Physics HW 3
The planet and its moon gravitationally attract each other. Rank the force of attraction between each pair from greatest to least.
(greatest) 2M-d-m = M-d-2m > M-d-m > M-2d-2m (least)
Rank the average gravitational forces from greatest to least between the following pairs of celestial bodies.
(greatest) sun and earth > sun and mars > sun and moon (least)
An object of mass 1 kg that is moving in a straight line on a level surface slows down from 4 m/s to rest. What is the net work done on the object?
-8 Joules
How many watts are expended when a force of 60 N moves a book 2 m in 3 s?
40 W
At a particular point in an elliptical orbit, a satellite has a gravitational potential energy of 5000 MJ with respect to Earth's surface and a kinetic energy of 5100 MJ. Later in its orbit, the satellite's potential energy is 6000 MJ. What is its kinetic energy at that point?
4100MJ
When the useful energy output of a simple machine is 100 J, and the total energy input is 200 J, the efficiency is _______.
50 percent
At what horizontal speed will the path of a projectile match the curvature of Earth?
8 km/s
Give an example in which a force is exerted on an object without doing work on the object.
A satellite is in a circular orbit about Earth.
Why doesn't the Leaning Tower of Pisa topple over?
A vertical line through the center of gravity passes inside its support base.
How do linear momentum and angular momentum differ?
Angular momentum depends on the distribution of mass, whereas linear momentum depends on the total mass.
What is required to change the angular momentum of a system?
External torque
If Earth shrank with no change in its mass, what would happen to your weight at the surface?
It would increase.
What is the magnitude of the gravitational field at Earth's center?
Zero N/kg
To double the rotational speed the centripetal force must _______.
be four times larger
When you lift twice the load twice as high, in half the time, the increase in potential energy is _______.
four times
The work that is done when twice the load is lifted twice the distance is _______.
four times as much
Satellites are orbiting at an altitude above ___________.
passenger airplanes altitude
To tighten a bolt, you push with a force of 75N at the end of a wrench handle that is 0.25m from the axis of the bolt. Do your answers depend on the direction of your push relative to the direction of the wrench handle?
yes
The amount of kinetic energy an object has depends on its mass and its speed. Rank the following sets of oranges and cantaloupes from least kinetic energy to greatest kinetic energy. If two sets have the same amount of kinetic energy, place one on top of the other.
(least) mass - 4m, speed - 1/4v mass - 2m, speed - 1/2v mass - m, speed - v (greatest) mass - 4m, speed - v (4 oranges) mass - 4m, speed -v (1 big one)
What will be the kinetic energy of a pile driver ram starting from rest that undergoes a 10-kJ decrease in potential energy?
10 kJ
What is the magnitude of Earth's gravitational force on a 1kg body at Earth's surface?
10N
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
Calculate the kinetic energy of a 100-kg scooter moving at 18 m/s. Kinetic energy = 1/2mass × speed^2 (KE = 1/2mv^2)
16200 J
To tighten a bolt, you push with a force of 75N at the end of a wrench handle that is 0.25m from the axis of the bolt. If you move your hand inward to be only 0.10m from the bolt, find the force that you should exert to achieve the same torque.
190N
Calculate the force of gravity on a 1-kg mass if it were 6.4×106m above Earth's surface (that is, if it were two Earth radii from Earth's center). F=(G)m1m2/r2
2.4N
Calculate the tension in a horizontal string that whirls a 1.6-kg toy in a circle of radius 2.0 m when it moves at 1.9 m/s on an icy surface. Centripetal Force: F = mv^2/r
2.9 N
Pretend you stood atop a ladder so tall that you were 6 times as far from Earth's center as you presently are. Find the ratio of your weight on the Earth surface to the weight at the top of the ladder.
36
Calculate the work done when a force of 8.0 N moves a book 1.1 mm. (1 N⋅m = 1 J) Work = force × distance
8.8 J
Calculate the torque produced by a 40-N perpendicular force at the end of a 0.22m long wrench. Torque = Lever Arm × Force
8.8 N*m
Rank their orbital speed from greatest to least.
A > B > C
If a spinning skater pulls her arms in so as to reduce her rotational inertia by half, by how much will her rate of spin increase?
Angular momentum remains the same, while her rate of spin doubles.
At what part of a meterstick should you place your finger so that it will balance in a horizontal position?
At the 50-cm mark
At what point do satellites moving with an elliptical path reach a maximum speed?
At the closest point to the planet
Rank the cups by the amount they depart from a straight-line path (most curved to least curved).
B > C > A
Rank the torque needed to keep the stick steady, from largest to smallest.
B > C > A
Why it is easier to remove an old bolt if we add an extension to the wrench handle?
Because a higher torque is applied
Rank their time to orbit Earth from greatest to least.
C > B > A
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.
How does escape speed from Mars compare with escape speed from Earth?
Escape speed from Mars is less than half that to escape Earth.
Why does the vertical component of velocity for a projectile change with time, whereas the horizontal component of velocity doesn't?
Gravity is a purely vertical force.
Why does the force of gravity change the speed of a satellite in an elliptical orbit?
Gravity slows the satellite as it moves away and speeds it up on its return.
How many meters does a tossed baseball fall beneath a straight-line path in traveling for 1 s? For 2 s?
In 1 second, it falls 5 m beneath the line. For 2 seconds, the ball falls 20 m beneath the line
When the speed of a moving car is doubled, how much more kinetic energy does it have?
It has four times as much.
As distance increases between most of the mass of an object and its center of rotation, how does rotational inertia change?
It increases
Why is centrifugal force in a rotating frame called a "fictitious force"?
It is not a fundamental force of nature. Instead, it is a force that only appears in an accelerating frame of reference.
What is meant by the "lever arm" of a torque?
