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In preparation for this problem, review Conceptual Example 7 From the top of a cliff, a person uses a slingshot to fire a pebble straight downward, which is the negative direction. The initial speed of the pebble is 9.0 $\mathrm{m} / \mathrm{s} .$ (a) What is the acceleration (magnitude and direction) of the pebble during the downward motion? Is the pebble decelerating? Explain. (b) After 0.50 $\mathrm{s}$ , how far beneath the cliff top is the pebble?

(a) The slingshot has an accelration of, $a=-9.8 \mathrm{m} \cdot \mathrm{s}^{-2}$ and its notdecelerating.(b) $y=-5.725 \mathrm{m} .$

Physics 101 Mechanics

Chapter 2

Kinematics in One Dimension

Motion Along a Straight Line

Rutgers, The State University of New Jersey

University of Michigan - Ann Arbor

Hope College

McMaster University

Lectures

04:16

In mathematics, a proof is…

04:48

In mathematics, algebra is…

04:20

Review Conceptual Example …

04:28

Multiple-Concept Example 6…

04:31

02:20

A slingshot fires a pebble…

05:01

See Multiple-Concept Examp…

03:24

Consult Multiple-Concept E…

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A rock is thrown off a cli…

02:37

In the problems, please as…

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Motion with gravity Consid…

08:44

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Consider the following des…

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Use $4.9 t^{2}+v_{0} t=s$.…

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04:52

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Suppose the object in Prob…

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Review problem. Suppose th…

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in this problem of person launches a pebble from the top of a cliff in the downward direction. The initial speed of that peple is nine meters per second, but because it's in the negative or the downward direction, we can call that negative 9.0 liters per second for the initial velocity. They want to know what the acceleration of the pebble will be nowhere near the surface of the earth. And there's no other forces acting on the public wallets in free fall. So we have the acceleration. We know the magnitude is going to be 9.8 meters per second squared, but that acceleration is always toward the center of the earth, which is down. So the acceleration also has a negative value. So the magnitude is 9.8, and, uh, direction is negative. Is the pebble decelerating? Well, sometimes people think when the acceleration is negative, that that means deceleration. But that's only true. If you're moving in the positive direction. Here we have a negative velocity and a negative acceleration. So on a graph of velocity craft, this would look something like this. Let's see, so the velocity is getting larger and larger in the negative direction. So it's speeding up, but it's speeding up in the negative direction. The acceleration, which is the slope of velocity versus time, is also negative. So in this case, the velocity is negative. The acceleration is negative. That means the speed is increasing, so that's not will be called typically in English language. We don't call that deceleration. We call that an acceleration in physics. Both are accelerations. Depending on the signs, you could be speeding up where you could be slowing down. Um, this next part of the question says after five point no seconds. What? How far beneath the cliff? How far is it traveled? How far beneath the cliff is that pebble? So we could just use the equation for position. Its initial velocity times time plus 1/2 times acceleration due to gravity. Tens t squared. So our initial velocity is negative. Nine times 1/2 a 2nd 0.5 seconds. What's 1/2 times the acceleration, which is negative times the time squared, So the first term is negative 4.5 liters. Second term is negative, 1.2 to 5 meters, so we have a total distance uh, negative. 5.725 meters. Rounded to two significant figures. We have negative 5.7 meters below the top of the cliff.

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