How tall is the bridge?

  • Thread starter keithcuda
  • Start date
  • #1
43
0

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
 

Answers and Replies

  • #2
phinds
Science Advisor
Insights Author
Gold Member
16,738
7,427
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.
 
  • #3
20,870
4,546
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
 
  • Like
Likes keithcuda
  • #4
phinds
Science Advisor
Insights Author
Gold Member
16,738
7,427
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:
 

Related Threads on How tall is the bridge?

  • Last Post
Replies
7
Views
556
  • Last Post
Replies
1
Views
6K
Replies
12
Views
2K
Replies
4
Views
992
Replies
22
Views
13K
Replies
13
Views
2K
Replies
2
Views
713
Replies
2
Views
640
Replies
3
Views
3K
Top