Inelastic Collision - Kinetic Energy

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Homework Help Overview

The discussion revolves around an inelastic collision involving a 6.0 kg metal ball and a 6.0 kg clay ball. The metal ball is initially moving at 4.0 m/s and collides with the clay ball, which is at rest. After the collision, both objects move together at 2.0 m/s. Participants are exploring the kinetic energy before and after the collision, as well as the energy lost by the metal ball and gained by the clay ball.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants discuss the calculation of kinetic energy using the formula KE = 1/2mv^2 for both the metal and clay balls before and after the collision. Questions arise regarding how to handle the energy calculations for separate objects post-collision and the implications of negative values in energy change calculations.

Discussion Status

Some participants have provided calculations for the kinetic energy of the metal ball before the collision and are attempting to apply the same principles to the clay ball. There is ongoing exploration of how to express energy loss and gain, with some participants questioning the interpretation of negative values in kinetic energy calculations.

Contextual Notes

Participants are navigating the complexities of inelastic collisions, particularly how to calculate kinetic energy for objects that stick together post-collision. There is a focus on understanding the implications of energy changes and the definitions of kinetic energy in this context.

JayaSuria
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A 6.0kg metal ball moving at 4.0m/s hits a 6.0kg clay ball at rest and sticks to it. The two move at 2.0m/s.

a. calculate the kinetic energy of the metal ball before it hits the clay ball.
b. calculate the kinetic energy of the metal ball after it hits the clay ball.
c. calculate the kinetic energy of the clay ball after being hit.
d. calculate the energy that the metal ball lose in the collision.
e. calculate the kinetic energy that the clay ball gain in the collision.

I have answered (a), how to do (b) - (e)?? Plz help
 
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How did you solve (a)?
 
@Doc Al == KE = 1/2mv^2.
(a) KE = 1/2 * 6 * 4^2 , equals to 48J
 
JayaSuria said:
@Doc Al == KE = 1/2mv^2.
(a) KE = 1/2 * 6 * 4^2 , equals to 48J
Good. Now use that same formula to do the rest.
 
Doc Al said:
Good. Now use that same formula to do the rest.

@Doc Al , that's the hiccup. For inelastic collision, after the hit, how to calculate for separate objects as before collision? In that case, is the value in (b) same as (c)?
 
JayaSuria said:
@Doc Al , that's the hiccup. For inelastic collision, after the hit, how to calculate for separate objects as before collision? In that case, is the value in (b) same as (c)?
The mass of each is the same but the velocity changes. You are given the velocity before and after the collision, so just plug away into the formula to compute the KE of each before and after the collision. Then you can figure out how the energy changes.
 
Doc Al said:
The mass of each is the same but the velocity changes. You are given the velocity before and after the collision, so just plug away into the formula to compute the KE of each before and after the collision. Then you can figure out how the energy changes.

@Doc Al owh thanks doc. Is this the solution?
(b) 1/2 * 6 * 2^2
(c) 1/2 * 6 * 2^2
(d) initial 48J, after 12J. Metal ball lost 36J
(e) initial 0J, after 12J. Clay ball gained 12J
 
JayaSuria said:
@Doc Al owh thanks doc. Is this the solution?
(b) 1/2 * 6 * 2^2
(c) 1/2 * 6 * 2^2
(d) initial 48J, after 12J. Metal ball lost 36J
(e) initial 0J, after 12J. Clay ball gained 12J
Looks good to me.
 
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Hi Doc, can u show me the calculation of kinetic energy loss by the metal ball?
Is there any calculation to this? instead by stating it only? for example KEloss=K1 - K2?
where k1 = intial KE of Metall Ball 48 J and K2 after collision 12 J, KEloss= 48-12 = 36 J
but if we do the same for clay ball where we calculate the KE gained by Clay ball using this formula
Kc1 = Kc1 intial KE which is Zero since the clay ball remain stationary, Kc2 = 12 J since it move together with the metal ball at 2.0 m/s
KEgain = Kc1 - Kc2
= 0 - 12 J
= - 12 J , As far as i know, Joule can never be negative, if we change the equation to Kc2-Kc1 we will get the right answer for the clay ball but for the metal ball it would become -36 J (which still negative numbers,except if I can explain that the negative symbol represent the loss of KE by metal ball, is it accepted that way? or KE can never be a negative numbers?)
 
  • #10
Doc Al said:
Looks good to me.
Hi Doc, can u show me the calculation of kinetic energy loss by the metal ball?
Is there any calculation to this? instead by stating it only? for example KEloss=K1 - K2?
where k1 = intial KE of Metall Ball 48 J and K2 after collision 12 J, KEloss= 48-12 = 36 J
but if we do the same for clay ball where we calculate the KE gained by Clay ball using this formula
Kc1 = Kc1 intial KE which is Zero since the clay ball remain stationary, Kc2 = 12 J since it move together with the metal ball at 2.0 m/s
KEgain = Kc1 - Kc2
= 0 - 12 J
= - 12 J , As far as i know, Joule can never be negative, if we change the equation to Kc2-Kc1 we will get the right answer for the clay ball but for the metal ball it would become -36 J (which still negative numbers,except if I can explain that the negative symbol represent the loss of KE by metal ball, is it accepted that way? or KE can never be a negative numbers?)
 
  • #11
I get it now, KEloss can be represent as KEloss=K1-K2 since we want to look for loss of energy therefore minus the intial, whereas KEgain = Ki + kf = 0 + 12
since we wanted to look for gain in kenetic energy therefore we add them together. Sorry pls ignore what i ask in the above
 
  • #12
hanz said:
I get it now, KEloss can be represent as KEloss=K1-K2 since we want to look for loss of energy therefore minus the intial, whereas KEgain = Ki + kf = 0 + 12
since we wanted to look for gain in kenetic energy therefore we add them together. Sorry pls ignore what i ask in the above
Surely, KE can never be negative. But the change in KE, which is always KEf - KEi, can be negative or positive depending on whether energy is lost or gained. (KEf + KEi has no meaning.)
 
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  • #13
Doc Al said:
Surely, KE can never be negative. But the change in KE, which is always KEf - KEi, can be negative or positive depending on whether energy is lost or gained. (KEf + KEi has no meaning.)
means it would be ok if metal ball KEloss=KEf-KEi= 12-48= -36 J? then just explain why it is negative?
 
  • #14
hanz said:
means it would be ok if metal ball KEloss=KEf-KEi= 12-48= -36 J?
The change in KE = - 36 J.

hanz said:
then just explain why it is negative?
A negative change means that the KE decreased. You can say that the metal ball lost 36 J of KE.
 
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