. This is College Physics Answers with Shaun Dychko.
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