How High Is the Bridge Based on Projectile Motion?

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Homework Help Overview

The problem involves determining the height of a bridge based on the projectile motion of a rock thrown horizontally. The rock travels 30 meters horizontally and is in the air for 2 seconds.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • The original poster attempts to apply the formula for distance fallen under gravity to calculate the height of the bridge. Some participants question the accuracy of the calculations and the assumptions regarding the motion of the rock.

Discussion Status

Participants are exploring different interpretations of the motion involved, particularly regarding the effects of gravity over time. There is a mix of agreement and disagreement about the original poster's approach, with some suggesting reconsideration of the assumptions made.

Contextual Notes

There is a noted confusion regarding the application of gravitational acceleration and how it relates to the distance fallen over time. The discussion reflects varying levels of familiarity with the concepts involved.

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Homework Statement


A rock thrown horizontally from a bridge hits the water below 30 meters away in the horizontal direction. If the rock was in the air for 2 seconds, how tall is the bridge?

Homework Equations


d=.5gt2

The Attempt at a Solution


I think I am going about this the right way. Just want to double check.

d=.5*9.81*4
d= 19.62m
The bridge would be 19.62m high?

cheers,
Keith
 
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It's been about 50 years since I've actually worked out a problem like that but it sounds wrong, and here's why: the acceleration due to gravity is 9.8 meters per second per second. That means that the rock will travel 9.8 meters in the first second, but much more in the next second, so the total can't be 9.8 x 2 which is what you got.
 
phinds said:
It's been about 50 years since I've actually worked out a problem like that but it sounds wrong, and here's why: the acceleration due to gravity is 9.8 meters per second per second. That means that the rock will travel 9.8 meters in the first second, but much more in the next second, so the total can't be 9.8 x 2 which is what you got.
It looks right to me. The first second it travels ~ 5 m, and the second second, it travels ~15 m.

Chet
 
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Chestermiller said:
It looks right to me. The first second it travels ~ 5 m, and the second second, it travels ~15 m.

Chet
Guess I have it wrong then that it travels 9.8m in the first second. I guess what the figure means is that at the end of the first second it is MOVING at 9.8 m/s, not that it has MOVED 9.8 m ... 50 years ago I bet I would have got it right :oldtongue:
 

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