Work-Kinetic Energy Homework: Solving for Force, Work & Kinetic Energy

[BuBbLeS01]

Homework Statement

With brakes fully applied, a 1540 kg car decelerates from a speed of 94.0 km/hr. What is the work done by the braking force in bringing the car to a stop? What is the change in the kinetic energy of the car?

Homework Equations

The Attempt at a Solution

W = -Fd
Not sure how to start this problem?

 

[paradigm]

This problem is, I think, most easily solved by finding the initial kinetic energy of the car.
Calculations are simplified if you convert 94.0 km/hr to m/s ==> 94.0 km/hr ~ 26.111111 m/s.
Now, K = 1/2 m v^2 , so the initial kinetic energy of the car is given by K = (1/2)x(1540 kg)x(26.111111 m/s)^2 = approx. 525 kJ.
Since the final velocity of the car is zero, the final kinetic energy is zero as well. Thus, the change in kinetic energy is 0 - 525 kJ = -525 kJ.
The work is equal to the total change in energy, so W = -525 kJ as well.
===> change in KE = W = -525 kJ <===

 

[BuBbLeS01]

I understand...but its saying its incorrect?

 

[paradigm]

well, i may have rounded a bit too much... also, it's common just to write the magnitude of the work, so try W = 525 kJ if -525 kJ doesn't work. A more exact answer is 524978.3951 J

 

[BuBbLeS01]

I got the change in kinetic energy right but, is the work is wrong when entered as positive. So do I enter is negative?

 

[paradigm]

yeah, negative was my first suggestion... in my original post i had it as positive but then edited it shortly thereafter to be negative... just a typo.

 

[BuBbLeS01]

Oh ok...that was right then, thank you much!