Register to reply 
One Dimensional Question 
Share this thread: 
#1
Dec2911, 08:58 PM

P: 45

[b]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 [b]3. Hi everyone! 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? 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! 1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution 1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution 


#2
Dec2911, 09:04 PM

Emeritus
Sci Advisor
HW Helper
PF Gold
P: 7,801

Assuming the cliff is on the earth, then yes, g = 9.8 m/s^{2}.
When it says the kid drops the rock, you can assume that the initial velocity is zero. 


#3
Dec2911, 09:23 PM

P: 45

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
Dec2911, 09:43 PM

Mentor
P: 11,626

One Dimensional Question
Also, velocity is just a fancy speed: it has magnitude and direction 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
Dec2911, 10:40 PM

P: 45

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 


#6
Dec3011, 08:39 AM

Mentor
P: 11,626




#7
Dec3011, 08:56 PM

P: 45




#8
Dec3011, 08:59 PM

P: 45

Wait...does the 3s from when he throws the rock ever come into play?



#9
Dec3011, 09:06 PM

Mentor
P: 11,626




#10
Dec3011, 09:10 PM

P: 45




#11
Dec3011, 09:28 PM

Mentor
P: 11,626




#12
Dec3011, 09:30 PM

P: 45




#13
Dec3011, 09:37 PM

Mentor
P: 11,626




Register to reply 
Related Discussions  
2 Dimensional Motion Question  Introductory Physics Homework  1  
Two dimensional vectors question  Introductory Physics Homework  2  
Question on ndimensional space  Calculus & Beyond Homework  2  
Dimensional analysis question  Introductory Physics Homework  1  
Dimensional analysis question  Introductory Physics Homework  6 