What is the initial speed of the rock when thrown off a cliff?

In summary, the kid dropped a rock off the edge of a cliff and it took 4.0s to reach the ground below. When he threw the rock down, it struck the ground in 3s. Using the equations ΔX=ViTf + 1/2ATf^2 and H=1/2gt^2, the initial speed of the rock can be calculated to be 4.9m/s. The initial velocity for the two scenarios is not the same as it takes a different amount of time for the rock to hit the ground.
  • #1
Let It Be
45
0
1. When a kid drops a rock off the edge of a cliff, it take s 4.0s to reach the ground below. When he throws the rock down, it strikes the ground in 3s. What inital speed did he give the rock?


ΔX=ViTf + 1/2ATf^2
H=1/2gt^2
Vf^2=Vi^2+ 2AΔX

3. Hi everyone! :smile: I know that when the kid drops the rock, you can use the free fall equation- H=1/2gt^2; but the problem doesn't say the height of the cliff and I don't know "g", unless it's just 9.8? :confused: Also, I don't know inital velocity, so I just don't know how to go about this! If anyone can help me, that'd be much appreciated!

 
Physics news on Phys.org
  • #2
Assuming the cliff is on the earth, then yes, g = 9.8 m/s2.


When it says the kid drops the rock, you can assume that the initial velocity is zero.
 
  • #3
Hmm...why would there be two different scenarios though? One with the kid dropping the rock, and the other him throwing it? And I don't understand what the question is asking with "intial speed", is that just inital velocity? Then wouldn't they both be zero?
 
  • #4
Let It Be said:
Hmm...why would there be two different scenarios though? One with the kid dropping the rock, and the other him throwing it?
Because that is part of the problem that is being posed.
And I don't understand what the question is asking with "intial speed", is that just inital velocity? Then wouldn't they both be zero?
When you throw something it's initial speed isn't zero. It's whatever speed it leaves your hand.

Also, velocity is just a fancy speed: it has magnitude and direction :smile: In this case you can take the initial speed to be directed downwards and call it the initial velocity if you like.

I suggest that you use the first scenario (rock is dropped) to find the height of the cliff. Then use the first of your equations to wrangle with the second scenario.
 
  • #5
Good advice!

Does this look right?

H=1/2gt^2
H=1/2(9.8)4s
H=19.6m

ΔX=ViTf + 1/2 ATf^2
19.6=Vi (4) + 1/2(o)Tf^2
Vi=4.9m/s

Thanks:smile:
 
  • #6
Let It Be said:
Good advice!

Does this look right?

H=1/2gt^2
H=1/2(9.8)4s
H=19.6m
Whoops. You forgot to square the time; that's a t2 up there.
 
  • #7
gneill said:
Whoops. You forgot to square the time; that's a t2 up there.

Ahh, I gotcha. Were those the right steps to solve it though?
 
  • #8
Wait...does the 3s from when he throws the rock ever come into play?
 
  • #9
Let It Be said:
Wait...does the 3s from when he throws the rock ever come into play?

Of course. It takes 3s to hit bottom when he throws it rather than drops it. That's the effect of the initial velocity.
 
  • #10
gneill said:
Of course. It takes 3s to hit bottom when he throws it rather than drops it. That's the effect of the initial velocity.

So in the ΔX=ViTf+1/2ATf2 the Tf would be 3s not 4s?
 
  • #11
Let It Be said:
So in the ΔX=ViTf+1/2ATf2 the Tf would be 3s not 4s?

Do you think it would take the same time for both scenarios?
 
  • #12
gneill said:
Do you think it would take the same time for both scenarios?

No..so do this problem twice to find his inital speed for both scenarios, using 4s for the first and 3s for the second?
 
  • #13
Let It Be said:
No..so do this problem twice to find his inital speed for both scenarios, using 4s for the first and 3s for the second?

When you DROP an object its initial speed is zero. The initial scenario allows you to calculate the height of the cliff. Use this height in the second scenario.
 

1. What is a one dimensional question?

A one dimensional question is a type of question that only has one possible answer. It usually involves a specific topic or concept and does not allow for any variation or complexity in the answer.

2. How is a one dimensional question different from other types of questions?

A one dimensional question is different from other types of questions because it only has one correct answer, whereas other types of questions may have multiple correct or acceptable answers.

3. What are some examples of one dimensional questions?

Examples of one dimensional questions include "What is the capital of France?", "Who wrote the novel Pride and Prejudice?", and "What is the chemical formula for water?"

4. When should I use a one dimensional question?

A one dimensional question is most appropriate when you want to test someone's knowledge or understanding of a specific topic or concept. It can also be useful for gathering basic information or facts.

5. What are the limitations of using one dimensional questions?

The main limitation of using one dimensional questions is that they do not allow for complex or critical thinking. They also do not provide much opportunity for the respondent to express their own ideas or opinions.

Similar threads

  • Introductory Physics Homework Help
Replies
2
Views
1K
  • Introductory Physics Homework Help
Replies
1
Views
712
  • Introductory Physics Homework Help
Replies
9
Views
2K
  • Introductory Physics Homework Help
Replies
3
Views
1K
  • Introductory Physics Homework Help
Replies
15
Views
1K
  • Introductory Physics Homework Help
Replies
4
Views
1K
  • Introductory Physics Homework Help
Replies
2
Views
5K
  • Introductory Physics Homework Help
Replies
1
Views
2K
  • Introductory Physics Homework Help
Replies
4
Views
2K
  • Introductory Physics Homework Help
Replies
6
Views
4K
Back
Top