PHYS CH 7 – Flashcards
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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
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Work is the force × distance.
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Cite an example in which a force is excreted on an object without doing work on an object.
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A weightlifter that holds a barbell weighing 1000 N overhead does no work on the barbell, because it is not being moved by the force he exerts, therefore he does no work on the barbell.
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Which requires more work-lifting a 50-kg sac a vertical distance of 2 m or lifting a 25-kg sac a vertical distance of 4 m
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The work done is equal in both cases
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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 in which the lighter sack is moved its distance in half the time
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The power would be the same if done in the same amount of time because the work done is equal. If the lighter sack is moved its distance in half the time, then the power would be greater than lifting the heavier sack. *
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Exactly what is it that enables an object to do work
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Energy enables an object to do work.
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A car is raised a certain distance in a service station lift and therefore has a potential energy relative to the floor. If it were raised twice as high. How much potential energy would it have relative to the floor
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If it were raised twice as high, then it would have twice as much potential energy relative to the floor because the gravitational potential energy is directly proportional to the height. PE=weight × height
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Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, how do their gains of potential energies compare
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The car that is twice as massive as the other car has more potential energy because its' weight is twice as massive than the other car.
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When is the potential energy of something significant
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The potential energy of something is significant only when it changes, meaning when it does work or transforms to energy of some other form.
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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
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It has 16 times more kinetic energy than it did before. Because if the speed of an object is doubled, its kinetic energy is quadrupled. (2^2=4) *
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Compared with some original speed, how much work must the brakes of a car supply to stop a car that is moving 4 times as fast, How will the stopping distance compare
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It takes 16 times as much work to stop and 16 times as much stopping distance.
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If you push a crate horizontally with 100 N across a 10 m factory floor and friction between the crate and the floor is a steady 70 N, how much kinetic energy is gained by the crate
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ΔkE=(100-70)(10)= (30) (10) = 300
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How does speed affect the friction between a road and a skidding tire
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Speed has little or no effect on friction.
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What will be the kinetic energy of a pile driver ram when it undergoes a 10kJ decrease in potential energy
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It will gain 10kj in kinetic energy.
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An apple hanging from a limb has potential energy because of its height. If it falls, what becomes of this energy just before it hits the ground, When it hits the ground
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The potential energy will convert to kinetic energy. When it hits the ground energy becomes thermal energy.
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What is the source of energy in sunshine
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The nuclear energy is transformed into radiant energy.
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an a machine multiply input force, Input distanc, Input energy
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A machine can multiply input force and input distance but NEVER input energy.
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If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much
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The distance is diminished by a factor of four.
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A force of 50 N is applied to the end of a lever, which is moved a certain distance. If the other end of the lever moves one-third as far, how much force can it exert
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150 N. F_d=ⅆ_F
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What is the efficiency of a machine that miraculously converts all the input energy to useful output energy
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100%
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If an input of 100 J in a pulley system increases the potential energy of a load by 60 J, what is the efficiency of the system
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Efficiency will be 60%
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What is recycled energy
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It is the reemployment of energy that otherwise would be wasted.
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In what sense are our bodies machines
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It is made up of smaller machines which are our living cells that need a source of energy, which feed on various hydrocarbon compounds that release energy when they react with oxygen.
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What is the ultimate source of energy for fossil fuels, dams, and windmills
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The sun is the source of practically all our energy.
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What is the ultimate source of geothermal energy
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The nuclear power in earth's interior.
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Can we correctly say that hydrogen is a new source of energy
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Why or why not?No because hydrogen is a carrier of energy, not a source.
