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couga
Oct 24, 2007, 04:30 PM
The engine of a 1950 kg car driving at 25m/s suddenly dies out as it approaches a 25 m high hill. Will the car have enough energy to clear this hill?

Capuchin
Oct 25, 2007, 12:06 AM
Work out the kinetic and the potential energy needed, compare and there's your answer.

catgita
Nov 22, 2007, 01:31 PM
With no air or mechanical drag the mass and energy of the car is irrelevant. If you were to drop the car from 25m, would it be going 25m/s when it hit the ground?

I can't recall the math at the moment, other than acceleration due to gravity is 9.8m/s/s. A napkin calculation says it comes up a little short. I think you could clear a 30m hill at 30m/s, but at 25m/s you only make about 22m up, 20m/s you make 15m, etc.

Capuchin
Nov 22, 2007, 02:27 PM
You're making it too complex catgita.

At the bottom of the hill, the car has KE = \frac{1}{2}mv^2 = 0.5*1950*25*25 J
At the top of the hill, the car must have PE = mgh = 1950*25*9.8 J

As the KE is > the PE, the car makes it over the hill. As it has enough KE to convert into PE.

I don't understand your method of doing the sum. I don't see how you are able to do it that way without knowing more information.

catgita
Nov 22, 2007, 04:31 PM
For one, the mass cancels out when you have:
1/2mv^2>mgh
giving you
1/2v^2>gh

Second, maybe you have heard of someone called Newton? Drop a hammer, drop a feather from 25m, which hits the ground first? Make the car 1kg or 1,000,000kg, no difference.

Regardless, you are correct, the car makes it to the top going about 11.6m/s. Thanks for the equasions, much better than my estimates.;)

Capuchin
Nov 22, 2007, 05:22 PM
Of course you're right, but you're just confusing things for our high school friend here. :)