Help with High School Physics Homework

[Mark Rice]

Hi, this is my first time using this forum, so i have no idea if this is even the correct place to be asking for help. I have a problem with my High School Higher Physics Homework. The question is as follow :

Homework Statement

During one run, a car and passengers of mass 800kg are released from rest at point A, a height of 20m above the ground. The car travels a distance of 120m along the track until it reaches point C, a height of 15m above the ground. A constant frictional force of 250N acts between the car and the track as the car moves from A to C. (A diagram of the track is included, see attachments). Find the Kinetic Energy of the Car on reaching point C.

Homework Equations

In the previous question i have been asked to calculate the work done against friction in moving from A to C, and i calculated this to be 30,000 N (W=fd W=120 * 250) (Not sure if this is correct though?) I think Ep=mgh is maybe relevant and obviously to work out the kinetic energy Ek=1/2mv^2 will be used.

The Attempt at a Solution

I genuinlly have no idea where to start on this question ? I am thinking that it will maybe have something to do with Potential energy loss, and the energy lost due to friction but i am completely confuzed. Can anyone please help me ?

 

[Doc Al]

Homework Equations

In the previous question i have been asked to calculate the work done against friction in moving from A to C, and i calculated this to be 30,000 N (W=fd W=120 * 250) (Not sure if this is correct though?)

Correct.

I think Ep=mgh is maybe relevant and obviously to work out the kinetic energy Ek=1/2mv^2 will be used.

Definitely.

The Attempt at a Solution

I genuinlly have no idea where to start on this question ? I am thinking that it will maybe have something to do with Potential energy loss, and the energy lost due to friction but i am completely confuzed.

Think in terms of mechanical energy. You start out with some total mechanical energy at A, but you end up with less at C due to work done against friction. Set up an equation expressing this.

 

[Mark Rice]

So would 30,000N previously worked out be the Energy 'lost to the surroundings', and also, if I am working out the kinetic energy, i assume i am not supposed to be working out the speed and using the kinetic energy equation, so how do i work it out? Sorry I'm really confuzed.

 

[Doc Al]

So would 30,000N previously worked out be the Energy 'lost to the surroundings',

Yes.

and also, if I am working out the kinetic energy, i assume i am not supposed to be working out the speed and using the kinetic energy equation, so how do i work it out?

Total mechanical energy = PE + KE

So what's the total mechanical energy at point A?
Then you can figure out what it is at point C. And use that to solve for the KE at point C.

 

[Mark Rice]

Ohhhhhhhh, thanks very much, I forgot that Total mechanical energy = PE + KE, Thanks for the help :)

 

[Mark Rice]

Just realized i should have written down 30,000J instead of 30,000N as it is energy but that's just a units error, but once again, thanks.