# Work and Energy of a car

How much work is required to stop a 1000 kg car traveling at 100 km/hr?

What I was thinking was that I would use the formula for KE,
KE=1/2mv2 (last two of course is for squared
since the KE would equal work. I have gotten as far as:

KE= 1/2(1000 kg)(100km/hr)2

I am assuming that somewhere there is a conversion needed, I'm just not getting the next steps.

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quasar987
Homework Helper
Gold Member
Which next steps? Just convert the km/hr into m/s and you're done.

Then maybe I'm not setting up the conversion correctly, I was using dimensional analysis and set it up

(100 km/hr) (1000m) (3600 sec) = 3.6 x 10^8
(1 km) (1 hr)

but I know this is wrong, by the time you square it and finish the equation, its way too big. The answer in the book is 386,000J, but I keep getting answers with 360 b/c of the sec. I'm sure this is a simple mistake I'm making, but I'm still stuck. Sorry.

lightgrav
Homework Helper
the hour is on the bottom!