chapter7 Flashcards
What is the unit of work?
joule
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
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.
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.
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.
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
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.
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?
300 J
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.
In a simple machine, how much work is done when an input of 10 N acts over a distance of 5 m?
50 J
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.
Can a machine multiply input force? Input distance? Input energy?
Force and distance, but not energy
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%
Fossil fuels, hydroelectric power, and wind power ultimately get their energy from _______.
the sun
Which energy production method does not ultimately depend on the Sun?
Nuclear fission
What is the ultimate source of the energy from fossil fuels, dams, and windmills?
the sun
A 1000-kg car and a 2000-kg car are hoisted to the same height. Raising the more massive car requires
twice as much work
The work you do when pushing a shopping cart twice as far while applying twice the force is
four times as much
No work is done by gravity on a bowling ball that rolls along a bowling alley because
the force on the ball is at right angles to the ball’s motion
One kilowatt-hour represents
3.6 million joules
The unit kilowatt-hour is a unit of
energy
The power expended doing 100 J of work in 50 s is
2 W
The power required to exert 4-N force over 3 meters in 2 seconds is
6W
Both a 50-kg sack is lifted 2 meters from the ground and a 25-kg sack is lifted 4 meters in the same time. The power expended in raising the 50-kg sack is
the same
A toy elevator is raised from the ground floor to the second floor in 20 seconds. The power needed using 1000 J of work, is
50 W
An object has gravitational potential energy due to its
location
A crate of grapes lifted 10 meters gains 200 J of potential energy. If the same crate is instead lifted 20 meters, its gain in potential energy is
twice as much
A 2-kg ball is held 4 m above the ground. Relative to the ground its potential energy is
80 J
A 2-kg box of taffy candy has 40 J of potential energy relative to the ground. Its height above the ground is
2 m
An object that has kinetic energy must be
moving
Two identical golf carts move at different speeds. The faster cart has twice the speed and therefore has
four times the kinetic energy
A melon is tossed straight upward with 100 J of kinetic energy. If air resistance is negligible the melon will return to its initial level with a kinetic energy of
100 J
A melon is projected into the air with 100 J of kinetic energy in the presence of air resistance. When it returns to its initial level its kinetic energy is
less than 100 J
If a Ping-Pong ball and a golf ball both move in the same direction with the same amount of kinetic energy, the speed of the Ping-Pong ball must be
more than a golf ball
A 2500-N pile-driver ram falls 10 m and drives a post 0.1 m into the ground. The average impact force on the ram is
250000 N
Two identical particles move toward each other, one twice as fast as the other. Just before they collide, one has a kinetic energy of 25 J and the other 50 J. At this instant their total kinetic energy is
75 J
When Joshua brakes his speeding bicycle to a stop, kinetic energy is transformed to
heat
A bicycle that travels four as fast as another when braking to a stop will skid
sixteen times as far.
A motorcycle moving at 50 km/h skids 10 m with locked brakes. How far will it skid with locked brakes when traveling at 150 km/h?
90 m
About 40 J is required to push a crate 4 m across a floor. If the push is in the same direction as the motion of the crate, the force on the crate is about
10 N
Which requires the most amount of work by the brakes of a car?
slowing down from 100 km/h to 70 km/h
Two identical arrows, one with twice the kinetic energy of the other, are fired into a bale of hay. Compared with penetration of the slow arrow, the faster arrow penetrates
twice as far
Two identical arrows, one with twice the speed of the other, are fired into a bale of hay. Compared with penetration of the slow arrow, the faster arrow penetrates
four times as far.
The bob of a simple pendulum has its maximum kinetic energy at the
bottom of its swing.
A ball rolling down an incline has its maximum potential energy at
the top
After rolling halfway down an incline, a marble’s kinetic energy is
the same as its potential energy.
A light aluminum ball and a heavy lead ball of the same size roll down an incline. When they are halfway down the incline, they will have identical
none
Strictly speaking, more fuel is consumed by your car if the air conditioner, headlights, or even a radio is turned on. This statement is
true
Acrobat Bart at the circus drops vertically onto the end of a see-saw, with his partner Art equidistant from the fulcrum at the other end. Art is propelled straight upward a distance twice that of Bart’s dropping distance. Neglecting inefficiencies we see
Art has half the mass of Bart
A 1-kg ball dropped from 2 m rebounds only 1.5 m after hitting the ground. The amount of energy converted to heat is about
more than 2.0 J.
A hydraulic jack is used to lift objects such as automobiles. If the input force is 200 N over a distance of 1 meter, the output force over a distance of 0.1 meter is ideally
2000 N
A machine puts out 100 watts of power for every 1000 watts put into it. The efficiency of the machine is
10 %
Earth’s primary energy source is
the sun
Hydro and wind power are indirect forms of
solar energy
A machine that promises more energy output than input is
a fantasy
Which of these is not a form of solar energy?
geothermal energy
A primary difference between momentum and kinetic energy is
momenta can cancel; kinetic energy cannot.
Impulse involves the time that a force acts, whereas work involves the
distance that a force acts.
A moving object has
- momentum.
- velocity.
- energy.
- speed.
Calculate the work done when a 14- N force pushes a cart 3.6 m .
50J
Calculate the work done in lifting a 480- N barbell 2.0 m above the floor.
960 J
What is the gain of potential energy of the barbell when it is lifted to this height?
960 J
Select the correct equations that show that 40 W of power is required to give a brick 120 J of PE in a time of 3 s .
P=W/t=(120J)/(3s)=40W
Select the correct equations that show that the gravitational potential energy of a 1000- kg boulder raised 3 m above ground level is 30,000 J . (You can express g in units of N/kg because m/ s 2 is equivalent to N/kg .)
PE=mgh=(1000kg)⋅(10N/kg)⋅(3m)=30,000J
Two identical cars are driving in opposite directions at the same speed. Their kinetic energies have ____.
the same magnitude and sign
A motorcycle drives up a steeply inclined ramp. The work done on the motorcycle by the Earth’s gravitational force is ____.
negative
During a certain time interval, the net work done on an object is zero joules. We can be certain that ____.
the object’s final speed was the same as its initial speed
When traveling twice as fast your kinetic energy is increased _______.
by four
When you lift twice the load twice as high, in half the time, the increase in potential energy is _______.
four times
Select the correct equations that show that 16
J
of work is done when a 2.0-
kg
block of ice is moved from rest to a speed of 4.0
m/s
.
W=m v 2 /2=((2.0kg)⋅(4.0m/s ) 2 )/2=16J
Nellie Newton applies a force of 60
N
to the end of a lever, which is moved a certain distance. If the other end of the lever moves one-third as far, find the force it exerts.
180 N
If you pull one end of the rope 1.3 m downward with a 44- N force, find the height you can lift a 160- N load.
.36 m
