Calculating Mechanical Energy Change of 1100kg Car

[uno]

1. A 1100 kg car at rest at the top of a hill accelerates down the road, reaching a speed of 20 m/s after descending a height of 120 m. What was its total change in mechanical energy (KE plus PEG) as of that moment in Megajoules?



2.KE = 1/2 mv^2 and PEg = mgh



3. I used the above formulas to figure out KE = 2.20E5 and PEg = 1.3E6. When I add both together, I get 1.51E6 and convert it to MJ = 1.51MJ. When I put in the answer, this is incorrect. Am I missing something?

 

[LowlyPion]

1. A 1100 kg car at rest at the top of a hill accelerates down the road, reaching a speed of 20 m/s after descending a height of 120 m. What was its total change in mechanical energy (KE plus PEG) as of that moment in Megajoules?

2.KE = 1/2 mv^2 and PEg = mgh

3. I used the above formulas to figure out KE = 2.20E5 and PEg = 1.3E6. When I add both together, I get 1.51E6 and convert it to MJ = 1.51MJ. When I put in the answer, this is incorrect. Am I missing something?

The KE at the bottom of the hill already includes the m*g*h from the descent. You probably want to just use the KE as the answer.

 

[krausr79]

It starts high and ends low, so it LOSES potential energy. It starts at rest and finishes in motion, so it gains kinetic energy. (I think)

 

[borgwal]

kraus is right: so how much mechanical energy does the car lose?