What the speed of the car

http://i1193.photobucket.com/albums/aa35…

The above link, is one question to the test I took.
Refer to part B, can someone explain what I did wrong? Should I had use negative 9.8m/s^2 instead? Not very sure when I need to use positive or negative gravity. Can someone also explain to me this confusion?

Need to review this for the final :)
Thxs.

Given kinetic energy at point 2 of:
K2 = ½mv^2
K2 = 0.5 * 1200 * 25^2
K2 = 375000 J

At point 3, kinetic energy will decrease by an amount equal to the gain in potential energy (this follows the law that mechanical energy is given by the sum of potential energy and kinetic energy):
E2 = U2 + K2 (assume U2 = 0)
E3 = U3 + K3
E2 = E3 (mechanical energy remains constant)

U2 + K2 = U3 + K3
0 + K2 = U3 + K3
K3 = K2 - U3

Given that E remains constant, K will decrease my an amount U:

U3 = mgh
U3 = (1200)(9.8)(15)
U3 = 176400 J

Finally, K3 = K2 - U3
K3 = 375000 - 176400
K3 = 198600 J

½mv^2 = 198600
v^2 = 2 * 198600 / 1200
v^2 = 331
v = 18.2 m/s


The mistake here in your answer is gy2 - gy3: = 0 - 15 = -15 (negative NOT positive):
2[9.8(-15) + 0.5(25)^2] = (V3)^2
Would yield correct result.

Hope it helps.

You are not to use negative gravity unless you are propelling something upwards.
Your concept is correct. Your calculations have not included the fact that gravity is UP TO A MAX constant, not continually

Hello Peacily, the potential at point 3 will be more than that at point 2. So mgh3 - mgh2 is to be there. That is why you get wrong answer.

15 should be -15 cuz its h2 - h3. Then answer would be 18 something