chapter 7 - homework questions
If you push a crate horizontally with 100 N across a 10-m factory floor and the friction between the crate and the floor is a steady 70 N, how much kinetic energy does the crate gain?
(a) You do F d = 100 N 10 m = 1000 J of work. (b) Because of friction, net work on the crate is less. KE = net work = net force distance = (100 N - 70 N)(10 m) = 300 J (c) So the rest, 700 J, goes into heating the crate and floor.
What will the kinetic energy of a pile driver ram be if it starts from rest and undergoes a 10 kJ decrease in potential energy?
10 kJ
What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?
100%
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 does it exert?
150 N
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?
60%
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 take the same 1000 J. (Work = FD = mg X distance = 10mD)
If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much?
Distance is diminished to one-quarter.
Exactly what is it that enables an object to do work?
Energy
Can a machine multiply input force? Input distance? Input energy?
Force and distance, but not energy
When the speed of a moving car is doubled, how much more kinetic energy does it have?
It has four times as much Kinetic energy = ((1/2)mass X (velocity squared))
Compared with a car moving at some original speed, how much work must the brakes of a car supply to stop a car that is moving twice as fast? How will the stopping distances compare?
It takes four times the work and four times the stopping distance. (Kinetic energy of car = 1/2 X mass x (change in velocity)^2 = Work to stop it = Work in Joules.)
A car is raised a certain distance in a service-station lift, thus giving it potential energy relative to the floor. If it were raised twice as high, how much more potential energy would it have?
It would have twice as much potential 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 in which the lighter sack is moved the same distance in half the time?
Lifted in the same time, the power is the same. The light sack moving in half the time requires double the power. (power = (workdone) /Tjme interval) = unit in watts)
Can we correctly say that hydrogen is a new source of energy? Why or why not?
No. It takes energy to make hydrogen.
What is the ultimate source of geothermal energy?
Nuclear power in Earth's interior
What is recycled energy?
Reused energy that otherwise would be wasted
How does speed affect the friction between a road and a skidding tire?
Speed has no effect on the friction.
What is the ultimate source of the energy from fossil fuels, dams, and windmills?
The Sun
Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, compare their gains of potential energy.
The car with twice the mass has twice the potential energy.
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?
The energy is kinetic energy before it hits the ground; it is thermal energy after.
What is the unit of work?
joule
What is the source of energy in sunshine?
Thermonuclear fusion energy
When is the potential energy of something significant?
When the potential energy changes
A force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, and an impulse changes the momentum of that object. What do we call the quantity force multiplied by distance?
Work
How much work is done on a satellite in a circular orbit about Earth?
Zero