Rock thrown from cliff, find initial and final velocities

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Homework Help Overview

The discussion revolves around a physics problem involving projectile motion, specifically a rock thrown horizontally from a cliff. The problem requires determining the initial speed of the rock and its final velocity after falling from a height of 25.0 m and landing 15.0 m away from the base of the cliff.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to calculate the time of flight using kinematic equations but expresses confusion over discrepancies between their results and those in their workbook. They question the method used to arrive at the time value.
  • Some participants point out potential errors in the original poster's calculations, specifically regarding the order of operations and the use of parentheses in the equations.
  • There is a discussion about the correct application of the kinematic formula for vertical motion and the implications of miscalculating the time of flight.

Discussion Status

The discussion is ongoing, with participants providing guidance on correcting the calculations. The original poster has expressed gratitude for the assistance received, indicating a productive exchange of ideas. However, there is no explicit consensus on the final solution yet.

Contextual Notes

The original poster mentions that their workbook consistently provides different answers from their calculations, which raises questions about the assumptions or methods used in the examples provided in their course materials.

kitkat87
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Homework Statement


I'm taking grade 12 U physics and I'm having a really difficult time understanding the answers given compared to the answers I arrive at.

For example:

The question reads:

A rock was thrown horizontally from a 25.0 m high cliff and landed 15.0 m from the base of the cliff. Determine the initial speed with which the rock was thrown, as well as its final velocity.

2. The attempt at a solution

Vertical
V1y = 0.0 m/s
ay=9.8 m/s^2
Displacement = 25 m

Solve for y-component

25m = (0.0 m/s)(change in time) + 1/2(9.8 m/s^2)(change in time)^2

Rearrange

(change in time)^2 = 2(25)/9.8m/s^2
change in time = Sqrt 2(25) / 9.8m/s^2
change in time = 0.7215375318230077

This is the answer I arrived at.

My workbook however, says the answer is 2.26. Can someone please explain to me how my workbook arrived that that answer? I cannot move on in the question until I solve this.

So far every example in my course provides the correct calculations but all of the final answers are completely incorrect.
 
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You did order of operations wrong. Work it out again from the first line and keep your parentheses.
 
kitkat87 said:

Homework Statement



The question reads:

A rock was thrown horizontally from a 25.0 m high cliff and landed 15.0 m from the base of the cliff. Determine the initial speed with which the rock was thrown, as well as its final velocity.

2. The attempt at a solution

Vertical
V1y = 0.0 m/s
ay=9.8 m/s^2
Displacement = 25 m

Solve for y-component

25m = (0.0 m/s)(change in time) + 1/2(9.8 m/s^2)(change in time)^2

Rearrange

(change in time)^2 = 2(25)/9.8m/s^2
change in time = Sqrt 2(25) / 9.8m/s^2
change in time = 0.7215375318230077

Welcome to PF!
You miss parentheses. (change in time)^2 = 2*25/9.8 = 5.102. Take the square root to get the time : The correct formula is (change in time) = Sqrt ((2*25) / 9.8)
You took the square root of 50 and divided it by 9.8, which is wrong.

ehild
 
Thank you both so much! you're amaz-o

My entire workbook leaves the 9.8m/s^2 throughout all of the examples so I thought that it stayed that way.

You've just made my life so much easier.
 

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