One-Dimensional Motion with Constant Acceleration

  • #1

Homework Statement


A rock is released from rest from the top of a very high cliff, and accelerates downward at g.
Approximately how far does the rock travel in the first 7 seconds of its free-fall? (Assume no air friction.)

Homework Equations


X=Xo+VoT+1/2AT^2

The Attempt at a Solution


I understand how to work out the problem, but I do not know why it works. You simply substitute the numbers in to get the distance. But for some reason, you are not given (nor do you use) initial position like the equation calls for. Can someone explain why it's simply thrown away? The initial position can't be 0, as it's being dropped from a mountain.[/B]
 

Answers and Replies

  • #2
Dick
Science Advisor
Homework Helper
26,260
619

Homework Statement


A rock is released from rest from the top of a very high cliff, and accelerates downward at g.
Approximately how far does the rock travel in the first 7 seconds of its free-fall? (Assume no air friction.)

Homework Equations


X=Xo+VoT+1/2AT^2

The Attempt at a Solution


I understand how to work out the problem, but I do not know why it works. You simply substitute the numbers in to get the distance. But for some reason, you are not given (nor do you use) initial position like the equation calls for. Can someone explain why it's simply thrown away? The initial position can't be 0, as it's being dropped from a mountain.[/B]
The question is asking how far the rock travels. That's the difference between the final position and the initial position. ##x_0## cancels when you take the difference.
 
  • #3
HallsofIvy
Science Advisor
Homework Helper
41,833
956
It sounds like you are memorizing formulas without really understanding them. That's a bad idea. You are free to take the "0" point any where you want. I suggest taking x= 0 at the top of the cliff. Further, the fact that rock is "released from rest" tells you that the initial velocity is 0. Finally, you can take the acceleration due to gravity to be either negative or positive- as long as you interpret the results in the same way- you are free to choose "positive" or "negative" either up or down.
 
  • Like
Likes Chestermiller
  • #4
311
23
You can ignore the mountain altogether : you have a ( constant ) acceleration rate, a starting velocity (u = 0) and a time ( 7 seconds ), so use newtons rules of motion.
 

Related Threads on One-Dimensional Motion with Constant Acceleration

Replies
1
Views
3K
Replies
2
Views
25K
  • Last Post
Replies
1
Views
984
  • Last Post
Replies
2
Views
870
  • Last Post
Replies
14
Views
2K
  • Last Post
Replies
2
Views
804
  • Last Post
Replies
3
Views
4K
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
4
Views
2K
Top