A few weeks ago I was enjoying one of my favorite parts of central NY, the peak foliage and rolling hills. I was on my way to New Hartford with my friends and passed by something that I normally don't see everyday: a run-away truck ramp. At first, this concept was scary. The first thing I thought was "how could anyone be that careless to use this?!" Then my physics brain kicked in. I realized that accelerating downhill in a semi with too much momentum (p=mv) for the breaks to effectively work is a pretty realistic scenario.
Here's how it works:
The ramp goes uphill and is covered in gravel. These two factors would each decrease the truck's velocity for different reasons (in an ideal worlds). Going uphill (increasing h) increases the truck's gravitational potential energy, which causes a decrease in kinetic energy because ideally mechanical energy is conserved (E= 1/2mv^2 + mgh). As the mass remains the same, the truck slows down. Now let's take friction into consideration: the gravel has a higher coefficient of friction than the road. This leads to a greater force of friction and thus greater work done by friction (Ffr= (uk)(Fn), Wfr= (Ffr)(d)). Utilizing the work-energy principle(ΔKE= -ΔPE + Wnc), one can see that adding a greater number to the left side of the equation leads to an greater change in KE, and thus a greater change in velocity- leading to the slowing of the semi.