It is the perpendicular distance from the rotational axis to the line along which the force acts.
What great discovery followed finding perturbations in the planet Uranus?
Neptune was discovered.
Which energy production method does not ultimately depend on the Sun?
Nuclear fission
What is the ultimate source of geothermal energy?
Nuclear power in Earth's interior
Select the correct equations that show that 60 W of power is required to impart 120 J of energy to something in 2 s. Power = work/time
P=W/t=(120J)/(2s)=60W
Select the correct equations that show that the gravitational potential energy of a 1000-kg boulder raised 3.0 m above ground level is 30,000 J.
PE=mgh=(1000kg)(10N/kg)(3.0m)=30,000J
Inertia depends on mass. Does rotational inertia depend on mass also?
Rotational inertia depends on the distribution of mass about the axis of rotation.
How is rotational inertia similar to inertia as studied in previous chapters?
Rotational inertia is the resistance to a change in rotational motion, which is similar to inertia which is a resistance to a change in velocity.
How does speed generally affect the amount of friction between a road and a skidding tire?
Speed has no effect on the friction.
Which has the higher tides: spring tides or neap tides? Why?
Spring tides are higher, because the tides from the Moon and Sun add together.
What is the ultimate source of the energy from fossil fuels, dams, and windmills?
The Sun
What kind of force acts on a ladybug inside a can that is whirled in a circular path?
The can exerts on the ladybug an inward centripetal force.
What is the evidence for saying whether or not work is done on an object?
The evidence is a gain in energy.
Why doesn't the force of gravity change the speed of a satellite in circular orbit?
The force is at a right angle to the velocity.
Why is kinetic energy a constant for a satellite in a circular orbit but not for a satellite in an elliptical orbit?
The force of gravity is perpendicular to the motion in a circular orbit but not in an elliptical orbit.
If two sacks, one twice as heavy as the other, are lifted the same vertical distances in the same time, how does the power required for each compare?
The heavier sack requires twice the power.
What did Kepler discover about the periods of planets and their distances from the Sun?
The period squared was proportional to the distance cubed.
A block is raised a certain distance by pushing it up an incline. How much potential energy does the block have compared to being raised vertically to the same height?
The potential energy is the same.
How does the tapered rim of a wheel on a railroad train rolling along a track allow one part of the rim to roll faster than another part?
The tangential speed is proportional to the radius. The inside of the wheel rolls on a larger radius than the outside of the wheel.
What is the source of energy in sunshine?
Thermonuclear fusion energy
How do clockwise and counterclockwise torques compare when a system is balanced?
They are equal in magnitude and opposite in direction.
When you whirl a can overhead by a string in a circular path, what is the direction of the force exerted on the can?
Toward the center of the circle.
True or false: One watt is the unit of power equivalent to 1 joule per second.
True, 1 W = 1 J/s
Select the correct equations that show that a 400000-J change in kinetic energy occurs for an airplane that is moved 200 m in takeoff by a sustained force of 2000 N.
W=Fd=(2000N)(200m)=400000J
Does twice as much work on a body mean twice as much change in energy?
Yes, it is.
How could your weight be greater than "mg" ? How could your weight be zero? Give examples.
Your weight is greater than "mg" when you are accelerating upward. Your weight is zero when you are in free fall.
What would the magnitude of the gravitational field be anywhere inside a hollow, spherical planet?
Zero N/kg
What did Newton discover about gravity that was not known at the time?
all of the above
When traveling twice as fast your kinetic energy is increased _______.
by four times
The horizontal and vertical components of velocity for a projectile are __________.
independent of each other
For a projectile launched horizontally, the magnitude of the vertical acceleration
is 9.8 m/s2.
The rotational speed of every point of a rotating rigid object _______.
is the same
What are the units of measurement for tangential speed and rotational speed?
m/s for tangential, rpm for rotational
A motorcycle drives up a steeply inclined ramp. The work done on the motorcycle by the Earth's gravitational force is ____.
negative
Kepler discovered that a planet moving in an elliptical orbit will ________________in equal time intervals.
sweep equal areas of space
Fossil fuels, hydroelectric power, and wind power ultimately get their energy from _______.
the Sun
Both the Moon and the Sun produce our ocean tides, but the tides are primarily due to the Moon. Why?
the difference in lunar pulls is greater than the difference in solar pulls
State Newton's law of universal gravitation in words.
the force is proportional to the product of the two masses and inversely proportional to the square of the distance between their centers
How does the force of gravity between two bodies change when the distance between them is halved?
the force quadruples
What happens to the force of attraction between two planets if their masses are doubled and the distance between them is also doubled?
the force would remain the same
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 one does twice the work in twice the time, the power expended is _______.
the same
Two identical cars are driving in opposite directions at the same speed. Their kinetic energies have ____.
the same magnitude and sign
Predict how the upward force exerted on the feet by the scale will compare to the man's weight if the elevator is moving downward at a constant speed.
the upward force on the feet will be equal to the man's weight
Predict how the upward force exerted on the feet by the scale will compare to the weight of the man if the elevator is moving upward at a constant speed.
the upward force on the feet will be equal to the man's weight
Predict how the upward force exerted on the feet by the scale will compare to the man's weight if the elevator is accelerating downward.
the upward force on the feet will be less than the man's weight
Predict how the upward force exerted on the feet by the scale will compare to the man's weight if the elevator is accelerating upward.
the upward force on the feet will exceed the man's weight
Of the four fundamental forces presently known, the force of gravity is
the weakest
Why are occupants of the International Space Station weightless?
they are in free fall
Do tides depend more on the strength of gravitational pull or on the difference in strengths? Explain.
tides depend only on the difference
Recycled energy is mainly _______.
use of energy otherwise wasted
