Calculating Work Done by Brakes

AI Thread Summary
To calculate the work done by the brakes on a car slowing from 92 km/h to 63 km/h, the driver needs to apply the work-energy principle, which relates work to changes in kinetic energy. The car's initial and final speeds can be converted to meters per second, and the mass of the car is given as 1368 kg. The change in kinetic energy can be calculated using the formula KE = 0.5 * m * v^2, and the work done by the brakes will equal the difference in kinetic energy. The discussion highlights confusion over the calculations and the lack of a friction coefficient, with the expected answer being 237 kJ, which the user finds too low and unclear. A step-by-step explanation of the energy conservation approach is requested to resolve the confusion.
Chica1975
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Homework Statement



the driver of a car mass 1368kg braked momentarily to slow down from 92km/h (25.556m/s) to 63 km/h (17.5m/s) how much work was done by the brakes (friction).

Homework Equations


w = Fd
F=ma
difference between initial acceleration and final acceleration s 8.056m/s (25.556 - 17.5)
mg = 13406.40



The Attempt at a Solution



I ave drawn the picture, I have tried a number of calculations - I am completely lost - I have no friction coefficient and don't know where to start. I have looked a book I have that has problems but has nothing like this. Apparently, the answer is 237.
 
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Try to use conservation of energy. I have no idea where 237 comes from, it's much to low (and has no units!)
 
Thanks - this is the answer the lecturer has given for this question. I have no idea how to get it out.
 
the units are KJ
 
Thanks I tried to use conservation energy equation. I can't get this out.

Please can some explain to me step by step how to do this - I have been on this question for the last hour and a half and have tried all sorts of things with different equations - its not working.

I need a reference point to start from I have no idea - I am stuck.
 

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