Spring Pushing an Object

  • Thread starter Zero
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  • #1
Zero
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Homework Statement:
Problem: A spring is compressed 50 cm from the equilibrium position. The spring constant is 12 N/cm. A 20 kg object is placed next to the spring initially at rest and it is released. After the object is released from the spring, the force of friction acting on the object is 30 N.

A) Calculate the force required to compress the spring by 50 cm. (Already got the answer which is 600N)

B) Calculate the energy that was stored in the spring. (Got this answer also which is 150 J)

C. Calculate the object's velocity 4m after it has left the spring. (This is where I stopped don't know what to do here)

What I tried: Unfortunately I don't know where to begin. I know what variables are which are work, spring constant, friction and potential energy in the spring which is the same as the work.

As for number 3, I'm totally lost but I do have a theory that I don't know if it would be true for this specific equation.

Theory: Would I have to find kinetic energy first.

The reason that I have such a theory is because of question D which I did not add since I did not attempt it as of yet but as of posting this, I will be attempting.
Relevant Equations:
F=Kx and W=1/2FX
Et1+w=Et2
1/2mv+w=1/2mv
0+150=1/2 (20)
300=(20)v
15=v
I know this answer is wrong.
 

Answers and Replies

  • #2
HallsofIvy
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When the object leaves the spring, the energy that was stored in the spring is converted into the kinetic energy of the object. So you can calculate the speed of the object at the time when it first leaves the spring. You are told that "After the object is released from the spring, the force of friction acting on the object is 30 N." Calculate the deceleration from that.
 
  • #3
Zero
17
4
I did what would you said and got -1.5003 m/s^2 but when I use the formula final velocity squared=initial volocity+two (acceleration)(distance) I couldn't do it because I can't square root a negative number. Is the formula used wrong or am I on the right track but I just forgot a very key detail. Also, I did get the kinetic energy of the object at 4m which is 30J
 
  • #4
berkeman
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I did what would you said and got -1.5003 m/s^2 but when I use the formula final velocity squared=initial volocity+two (acceleration)(distance) I couldn't do it because I can't square root a negative number. Is the formula used wrong or am I on the right track but I just forgot a very key detail. Also, I did get the kinetic energy of the object at 4m which is 30J
Can you show your equation? Just because the velocity is decreasing and doesn't reach zero, doesn't mean that you need to take the square root of a negative number...
 
  • #5
haruspex
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final velocity squared=initial volocity+two (acceleration)(distance)
That should be
final velocity squared=initial velocity squared + two (acceleration)(distance)
What did you get for initial velocity?
I did get the kinetic energy of the object at 4m which is 30J
So deduce the velocity from that.
 
  • #6
Zero
17
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That should be
final velocity squared=initial velocity squared + two (acceleration)(distance)
What did you get for initial velocity?

So deduce the velocity from that.
isn't initial velocity 0 so it wouldn't matter. Yes i know the kinetic energy is 30j and the velocity is 1.73 m/s but i just don't know how to get the answer for velocity unless you did a different formula than Ek=1/2mv^2 to find kinetic energy then that is possible to deduce that way
 
  • #7
haruspex
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isn't initial velocity 0 so it wouldn't matter.
The acceleration in the equation you are using results from friction, and that does not start until the object leaves the spring. So for this equation the initial velocity is the velocity as it leaves the spring.
 
  • #8
Zero
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The acceleration in the equation you are using results from friction, and that does not start until the object leaves the spring. So for this equation the initial velocity is the velocity as it leaves the spring.
Got the answer thank you all for the effort
 
  • #9
Zero
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Can you show your equation? Just because the velocity is decreasing and doesn't reach zero, doesn't mean that you need to take the square root of a negative number...
Got the answer thank you all for the effort
 

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