Speed of rocks when they hit the ground

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SUMMARY

The speed of rocks thrown upwards and downwards just before they hit the ground is the same, despite differing times of flight. This conclusion is based on the conservation of mechanical energy principle, which states that the initial kinetic energy of the rocks converts to potential energy and then back to kinetic energy as they fall. The mathematical demonstration involves using the equations of motion under gravity, specifically \( v^2 = u^2 + 2gh \), where \( v \) is the final velocity, \( u \) is the initial velocity, \( g \) is the acceleration due to gravity, and \( h \) is the height.

PREREQUISITES
  • Understanding of basic physics concepts, particularly kinematics.
  • Familiarity with the conservation of mechanical energy principle.
  • Knowledge of equations of motion in a gravitational field.
  • Ability to perform calculations involving initial and final velocities.
NEXT STEPS
  • Study the equations of motion under gravity, focusing on \( v^2 = u^2 + 2gh \).
  • Explore the conservation of mechanical energy in different scenarios.
  • Learn about the effects of air resistance on falling objects.
  • Investigate the differences in time of flight for objects thrown upwards versus downwards.
USEFUL FOR

Students studying physics, educators teaching kinematics, and anyone interested in understanding the principles of motion under gravity.

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Homework Statement



If I throw a rock upwards with an initial velocity and throw a rock downwards, what is the speed of the rocks just before they hit the ground?

Homework Equations





The Attempt at a Solution

\

They should hit the ground at the same speed and different times. I would use the conservation of mechanical energy, but someone I'm speaking to disagrees with me that they have same speed.

How would you show this mathematically?
 
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Use the formulas for objects traveling in a gravity field. You are studying physics, I presume.
 
See your initial kinetic energy is the final potential energy.
use this relation.
 

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