Conservation of Momentum & Energy

Conservation of Momentum & Energy

A Joule of energy = energy expended in applying a force of one newton through a distance of one meter (1 newton meter or N·m) =


kg • m2___________s21J =




v2 = 2g(H-y)


Roller Coaster Physics

We can say that the sum of potential energy plus kinetic energy remains constant at any point in time, again, for a closed system. Let’s take a look at how this applies in one of the most personally exhilarating applications of physics.

Left: Dragon Khan roller-coaster in Universal Port Aventura park in Spain, Wikipedia


A 100 kg roller coaster that does a loop-d-loop must be designed such that the coaster stays on the tracks at all times especially when it is upside down. Using equation 2.4 above, we can write:




That is, the sum of the potential and kinetic energy will be the same for all points H, A, B and C on the roller coaster track assuming the ideal case where no energy is lost due to friction. PE can be converted into KE and back, but the sum will always be a constant.


Initially, at point H where y = 20 m above the ground, the roller coaster has no kinetic energy but lots of potential energy. We know it has no kinetic energy because its velocity = 0! Its potential energy can be calculated from Equation 3.3 as:


PE = mgh = 100 kg • 9.8 m/s2 • 20 m = 19,600 J


At the bottom of the roller coaster track (point A), the coaster has no potential energy (h = 0) and lots of kinetic energy. In fact, it has a maximum of kinetic energy and so a maximum velocity. Its kinetic energy can be described as:


KE = ½ mv2 = ½ * 100 kg * v2 (m/s)2

But what is the coaster’s velocity?


Since we know that PE + KE = constant, we know that the KE at the bottom is just equal to the potential energy of the coaster at the top (PE = 19,600 J), and the maximum value of the PE equals the maximum value of KE (assuming there is no friction).

(Left: Rollercoaster © Zechal -



PE = 19,600 J = KE = ½ mv2

19,600 J = ½ * 100 * v2 = 50 v2

and so v = (19,600 J/50)½ = 19.8 m/s


For the roller coaster at any point on the track:

    ETotal = PE + KE = Potential Energy at maximum height, H.


mgH = mgy + ½ mv2


where y = the height of the roller coaster at that point.


Collecting terms of the last equation gives us a general equation for the velocity of a roller coaster at any point – not very important in the scheme of things, but something you derived and now understand:


velocity squared = 2 x gravity x(maximum height - height at any given point) v2 = 2g(H-y)


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