Gravitational, kinetic, and elastic energy question

[combatwombat]

A large rock is tied to a rubber band and dropped straight down. As the rock falls, the rubber band gradually stretches, eventually bringing the rock to a stop. Which of the following energy transfers is taking place in this process?

A. Kinetic to gravitational potential to elastic potential
B. Kinetic to elastic potential to gravitational potential
C. Gravitational potential to elastic potential to kinetic
D. Gravitational potential to kinetic to elastic potential



Correct answer: D. Why? I don't see how kinetic energy should factor in at all - it seems to be that all the energy should start out as gravitational potential, and then slowly be transferred to elastic potential.

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[Marjan Islam]

Well, the rock is increasing in velocity as it is being dropped. If there was no rubber band, it would be gravitational potential to kinetic energy. But with the rubber band there, at the very END of the drop, at that exact instant, there is elastic potential energy. Right after that instant, the rock would shoot back up going from elastic to kinetic back to gravitational potential.

As a side not, even if you couldn't figure out the kinetic energy role, you could still pick C. since when the rock stops, you KNOW it's elastic potential at the end.

 

[LeftyUM]

The way I do these types of questions is that I look at the extremes first. At the start you KNOW its got to be gravitational potential, and at the end (that LAST instant) its elastic potential. Just knowing those, gets you to the right answer. I think Marjan nailed the explanation though.

 

[G1SG2]

Correct answer: D. Why? I don't see how kinetic energy should factor in at all - it seems to be that all the energy should start out as gravitational potential, and then slowly be transferred to elastic potential.

ANY moving object has kinetic energy. KE=1/2mv2. The rock moves as it drops down; therefore, it MUST have kinetic energy.

 

[physics junkie]

Kinetic energy is 1/2mv^2, as long as velocity is increasing the kinetic energy is being increased.
Gravitational potential energy is mgh, as long as height is decreasing gravitational energy is being decreased.
Elastic potential energy is (1/2)kx^2 where 'x' is the deviation from the expanded from full length. As soon as the rubber band starts to stretch you see kinetic energy start to decrease.

Technically gravitational energy is still being converted into elastic potential energy the entire time.

 

[naphthalene]

why is it not A? HOw do u know that gravitational comes before kinetic?

 

[whiteshadodw]

Well, the rock is increasing in velocity as it is being dropped. If there was no rubber band, it would be gravitational potential to kinetic energy. But with the rubber band there, at the very END of the drop, at that exact instant, there is elastic potential energy. Right after that instant, the rock would shoot back up going from elastic to kinetic back to gravitational potential.

As a side not, even if you couldn't figure out the kinetic energy role, you could still pick C. since when the rock stops, you KNOW it's elastic potential at the end.

if anything, the magnitude of velocity is decreasing as it gets converted into elastic potential energy.

OP, imagine if the rock dropped from some height onto the rubber band or w/e. at the top of the height/cliff its all gravitational potential then in midflight its all kinetic and at the bottom of the rubber band its all elastic.