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Physical Science - Chapter 6 Review
Terms in this set (30)
energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes. In an ideal machine, where no energy is transformed into heat, work input = work output and (Fd) input = (Fd) output.
Conservation of Energy
the percent of the work put into a machine that is converted into useful work output
the property of a system that enables it to do work
energy of motion, described by the relationship: KE = ½ mv2
a device such as a lever or pulley that increases (or decreases) a force or simply changes the direction of a force
the stored energy that a body possesses because of its position
the time rate of doing work: P = work/time
the unit of power, the joule per second
the product of the force and the distance through which the force moves: W = Fd
the work done on an object is equal to the energy gained by the object; Work = ∆E
Work - Energy Theorem
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 X distance?
Work; changes energy
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.
Which requires more work - lifting a 50-kg sack a vertical distance of 2 m or lifting 25-kg sack a vertical distance of 4 m?
Both are the same amount of work because the force x distance is the same for each.
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.
What are the two main forms of mechanical energy?
Potential and Kinetic Energy
Exactly what is it that a body having energy is capable of doing?
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?
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 potential energy.
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 x 4 m = 40 J. At 8 m the PE = 80 J.
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 4(2) or 16 times the KE.
Compared to some original speed, how much work must the brakes of a car supply to stop a car moving four times as fast? How will the stopping distance compare?
A four times as fast car has 16 times as much DE and will require 16 times as much work to stop, and 16 times as much stopping distance.
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= 10kj
Can a machine multiply input force? Input distance? Input energy?
yes; yes; no
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 four-fold force is one fourth as much.
If the man in Figure 6.21 pulls 1 m of rope downward with a force of 100 N, and the load rises 1/7 as high, what is the maximum load that can be lifted?
Seven times as much; 700 N
What is the efficiency of a machine that miraculously converts all the input energy useful output energy?
Is a machine physically possible that has an efficiency greater than 100%? 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 energies of fossil fuels, dams, and windmills?
What is the source of geothermal energy?
Nuclear energy from radioactive decay
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?
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.
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