# Conservation of energy of a car problem

1. Mar 19, 2008

### myoplex11

1. The problem statement, all variables and given/known data
Determine the height h to the top of the incline D to which the 200-kg roller coaster car will reach, if it is launched at B with a speed just sufficient for it to round the top of the loop at C without leaving the track. The radius of curvature at C is
ρC = 25 m. I have attached a picture of this problem below.

2. Relevant equations
KE1+PE1=KE2+PE2

3. The attempt at a solution
.5mv1^2+mgh=.5mv^2+mgh
.5mv1^2=mgh
0.5v1^2=gh

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Last edited: Mar 19, 2008
2. Mar 19, 2008

### tiny-tim

… physics for "hug" is "force" …

Your method would only give you the speed needed for the car to reach the top with zero speed.

The question requires the speed at the top to be so great that the car still "hugs the track".

Physics for "hug" is "force".

(Reminds me of: Principal "How is you mathematics?" Groucho Marx: "Sir, I speak it like a native!" )
So … what are the forces on the car at the top of the track?

3. Mar 19, 2008

### myoplex11

centipetal force and the normal force i still have no idea how to do this

4. Mar 19, 2008

### tiny-tim

… the acceleration is always downward …

"Centipetal force" is not a force!!

And you've missed out gravity …

The only two forces are gravity, and the normal force (the reaction force).

Those two forces determine the acceleration.

Paradoxically, they're both downward (at the top of the loop, C) … which means that the acceleration is always downward (centripetal)!

But that doesn't matter, because circular motion (at the top of the circle) requires downward acceleration!

This is geometry, not physics (and you can easily prove it using vectors) … an object with speed v in a circle or radius r has an acceleration of v^2/r towards the centre of the circle.

So, at C, you want gravity and the normal force together to equal m.v^2/r (Newtons' second law).

The question asks for the minimum case, which obviously must be when the normal force is zero: then the force from gravity alone equals mv^2/r.

And so … ?