A car of mass 1200kg slows down from a speed of 20 m/s to 10 m/s how much kinetic energy does the car lose

Answer 1

Hi, Lots,

In this problem you use the KE=(mv²/2) expression for kinetic energy and the difference in KE , ΔE (as usually defined) as the final KE_f=(mv_f²/2) minus the initial KE_in=(mv_i²/2)

1. ΔE= KE_fin-KEin

Use v_i=20m/s ; v_f=10m/s . As the mass and speed are given in SI system, all energies would be in Joules (J).

Some general comments:

The problem is rather straightforward yet one could learn from it some more.

The negative result would mean that the KE of the car ihas decreased.

However, that is not a "loss" (as energy is not known to ever be "lost" but rather it is getting transfered to a different form. For example the car may slow down by going uphill so, it's KE woul decrease but at the same time uts potential energy mgh would increase. Or, if the car was moving on a horizontal road but the driver just pushed the brakes, then the car's KE decreased but didn't get "lost" only channeled into increasing the speed of random motion of molecules of the road, tires and breaks. Yes, that part of KE stops wearing the coat of an ordered motion (of the car as a whole) but "reappeares as" or "turned into" a tiny increase of the average speed of random (or "thermal") motion of a huge number of tiniest molecules.

One could use an analogy : you put a $1 bill into a changing machine. The $1' bill "disappeared" swallowed by the machine but out jump 4 shiny quarter counts. Yes, a $1 (in the form of a single Bill) did disappear from our pocket, from your hands, from your sight. But the $1 VALUE did NOT just switching the form and reappearing as 4 quarters

Hope that was helpful

Dr Ariel B.