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# M2 question - energy watch

1. Hi, I'm a bit stuck on a M2 energy question and was hoping this was the place to get some help!

"A car, of mass 1100kg, is travelling down a hill, inclined at an angle of 5degrees to the horizontal. The driver brakes hard and skids 15m as the car stops. The coefficient of friction between the tyres and the road is 0.7. Find the initial kinetic energy and speed of the car."

I tend to be ok with most energy questions but this one is just really bugging me, I can't figure out how to work with the 2 unknowns.

I honestly am not completely sure how to start it, but I was thinking of working out the GPE 15m up and the KE at the beginning (with the unknown initial velocity), and I know at the end there will be no kinetic energy or GPE, so all of it will have been lost in work done by the braking force, assisted by friction?

I am going to stop rambling my ideas because I'm honestly pretty stuck, but help would be appreciated! Thanks in advance!
2. First off, I'm out of my depth here.

I will be doing M2 and the title caught my curiosity to see what I have to expect.

Wouldn't the braking force be just friction?

Then you could find the component of g down the slope, subtract the frictional force from the brakes, to find the decceleration.

The you'd have s, a and v, and could find u and then energy?

(Sorry if this is total crap)
3. It worked, thanks so much! Here's silly me thinking you have to use all the energy equations when it is a lot easier than that! Thanks again!
4. (Original post by alexx :))
I tend to be ok with most energy questions but this one is just really bugging me, I can't figure out how to work with the 2 unknowns.
I can only see 1 unknown - v (KE=mv2/2)

If you wanted to use energy

R=mgcos(5)
F=0.7R
F=0.7mgcos(5)

height lost = 15sin(5)

GPE + KE = WD
mgh + mv2/2 = Fs
5. Am glad it worked, good luck on the exam, whenever it is..
6. oh, it was last year for me, but good luck to you too

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