Help Total mechanical energy question

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The discussion centers on calculating the speed of a rock just before it hits the ground after being thrown upwards from a height of 30 meters. Total mechanical energy is conserved, meaning the energy at the moment of release equals the energy just before impact. The gravitational potential energy is zero at ground level, while the kinetic energy will determine the speed at that point. The confusion arises from the misconception that the speed is zero when the rock hits the ground; in fact, it is at its maximum just before impact. Understanding the conservation of energy principle is key to solving the problem correctly.
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[SOLVED] Help! Total mechanical energy question:)

Q. Suppose you are standing on a 30 m building and throw a 1 kg rock upwards at 20 m/s. Knowing that total mechanical energy is conserved, calculate the speed of the rock when it hits the ground.


Total Mechanical Energy equation:

Etotal = Egravitational + Ekinetic
Et = Eg + Ek
= mgh + ½mv2


What is the speed of the rock when it hits the ground? Well, the gravitational potential energy would be 0, if the reference point is the ground, yes? The upwards motion of the rock has thrown me off:confused:
 
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Correct me if I'm wrong, but when the rock hits the ground, wouldn't the speed be zero? It's not moving.
 
adv said:
Correct me if I'm wrong, but when the rock hits the ground, wouldn't the speed be zero? It's not moving.
The problem is asking you to determine the rock's speed at the instant just before it hits the ground.
Since total mechanical energy is conserved, the mechanical energy that the rock has at the instant it leaves the thrower's hand must be equal to its total mechanical energy at the instant just before it hits the ground.
 

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