Graphing Kinetic Energy of a Toy Car Released from a Compressed Spring

[sp3sp2sp]

Homework Statement

A toy car is held at rest against a compressed spring, as shown in the figure. When released, the car slides across the room. Let x=0 be the initial position of the car. Assume that friction is negligible.
Sketch a graph of the car's kinetic energy from the moment it is released until it passes the equilibrium position of the spring. Your graph should be consistent with the given plots of total energy (gray line in graphing window) and potential energy (gray parabola in graphing window).

Part A and B asked for total Energy and gravitational PE. This is shown in attached file. The problem is with plotting KE. I can't tell if I got it wrong because my graph is sloppy or if I actually have the shape completely wrong. The line that is marked wrong is the red line, which is supposed to be KE
thanks for any help

Homework Equations

As PE decreases KE will increase, so that total E always constant. This is what I thought I plotted (?) thanks

The Attempt at a Solution

 

[phinds]

Homework Statement

A toy car is held at rest against a compressed spring, as shown in the figure. When released, the car slides across the room. Let x=0 be the initial position of the car. Assume that friction is negligible.[/ATTACH]

As I read this problem statement, you have a flat floor and no friction. That being the case, why would the car ever slow down? Is that supposed to be due to wind resistance?

 

[sp3sp2sp]

Just to clarify because it might be misunderstood.

Part A:Sketch a graph of the total energy of the spring and car system. There is no scale given, so your graph should simply reflect the qualitative shape of the energy vs. time plot.

Part B: Sketch a plot of the elastic potential energy of the spring from the point at which the car is released to the equilibrium position of the spring. Make your graph consistent with the given plot of total energy (the gray line given in the graphing window).

 

[phinds]

You have not answered my question

 

[sp3sp2sp]

thank you for the reply. So because this is just from time car is released to when spring reaches its eq-position that means that car has acceleration and KE will increase but then be constant at springs eq-position (black line)?

 

[phinds]

thank you for the reply. So because this is just from time car is released to when spring reaches its eq-position that means that car has acceleration and KE will increase but then be constant at springs eq-position (black line)?

If I understand you correctly, then yes, that sounds correct.

 

[sp3sp2sp]

No the car would not slow down. Problem says friction negligable

 

[phinds]

No the car would not slow down. Problem says friction negligable

right. I misunderstood you. I think posts 5 and 6 clarify everything.

 

[sp3sp2sp]

OK thanks but if you were to extend the graph (--> pos x) then would the KE simply = the total energy of the system?

 

[phinds]

OK thanks but if you were to extend the graph (--> pos x) then would the KE simply = the total energy of the system?

Well, yeah, I suppose once the spring has done its thing, the only energy left in the system is the KE of the car. The energy of the system starts out as the energy of the coiled spring and that gets converted into the KE of the car.

 

[sp3sp2sp]

I used all my answers up and they were all wrong. Then I clicked on show answer and it showed the graph that I originally got marked as wrong.

 

[haruspex]

I used all my answers up and they were all wrong. Then I clicked on show answer and it showed the graph that I originally got marked as wrong.

No, there's a difference.
In the PE graph, there are three kinks, so four straight sections.
In your failed attempt, the KE graph only has two kinks, so it is not an exact inversion of the PE graph.
The displayed correct version has the full three kinks.

 

[skylersmith9]

I got this for an answer for the kinetic energy problem (last one). This is the correct answer

 

[skylersmith9]

I go this though for the B part of this problem which was conflicting with part C. This is the correct answer