Help with kinematics in one dimension free fall

In summary, the conversation discusses finding the time it takes for a wrecking ball to fall from rest to the ground after the cable breaks. The equation used is h = vo*t + 1/2*g*t^2, with the initial velocity (vo) being 0. The conversation also explores using the equation v=vo + at to find the final velocity and then using it in the formula y=1/2(v+v)t to calculate the final height. Ultimately, the conversation suggests using the equation 1/2 a*t^2 to simplify the process.
  • #1
xlaserx7
13
0

Homework Statement


A wrecking ball is hanging at rest from a crane when suddenly the cable breaks. The time it taktes for the ball to fall halfway to the ground is 1.2s. Find the time it takes for the ball to fall from rest all the way to the ground

Homework Equations


v=vo + at


The Attempt at a Solution


0 = 4.6 + (-9.80)t
t = - 5.2 s. time can't be negative so 5.2s
 
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  • #2
Use the equation h = vo*t + 1/2*g*t^2
Using vo = 0 and t = 1.2s find the half height and then full height. From that find the time to fall from rest to the ground.
 
  • #3
i don't understand why can't i use v=vo + at
and did i mess up on the time?
 
Last edited:
  • #4
0 times 1.2 + .5 (-9.80)(1.44) = -7.056 gives me the height. full height is 7.056 times 2 = 14.112
now i am stuck again >.<

wait... now i use v=vo + at to find the final so

0 + (-9.80)1.2 = 11.76 = final velocity

then i use y = 1/2 ( v + v )t

y = 1/2(0 + -11.76)1.2
y = -7.056 omg i got the same thing?
 
Last edited:
  • #5
Maybe look at it this way:

The distance dropped from rest is equal to 1/2 a*t2.

Exploiting this observe then that

[tex]\frac{H_1}{H_2} =\frac{1/2*g*t_1^2}{1/2*g*t_2^2}[/tex]

This simplifies to - plugging in that H1 = H , and H2 = 2H

[tex]\frac{H}{2H} = \frac{1.2^2}{t_2^2} = \frac{1}{2}[/tex]

[tex]t_2 = 1.2 * \sqrt{2}[/tex]

Evaluating this equation looks remarkably easier to me.
 
  • #6
ahh thank you
 
  • #7
xlaserx7 said:
ahh thank you

So long as you learn the technique, you're welcome.

Good luck.
 

What is kinematics in one dimension free fall?

Kinematics in one dimension free fall is a branch of physics that deals with the motion of objects in a single direction under the influence of gravity. It involves studying the position, velocity, and acceleration of an object as it falls freely towards the ground.

What are the key equations used in kinematics in one dimension free fall?

The key equations used in kinematics in one dimension free fall are the equations of motion:

  • Position (x) = Initial position (x0) + Initial velocity (v0)t + 1/2 acceleration (a)t^2
  • Final velocity (v) = Initial velocity (v0) + acceleration (a)t
  • Final position (x) = Initial position (x0) + Average velocity (vavg)t
  • Average velocity (vavg) = (v0 + v)/2

How do you calculate the acceleration of an object in free fall?

The acceleration of an object in free fall is always equal to the acceleration due to gravity, which is approximately 9.8 m/s^2. It can be calculated using the equation:

  • Acceleration (a) = Change in velocity (v)/Change in time (t)

What is the difference between velocity and acceleration in kinematics in one dimension free fall?

Velocity is a measure of an object's speed and direction of motion, while acceleration is a measure of how an object's velocity changes over time. In kinematics in one dimension free fall, an object's velocity increases or decreases due to the constant acceleration of gravity, while its acceleration remains constant.

How do you solve problems involving kinematics in one dimension free fall?

To solve problems involving kinematics in one dimension free fall, you can follow these steps:

  1. Identify the known and unknown variables.
  2. Choose the appropriate equation(s) to solve for the unknown variable.
  3. Plug in the known values and solve the equation.
  4. Check your answer and make sure it makes sense.

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