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Where does the potential energy come from
That raises another issue. What happens to the clock when the spring or weights expend all of their stored energy. The answer is that eventually the pendulum will stopswinging and the clock will stop running unless a human imparts more energy intothe spring or weight system that keeps the clock running.
Twenty-four hours of controlled energy transfers
I can still remember my grandfather winding the spring on his pendulum clock as the last thing he did before going to bed each night. He was convertingpotential energy derived from food into elastic potential energy in the clock's spring.
During the next 24 hours, that elastic potential energy would be converted to a combination of gravitational potential energy and kinetic energy in thependulum, which would move the hands on the clock to indicate the correct time.
The next night, my grandfather would start the process all over again by imparting energy into the clock's spring.
I will present and explain sample calculations involving both kinetic energy and mechanical energy in this section.
A roller coaster car
What is the kinetic energy possessed by a 1378 pound roller coaster car that is moving with a speed of 60 ft/sec?
Answer:
Enter the following expression into the Google search box:
0.5*1378lb * (60ft/s)^2
The resulting kinetic energy is 1.045 * 10^5 joules.
Similarly, you could enter the following into the Google search box:
0.5*625kg * (60ft/s)^2
and you would get the same answer of 1.045 * 10^5 joules.
As you can see, it doesn't matter whether you use English units or SI units for mass and velocity,as long as you do the calculation correctly. The answer is the same, because the kinetic energy of the object doesn't change just because you express thosequantities in different units.
More on the roller coaster car
What would be the kinetic energy of another car with the same mass but with three times the speed?
Answer:
We don't even need to evaluate the equation to answer this question. We know that the kinetic energy varies as the square of the speed, so if we triple thespeed, we will increase the kinetic energy by a factor of 3*3=9. Therefore,
KE = 9*1.045 * 10^5 joules = 9.405 * 10^5 joules
Dead Fred the daredevil
Dead Fred, the daredevil possessed a kinetic energy of 18000 joules just prior to hitting a solid brick wall on his motorcycle. If his mass was 176 pounds, what was his speed?
Answer:
KE = 0.5*m*v^2, or
v^2 = KE/(0.5*m), or
v = sqrt(KE/(0.5*m)), or
v = sqrt(18000joules/(0.5*176pound))
Enter the right-hand expression above into the Google search box and you should get a velocity of
v = 21.24 m/s
A dart gun
A dart with a mass of 0.1 kg leaves the muzzle of a dart gun with a velocity of 10 m/s. The child is holding the dart gun at a position that is 1 m above theground. At that instant,
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