Homework help: Work and Energy, Free fall

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Homework Help Overview

The discussion revolves around a problem in the subject area of work and energy, specifically focusing on the motion of a 200 kg object in free fall from a height of 20 m to 5 m, and the calculation of its new speed after falling.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants explore the use of kinematic equations and conservation of energy to determine the new speed of the object. Questions arise regarding the direction of the initial speed and the appropriateness of using gravitational acceleration in the calculations.

Discussion Status

There is ongoing exploration of different methods to approach the problem, with some participants suggesting the use of energy conservation as a potential method. Clarifications about the direction of motion and the validity of the calculations have been raised, indicating a productive dialogue without a clear consensus on the preferred method.

Contextual Notes

Participants note that the original poster's calculations may need to consider significant digits due to the nature of the given values. There is also mention of potential confusion caused by multiple threads on the same topic.

Syeda
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Homework Statement


A 200 kg object moves at 10 m/s at 20m (vertically). It falls to a height of 5 m. Find the new speed.

Homework Equations


vf²=vi²+2ad

The Attempt at a Solution


vf²=(10m/s)²+2(9.8m/s²)(15m)
vf=19.8m/s

To find the distance for the new speed I did 20-5, but I'm not sure it's correct.
Also I'm not sure if I'm supposed to use 9.8 for the acceleration
 
Last edited:
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Use energy conservation.

On edit: I noticed that you have posted two different threads with the same title. Please try to avoid this because it can be confusing.
 
"moves at 10m/s" in which direction? Vertically? Horizontally?
 
CWatters said:
"moves at 10m/s" in which direction? Vertically? Horizontally?
Vertically, from 20 m to 5 m
 
Then I believe your working in #1 is correct.

You could use conservation of energy method to check you get same answer if you want.

Edit: I just noticed the title says work and energy so perhaps your tutor intended you to use conservation of energy method rather than the equations of motion.
 
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CWatters said:
I just noticed the title says work and energy so perhaps your tutor intended you to use conservation of energy method rather than the equations of motion.
Arguably, OP's relevant equation, vf²=vi²+2ad is the energy conservation equation with a factor of (½)m multiplying both sides canceled out.
 
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Everything you have done in step #3 is correct.
Yes, it moves 15 metres.
Yes, it is freefall, so you can use 9.8 for the acceleration.
Remember that if the solution has to be in proper significant digits, you will need to round it to 1 digit because the lowest number of significant digits in your original given values is 1 digit. In fact, ALL of the given numbers are 1 significant digit long.
 
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