How Do You Calculate the Initial Velocity of a Rock Thrown from a Bridge?

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Homework Help Overview

The discussion revolves around calculating the initial velocity of a rock thrown from a bridge 30 meters high, with a total time of 3 seconds for the rock to return to the water. The problem is situated within the context of uniformly accelerated motion, specifically involving kinematic equations.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the use of kinematic equations to relate displacement, time, and acceleration due to gravity. Some express uncertainty about how to approach the problem, while others attempt to clarify the variables involved, such as displacement and initial velocity.

Discussion Status

There is ongoing exploration of the problem, with some participants providing hints and guidance on relevant equations. However, there is no explicit consensus on the correct approach or final answer, as participants continue to question assumptions and share their attempts.

Contextual Notes

Some participants express frustration over the perceived lack of support from others, while others question the original poster's background in physics and the appropriateness of the problem being assigned. The discussion reflects varying levels of experience and understanding among participants.

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Homework Statement


A human throws a rock off of a bridge that is 30 meters above the water. It takes 3 seconds from the time the human throws the rock up in the air and falls all the way back down to the water. What is the initial velocity of the rock leaving the persons hand?

Homework Equations



i have know idea

The Attempt at a Solution



idk. my physics teacher gave me this problem today at school cause he believes me to be one of his better students. he told me to come home and attempt it because he found this problem in an old physics book today and could not figure it out. please help if possible
 
Last edited by a moderator:
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Here the displacement of the rock is 30 m.
The displacement and acceleration due to gravity are in the same direction but the velocity is in the opposite direction.
Use the kinematic equation h = -vo*t + 1/2*g*t^2 and find velocity.
 
Welcome to PF dantheman3199.

Please, in the future, make some attempt, no matter how small, at solving a problem before seeking help.

As you're aware, this is a problem in uniformly accelerated motion. That automatically implies several equations that could be relevant :smile:

p.s. Calculus was developed about 200 years before Einstein was born.
 
well i had no idea how to attempt it. i tried for like an hour on scratch paper then found this. so if you know how to do the problem then be my guest
 
hey rl.bhat if you could help me out with solving this it would be good. i don't have enough experience in physics to know what h would be?
 
dantheman3199 said:
hey rl.bhat if you could help me out with solving this it would be good. i don't have enough experience in physics to know what h would be?
rl.bhat is helping you, but he isn't going to do your homework for you.

What level of education are you at?
 
Im in twelvth grade in high school and i have only been taking physics for 3 weeks. and i guess i put this problem in the wrong folder cause its not homework. my teacher couldn't figure out how to do it so he asked me to try
 
dantheman3199 said:
Im in twelvth grade in high school and i have only been taking physics for 3 weeks. and i guess i put this problem in the wrong folder cause its not homework. my teacher couldn't figure out how to do it so he asked me to try
You're in twelfth grade and you have only just started studying Physics? So, you're 18 years old and have never studied Physics until three weeks ago?

And I'm sorry, but I don't believe that your 12th grade Physics teacher cannot solve this problem. Furthermore, even if your teacher couldn't solve this problem, I don't think that they would give it to one of their students to solve.
 
ok no << profane insult removed by berkeman >>im 17. and clearly it doesn't matter if you don't believe me. don't write **** down on my thread if your not going to help out.
 
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  • #10
dantheman3199 said:
and clearly it doesn't matter if you don't believe me. don't write **** down on my thread if your not going to help out.
Question: what made you choose this specific title for this thread?
 
  • #11
what?
 
  • #12
is the intial velocity on my problem 4.7 m/sec?
that is what i ended up getting
 
  • #13
dantheman3199 said:
what?
Why did you choose the title "Uniformaly accelerated motion"?
 
  • #14
dantheman3199 said:
ok no << profane insult removed by berkeman >>im 17. and clearly it doesn't matter if you don't believe me. don't write **** down on my thread if your not going to help out.

Insults are not allowed here.

We will not solve your schoolwork problem for you here. You must do it yourself. We can give you hints (as has already been done in this thread), but you must do your schoolwork for yourself.

As already mentioned, you should use the kinematic equations of motion for a constant acceleration field (like gravity). Here is some further reading that you can do to help you figure out how to solve this class of problems:

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L6a.html

.
 
Last edited by a moderator:
  • #15
ok thanks berkeman. i got 4.7m/sec as intial velocity. could you confirm that?
 
  • #16
dantheman3199 said:
ok thanks berkeman. i got 4.7m/sec as intial velocity. could you confirm that?

I'd be happy to. Could you please show the equation that you used, and show the numbers plugged in? Thanks.
 
  • #17
h = -vo*t + 1/2*g*t^2and i plugged 30 meters in for h. plugged 9.8m/sec^2 for g. and 3 seconds for time
 

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