A 120-kg roller coaster cart is being tested on a new track, and a crash-test dummy is loaded into itThe roller coaster starts from rest at point A which is 35 m above the ground. The cart is observed to roll down the track to point Bwhich is 10 m above the ground . In your own words, what physics ideas would you be able to use to predict how fast the cart is moving at point B? What calculations would you use, what things would you be measuring , and what physics principles or laws would you use to make this calculation ?

a) variation of the energy is equal to the work of the friction force

b) W = Em_{f} -Em₀ ,  c) he conservation of mechanical energy

Explanation:

a) In an analysis of this problem we can use the energy law, where at the moment the mechanical energy is started it is totally potential, and at the lowest point it is totally kinetic, we can suppose two possibilities, that the friction is zero and therefore by equalizing the energy we set the velocity at the lowest point.

 Another case is if the friction is different from zero and in this case the variation of the energy is equal to the work of the friction force, in value it will be lower than in the calculations.

b) the calluses that he would use are to hinder the worker's friction force and energy

          W = Em_{f} -Em₀

          N d = ½ m v² - m g (y₂-y₁)

          y₂-y₁ = 35 -10 = 25m

c) if there is no friction, the physical principle is the conservation of mechanical energy

 If there is friction, the principle is that the non-conservative work is equal to the variation of the energy

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