Need help with Basic Kinematics question

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In summary, the question is asking for the force of a stainless steel coil, weighing 497.3 slugs, when it falls for 3/4 seconds after the crane's cable snaps. The equation F=ma is used to calculate the force, and the converted value is 16013.06 lbf. However, the force of collision depends on the type of collision and time of collision, and the given information is not enough to accurately calculate it. The calculated final velocity is 24.15 ft/s, and the momentum can be calculated by the equation p=mv.
  • #1
gretchenm
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Homework Statement



Here is my question: A crane is moving a stainless steel coil weighing 497.3 slugs. The crane's cable snaps and the coil falls for 3/4 seconds before hitting the ground. What is the FORCE the coil hits the ground with?



Homework Equations



F=ma
PE=KE??


The Attempt at a Solution



I have already converted slugs to lbf...
F = ma = 497.3 slugs= 497.3 lbf s^2/f * 32.2 f/s^2= 16013.06 lbf

Now I am feeling pretty stupid trying to figure out how to calculate the force that it hits the ground with. I am sure I have to use the .75 seconds somewhere, but for some reason can't figure it. Any help? Thanks! :)
 
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  • #2
The force it hits the ground at depends on how easily the object deforms. Are you sure the question wasn't asking for the momentum with which it hits the ground?

You've already written down the force on the object due to gravity, and you have the time it falls for. Now it is straightforward to calculate the accumulated momentum of the object.
 
  • #3
gretchenm said:

Homework Statement



Here is my question: A crane is moving a stainless steel coil weighing 497.3 slugs. The crane's cable snaps and the coil falls for 3/4 seconds before hitting the ground. What is the FORCE the coil hits the ground with?



Homework Equations



F=ma
PE=KE??


The Attempt at a Solution



I have already converted slugs to lbf...
F = ma = 497.3 slugs= 497.3 lbf s^2/f * 32.2 f/s^2= 16013.06 lbf

Now I am feeling pretty stupid trying to figure out how to calculate the force that it hits the ground with. I am sure I have to use the .75 seconds somewhere, but for some reason can't figure it. Any help? Thanks! :)

Well i guess you must use, F = dp/dt

but dt here won't be 3/4. 3/4 is time of falling from which you will calculate final velocity ...
 
  • #4
Hmm, I agree with your statement about the force depending on how easily the object deforms. Do we have to use the elasticity modulus of stainless steel in there somewhere, maybe? I think it is 180 x 10^9 N/m^2 if I looked it up correctly.
 
  • #5
So then final velocity would be...
v(f)=0+32.2 (.75)=24.15 ft/s...right? Then where can we go with this?
 
  • #6
gretchenm said:
So then final velocity would be...
v(f)=0+32.2 (.75)=24.15 ft/s...right? Then where can we go with this?

As BruceW said, the force of collision depends of type of collision (elastic or inelastic) and time of collision.
 
  • #7
So I think what I am getting from the answers that have been posted, is that this is a question that does not give enough information to correctly answer...how would I go about calculating the momentum then? Would it just be 16013.06 lbf times the final velocity of 24.15 ft/s? (386715.399 lbf ft/s*1.355817948=524315 J/s) I just used a conversion factor I looked up to get SI units. Anyone agree with this calculation? (or not..)
 
  • #8
gretchenm said:
So I think what I am getting from the answers that have been posted, is that this is a question that does not give enough information to correctly answer...how would I go about calculating the momentum then? Would it just be 16013.06 lbf times the final velocity of 24.15 ft/s? (386715.399 lbf ft/s*1.355817948=524315 J/s) I just used a conversion factor I looked up to get SI units. Anyone agree with this calculation? (or not..)

I did not see the values but method is fine. p=mv !
And yes the info. given is not enough. But maybe the questions has some assumptions that author thought will be obvious to a reader, perhaps a note before starting a series of questions.
 
  • #9
Yeah, I think I am just going to stick with calculating momentum and be done with this! Thanks everyone for all of the input! :)
 

What is kinematics?

Kinematics is the branch of physics that studies the motion of objects without considering the forces that cause the motion.

What is a basic kinematics question?

A basic kinematics question typically involves finding the displacement, velocity, or acceleration of an object given certain information about its motion.

What are the three basic kinematic equations?

The three basic kinematic equations are:

  • Displacement: x = x0 + v0t + 1/2at2
  • Velocity: v = v0 + at
  • Acceleration: v2 = v02 + 2a(x-x0)

How do you solve a basic kinematics question?

To solve a basic kinematics question, you will need to identify the given information, determine which kinematic equation(s) to use, and plug in the values to solve for the unknown variable.

What are some common mistakes to avoid when solving kinematics problems?

Common mistakes to avoid when solving kinematics problems include not using the correct units, not considering the direction of motion, and not carefully reading and interpreting the given information.

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