Conservation Laws Homework help

1. Nov 20, 2004

Lorax

Hi, I'm having a few problems with my Homework I'll post one question for now then if I find out where I got stuck I'll go on to my next question. Alright...

"A 1500kg car accelerates up a hill in an attempt to pass a semi truck, it had an initial velocity of 12m/s if accelerated at a rate of 2m/s^2 for the 180m up the slope. If the hill had a vertical height of 15m and ther was 400j of work done by the friction how much work was done by the cars engine?"

Alright, well I know the Mass, Vi, A, D, G, H, and Vf and the W done by the friction.

But I'm a little unsure about how to go about figuring this question out. If someone could start me off that would be great.

2. Nov 21, 2004

Parth Dave

First of all, energy must be conserved (as you suggested). So, try to find an expression for the initial energy and the final energy.

3. Nov 21, 2004

rayveldkamp

Hi,
The total energy of the system must be conserved, now at the start the car has velocity 12m/s and hence kinetic energy: 1/2mv^2. Since the change in the potential energy depends on CHANGE in height, assume it has initial height of zero, and so the total initial energy is: 1/2 m(12^2).
Now since acceleration is constant you can use
Put your values in to get vf, then u can get the final kinetic energy. Then calculate the change in potential energy: mg(15)
Now the final kintetic energy plus the potential energy, subtract 400J should equal the initial kinetic energy.
Hope that helps

4. Nov 21, 2004

Lorax

yeah thanks you two, I had figured out the final kinetic energy, and potential energy but was a biy unsure as to what to do with the 400j. Thanks! I'll add my next question in an hour or so.

5. Nov 21, 2004

Lorax

Actually now that I relooked over my answer that doesnt make any sense to me I get a number like 8687250-400. Can someone verify if what rayveld said is correct. Also if someone could maybe rewrite what he said that would be great help.

6. Nov 22, 2004

rayveldkamp

Hi sorry if my last post was confusing i was in a rush. My answer to it came out as:
3,254,730 - 400=3,254,330 J
I don't know if its right but there should be a lot of work done by the car's engine to accelerate it at 2m/s.

7. Nov 22, 2004

Lorax

can someone verirify if rayveld has the right asnwer I did it and keep getting something different