00:01
In this question, we're given that a car with a mass of 1 ,250 kilograms, 1 ,250 kilograms, goes up a hill with a height of 16 .2 meters, and over that distance, the engine does 6 .44 times 10 to the 5 joules of work, while friction does negative 3 .11 times 10 to the 5 joules of work.
00:23
The negative sign here indicates that friction is taking energy out of the car, rather than putting energy in, which is what we expect since friction slows things down rather than speeding them up.
00:34
This question is asking us to calculate, what is the change in kinetic energy of the car over the course of this travel? so first, we need to start with the knowledge that the sum of the non -conservative work in the system, that is the work done by friction plus work done by the engine, the sum of those non -conservative works is equal to the change in the car's total energy.
00:59
So in other words, work done by friction plus work done by the engine is equal to the change in the energy.
01:12
Now the change in the car's energy has two components.
01:16
First, the car's gravitational potential energy changes, plus its kinetic energy changes.
01:27
So since we're interested in the kinetic energy here, we just need to divide both sides.
01:33
By the change in potential energy, so that kinetic energy is on its own on one side.
01:39
So delta ek is equal to work done by friction plus work done by the engine minus the change in potential energy...