The answer is not 'A'. Using the numbers given in the question, it's 'B'.
It WOULD be 'A' if the number in B were 71 or less. ________________________________
<span>This is not as simple as it looks.
What quantity are we going to compare between the two cases ? Yes, I know ... the "amount of work". But how to find that from the numbers given in the question ? Is it the same as the change in speed ? Well ? Is it ? NO. IT's NOT.
In order to reduce the car's speed, the brakes have to absorb the KINETIC ENERGY, and THAT changes in proportion to the SQUARE of the speed. ( KE = 1/2 m V² )
Case 'A' : The car initially has (1/2 m) (100²) = (1/2m) x 10,000 units of KE.
It slows down to (1/2 m) x (70²) = (1/2m) x 4,900 units of KE.
The brakes have absorbed (10,000 - 4,900) = 5,100 units of KE.
Case 'B' : The car initially has (1/2 m) (79²) = (1/2m) x 6,241 units of KE.
It slows down to a stop . . . NO kinetic energy.
The brakes have absorbed all 6,241 units of KE.
Just as we suspected when we first read the problem, the brakes do more work in Case-B, bringing the car to a stop from 79, than they do when slowing the car from 100 to 70 .
But when we first read the problem and formed that snap impression, we did it for the wrong reason. Here, I'll demonstrate:
Change Case-B. Make it "from 71 km/h to a stop".
Here's the new change in kinetic energy for Case-B:
The car initially has (1/2 m) (71²) = (1/2m) x 5,041 units of KE. It slows down to a stop . . . NO kinetic energy. The brakes have absorbed all 5,041 units of KE.
-- To slow from 100 to 70, the brakes absorbed 5,100 units of KE.
-- Then, to slow the whole rest of the way from 71 to a stop, the brakes absorbed only 5,041 units of KE.
-- The brakes did more work to slow the car the first 30 km/hr than to slow it to a complete stop from 71 km/hr or less.
That's why you can't just say that the bigger change in speed requires the greater amount of work. ______________________________________
It works exactly the same in the opposite direction, too.
It takes less energy from the engine to accelerate the car from rest to 70 km/hr than it takes to accelerate it the next 30, to 100 km/hr !</span>
(The exact break-even speed for this problem is 50√2 km/h, or 70.711... km/hr rounded. )
The correct answer is 10 billion years. The Sun is expected to undergo hydrogen fusion for a total of 10 billion years. The Sun generates its energy by nuclear fusion of hydrogen and produces helium nucleus. It fuses 620 million metric tons every second.
If bonds are broken, the energy is released, and if bonds are formed, energy is absorbed. During conversions from chemical energy to thermal energy, the energy stored in the chemical bonds are released and this energy causes surrounding molecules to move faster thus increasing the thermal energy of a substance.
