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Free Fall Problem

  • #1
226
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Homework Statement


Calculate the length of time it would take for an object to fall 10.0m if g were one sixth the value of earth's g.

g=1/6(-9.8)= -1.633333333 m/s^2

a=-1.633333333m/s^2
displacement=10.0m[down]

Homework Equations


Maybe one of the big 5 kinematics equations? Or maybe a=change in velocity/time.

The Attempt at a Solution


I didn't have enough variables to use any of the listed equations. This is because the question does not give any velocity values and I don't know how to solve for them. How would you do this problem. Note: According to my book, the correct answer is 3.5s.
 

Answers and Replies

  • #2
ehild
Homework Helper
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1,876

Homework Statement


Calculate the length of time it would take for an object to fall 10.0m if g were one sixth the value of earth's g.

g=1/6(-9.8)= -1.633333333 m/s^2

a=-1.633333333m/s^2
displacement=10.0m[down]

Homework Equations


Maybe one of the big 5 kinematics equations? Or maybe a=change in velocity/time.

The Attempt at a Solution


I didn't have enough variables to use any of the listed equations. This is because the question does not give any velocity values and I don't know how to solve for them. How would you do this problem. Note: According to my book, the correct answer is 3.5s.
The object falls - it means its initial velocity is zero. What relation exist between constant acceleration, time, and displacement?
 
  • #3
226
8
The object falls - it means its initial velocity is zero. What relation exist between constant acceleration, time, and displacement?
I'm not sure. what do you mean?
 
  • #4
SteamKing
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I'm not sure. what do you mean?
Knowing that the initial velocity is zero, which one of the five SUVAT equations relates constant acceleration, time, distance, and initial velocity?

Scan the list of these equations and use a process of elimination to narrow your choices to the correct equation.
 
  • #5
226
8
Knowing that the initial velocity is zero, which one of the five SUVAT equations relates constant acceleration, time, distance, and initial velocity?

Scan the list of these equations and use a process of elimination to narrow your choices to the correct equation.

Don't objects have a non-zero initial velocity when dropped?
 
  • #6
ehild
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Don't objects have a non-zero initial velocity when dropped?
The problem meant that the object was hold and then released and let to fall. What is the initial velocity?
 
Last edited:
  • #7
226
8
The problem meant that the object was hold and then released and let to fall. What is the initial velocity?
I thought that the initial velocity meant the velocity right after the motion began, just like final velocity is the velocity right before motion ends, not the velocity when the motion is over and the object is at rest.
 
  • #8
CWatters
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The "final Velocity" isn't always the Velocity just before the object stops moving. It depends on the problem. For example there is a common problem that involves an object falling past a window - after passing the window it keeps falling. When ,you break the problem down the final Velocity you plug into one of the suvat equations is typically the Velocity as it passes the bottom of the window.

The initial velocity when an object is dropped is usually zero but there are situations where it isn't. For example when it's dropped from a helicopter that is ascending or descending.
 
  • #9
226
8
The "final Velocity" isn't always the Velocity just before the object stops moving. It depends on the problem. For example there is a common problem that involves an object falling past a window - after passing the window it keeps falling. When ,you break the problem down the final Velocity you plug into one of the suvat equations is typically the Velocity as it passes the bottom of the window.

The initial velocity when an object is dropped is usually zero but there are situations where it isn't. For example when it's dropped from a helicopter that is ascending or descending.
Why is the final velocity in the window problem the velocity as it passes the bottom of the window?
 
  • #10
ehild
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I thought that the initial velocity meant the velocity right after the motion began,
What is the velocity of the object right after the accelerating motion began if it was in rest before? You keep a pebble in your hand. What is the velocity of the pebble? Then you open your hand and let the pebble fall out. What is the velocity of the pebble at the instant when you release it?
"Initial" and "final" are defined by the problem. If you observe a pot falling in front of a window, you observe it from the room during the time it moves from the top of the window down the bottom.
 
  • #11
CWatters
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Why is the final velocity in the window problem the velocity as it passes the bottom of the window?
Why not? It depends on the actual version of the question but the most common version of the problem requires you to solve it in two phases, in the first phase the final velocity is the top of the window. That value is then used as the initial velocity for the second phase of the solution. In the second phase it seems reasonable to label the velocity at the bottom of the window the final velocity although you can solve the problem without calculating it.
 

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