Finding the impluse of a stopping car

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Homework Help Overview

The discussion revolves around calculating the impulse experienced by a car that comes to a stop after colliding with a tree. The car's mass is given as 1400 kg, and it was initially traveling at 60 km/h before the impact, which raises questions about the conversion of units and the definitions of impulse and momentum.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the calculation of change in momentum using the formula m(v-u) and question the sign of the impulse. There is also a focus on the conversion of speed from km/h to m/s and the distinction between impulse and force.

Discussion Status

Some participants have provided guidance on the correct interpretation of the velocity change and its implications for calculating impulse. There is an ongoing exploration of the definitions and relationships between impulse, momentum, and force, with no explicit consensus reached on the final interpretation.

Contextual Notes

There are mentions of potential confusion regarding the definitions of initial and final velocities, as well as the need for unit conversion, which may affect the calculations. The discussion reflects a mix of understanding and uncertainty about the concepts involved.

Ibby
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A car with a total mass of 1400kg,travling at 60kmh hits a large tree and stops in 0.080 s .



2. change in momentium: m(v-u)



3. 1400(0-60)

=-84000
 
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That should be correct, I am not sure if impulses are positive or negative though. Based on it being the change in momentum, it should be negative.

EDIT: You will need to convert 60 km/h to m/s or else your units will be off.
 
at the back of the book its 2.3 x 10 ^4 opposite tothe intial dircetion of the car
 
got it f= ma 1400 x 16.6ms= 2.3 x 10^4 its right but isn't thsi meant to be the force or net force not the impulse?
 
Ibby said:
got it f= ma 1400 x 16.6ms= 2.3 x 10^4 its right but isn't thsi meant to be the force or net force not the impulse?

That's not ma. That's m(vfinal - vinitial), which IS the impulse. The velocity change is 16.67 m/s. This is not an acceleration.
 
cepheid said:
That's not ma. That's m(vfinal - vinitial), which IS the impulse. The velocity change is 16.67 m/s. This is not an acceleration.

iam not getting it . how is the velocity chnage 16.67? isn't that its inital speed since the car stopped when it hit the tree .so its final velocity should be 0 and its inital 16.67 ?
 
Ibby said:
iam not getting it . how is the velocity chnage 16.67? isn't that its inital speed since the car stopped when it hit the tree .so its final velocity should be 0 and its inital 16.67 ?

The change in velocity is the difference between the final velocity and initial velocity. I hope you understand this.

change in velocity = 0 m/s - 16.667 m/s = -16.667 m/s

Yeah, sure, 16.667 m/s is the initial velocity. But since the final velocity is zero, the change in velocity will be equal in magnitude to the initial velocity (and opposite in direction). Since I was only referring casually to the magnitude of the change, I was a bit lax with the negative sign in my previous post (but so were you). Anyway, multiply this change in velocity by the mass of the car to get its change in momentum (which, by definition, is the impulse).

Nitpicking aside, the main point I was trying to make in post #5 (which I hope you understood) was that for some strange reason you were claiming that 16.667 m/s was an acceleration when, in fact, it is not. It doesn't even have the right units to be an acceleration.
 
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