How far does the car travel while slowing down?

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Homework Help Overview

The discussion revolves around a physics problem involving a car decelerating from an initial speed to a final speed while experiencing a net force. Participants are exploring how to set up the problem and calculate the distance traveled during deceleration using various approaches, including kinematics and energy principles.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss using Newton's laws to find acceleration and apply kinematic equations to determine distance. Others suggest using the work-energy theorem, questioning how to incorporate kinetic and potential energy in the context of the problem.

Discussion Status

There is an ongoing exploration of different methods to approach the problem. Some participants have provided guidance on using both energy methods and kinematic equations, while others are clarifying the signs of the velocities and forces involved. The discussion reflects a mix of interpretations and attempts to reconcile different approaches.

Contextual Notes

Participants are working within the constraints of a homework assignment, which may impose specific rules regarding the use of significant figures and the treatment of forces. There is some uncertainty regarding the correct application of formulas and the interpretation of initial and final velocities.

StephenDoty
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A 1000.0 kg car experiences a net force of 9500N while decelerating from 30.0m/s to 23.4m/s. How far does the car travel while slowing down?

I do not even know how to start to setup the problem. I know that W=Fd but I do not know how to apply this with the velocities.

Any help would be appreciated. Thank you.

Stephen
 
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You can calculate the car's acceleration from the given data using Newton's laws, then use the standard kinematic equations of motion to calculate the distance.
 
since we are on the energy and momentum chapter is there any way to do it with KE and PE?
 
StephenDoty said:
since we are on the energy and momentum chapter is there any way to do it with KE and PE?
Oh, sure. Use the work energy theorem
[tex]W_{total} = W_{net} = \Delta KE[/tex]. In general , you've got to be a bit careful when using this equation, because W_net includes work done by both conservative (like gravity) and non-conservative (friction, etc.) forces, but it this case, those forces are given as one net number, so you don't have to worry about PE change.
 
Last edited:
so...
Fd=1/2mvf^2 - 1/2mvi^2

?
 
StephenDoty said:
so...
Fd=1/2mvf^2 - 1/2mvi^2

?
yes, where the value you use for F is the given F_net. Check it out both ways using the energy method vs. Newton 2 and the kinematic equations.
 
Would vi be 23.4m/s
and vf= 30m/s
?

If it is the other way around then F*D= a negative number

any help would be appreciated.
 
vf is 23.4m/s since you're slowing down, right? so the change in kinetic you get should be a negative number, and keep in mind that the force of friction is always negative, so the work done is negative too.
 
so vi= 30.0m/s
vf=23.4m/s

and the force of 9500N is negative?

-9500N*d=(1/2)(1000kg)(23.4m/s)^2 - (1/2)(1000kg)(30m/s)^2

?
 
  • #10
yep.
 
  • #11
d= 18.5m or 19m with sig figs.?
 
  • #12
A general rule of thumb is to use the least amount of sig figs as the values given to you in the question.
 
  • #13
which would be 2 from 9500N
 
  • #14
do i have the right number of sig figs
 

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