Physical Science Chapter 6
What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?
100%—the efficiency of an ideal machine.
Cite an example where a force is exerted on an object without doing work on the object.
A force is needed to hold a barbell overhead, but this force does no work on the barbell while the barbell is at rest.
Compared to some original speed, how much work must the brakes of a car supply to stop a four-times-as-fast car? How will the stopping distance compare?
A four-times as fast car has 16 times as much KE and will require 16 times as much work to stop, and 16 times as much stopping distance.
A moving car has kinetic energy. If it speeds up until it is going four times as fast, how much kinetic energy does it have in comparison?
A four-times as fast car has 42 or 16 times the KE.
Which requires more work—lifting a 50-kg sack a vertical distance of 2 m or lifting a 25-kg sack a vertical distance of 4 m?
Both require the same work because the force × distance is the same for each.
If a machine multiplies force by a factor of four, what other quantity is diminished, and how much?
Increasing force means decreasing distance, so the distance moved by the four-fold force is one fourth as much.
Is a machine physically possible that has an efficiency greater than 100 percent? Discuss.
Not in the world as we know it. Such a machine would clearly violate the conservation of energy, a law that has never been violated.
What is the ultimate source of geothermal energy?
Nuclear energy from radioactive decay.
How many joules of potential energy does a 1-N book gain when it is elevated 4 m? When it is elevated 8 m?
PE = 1 N × 4 m = 4 J. At 8 m the PE = 1N × 8 m = 8 J.
What will be the kinetic energy of pile driver ram when it undergoes a 10 kJ decrease in potential energy? (Assume no energy goes to heat.)
PE = KE = 10 kJ.
What are the two main forms of mechanical energy?
Potential energy and kinetic energy.
If both sacks in the preceding question are lifted their respective distances in the same time, how does the power required for each compare? How about for the case where the lighter sack is moved its distance in half the time?
Power for each is the same because the same work is done in the same time. Twice the power is required to do the same work on the lighter sack in half the time.
15. If the man in Figure 6.22 pulls 1 m of rope downward with a force of 100 N, and the load rises 1/7 as high (about 14 cm), what is the maximum load that can be lifted?
Seven times as much, 700 N.
What is the ultimate source of energies for the burning of fossil fuels, dams, and windmills?
Solar energy.
Two cars are lifted to the same elevation in a service station. If one car is twice as massive as the other, how do their potential energies compare?
The twice-as-massive car has twice the PE.
A car is lifted a certain distance in a service station and therefore has potential energy relative to the floor. If it were lifted twice as high, how much potential energy would it have?
Twice.
The energy we require for existence comes from the chemically stored potential energy in food, which is transformed into other forms when it is metabolized. What happens to a person whose work output is less than the energy he or she consumes?Whose work output is greater than the energy he or she consumes? Can an undernourished person perform extra work without extra food? Briefly discuss.
When work output is less than input energy, the person gains weight. When work output is more than input energy, weight loss results. An undernourished person performing extra work without extra food violates energy conservation—and perishes.
Exactly what is it that a body having energy is capable of doing?
Work.
Can a machine multiply input force? Input distance? Input energy? (If your three answers are the same, seek help, for the last question is especially important.)
Yes; yes; NO! (Machines are good at multiplying force or distance, but no machine can multiply energy—a conservation of energy no no!)
A force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, which changes the momentum of that object. What do we call the quantity force distance?
work
