chapter8 Flashcards
What are the units of measurement for tangential speed? For rotational speed?
m/s for tangential, RPM for rotational
On a rotating turntable, how do tangential speed and rotational speed vary with distance from the center?
Tangential speed increases with distance. Rotational speed is constant.
Rotational inertia about the midpoint of an object becomes greater with _______.
increased mass and increased distance to mass concentration
Inertia depends on mass; rotational inertia depends on mass and something else. What?
The distribution of mass about the axis of rotation
How does the lever arm change if you decrease the angle of the force?
It decreases
How does the lever arm change when you decrease the distance to the nut?
it decreases
How does the force needed to turn the wrench change if you increase the lever arm?
it decreases
What does a torque tend to do to an object?
Torque tends to twist or change the state of rotation of the object.
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.
How do clockwise and counterclockwise torques compare when a system is balanced?
They are equal in magnitude and opposite in direction.
The center of gravity of a basketball is located _______.
at its geometrical center
If you toss a stick into the air, it appears to wobble all over the place. Specifically, about what place does it wobble?
It will rotate about the center of mass.
What is the relationship between the center of gravity and the support base for an object that is in stable equilibrium?
A vertical line through the center of gravity must pass inside the support base of the object
Why doesn’t the Leaning Tower of Pisa topple over?
A vertical line through the center of gravity passes inside its support base
When a rock tied to a string is whirled in a horizontal circle, doubling the speed _______.
quadruples the tension in the string
When you whirl a can at the end of a string in a circular path, what is the direction of the force you exert on the can?
Toward the center of the circle
If the string that holds a whirling can in its circular path breaks, what kind of force causes it to move in a straight- line path: centripetal, centrifugal, or no force? What law of physics supports your answer?
No force; Newton’s first law
If you are not wearing a seat belt in a car that rounds a curve, and you slide across your seat and slam against a car door, what kind of force is responsible for your slide: centripetal, centrifugal, or no force?
There is no force as viewed by someone outside the car. To them you move in a straight line
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
How can gravity be simulated in an orbiting space station?
- Rotate a cylinder to create centrifugal force as viewed by a person on the inside of the curved outer wall
- Spin two pods connected by a cable
- Spin a station shaped like a bicycle wheel
Distinguish between linear momentum and angular momentum
Angular momentum depends on the distribution of mass, whereas linear momentum depends on the total mass.
What is the law of inertia for rotating systems in terms of angular momentum?
Angular momentum does not change without an applied external torque.
Consider someone in a rotating space habitat. Rotational motion provides
a support force
What is required to change the angular momentum of a system?
external torque
When the rotational speed of a rotating system doubles, its angular momentum _______.
doubles
If a skater who is spinning pulls her arms in so as to reduce her rotational inertia by half, by how much will her angular momentum change? By how much will her rate of spin change?
Her angular momentum does not change. Her spin rate doubles
Your pet hamster sits on a record player that has constant angular speed. If the hamster moves to a point twice as far from the center, then its linear speed
doubles
The circumference of a bicycle wheel is 2 meters. If it rotates at 1 revolution per second then its linear speed is
2 m/s
Consider a string with several rocks tied along its length at equally spaced intervals. You whirl the string overhead so that the rocks follow circular paths. Compared with a rock at the end of the string, a rock at the middle moves
half as fast
The net force exerted on a car traveling in a circular path at constant speed is
directed toward the center of the curve
If you’re on a Ferris wheel at a carnival, seated 10 m from the Ferris wheel’s axis that makes a complete rotation each minute, your linear speed is
62.8 m/min.
The rotational inertia of your leg is greater when your leg is
straight
A ring and a disk both at rest roll down a hill together. Which rolls slower?
ring
A ring, a disk, and a solid sphere begin rolling down a hill together. Which reaches the bottom first?
sphere
A vertically-held sledge hammer is easier to balance when the heavier end is
at the top, away from your hand.
Which tin can will roll down an incline in the shortest time, one filled with water or one filled with ice? (Hint: water ‘slides’ inside the can.)
with water
Compared with a force, a torque involves
- distance from an axis of rotation.
- leverage.
- rotation.
A torque acting on an object tends to produce
rotation
A ball that gains speed as it rolls down a hill experiences a
net force and net torque
If you place a pipe over the end of a wrench when trying to rotate a stubborn bolt, effectively making the wrench handle twice as long, you’ll multiply the torque by
two
For a system in mechanical equilibrium, the resultant
force must be zero.
torque must be zero
Two people are balanced on a seesaw. If one person leans inward toward the center of the seesaw, that person’s end of the seesaw tends to
rise
To rotate a stubborn screw, it is best to use a screwdriver that has a
wide-handle
On a balanced seesaw, a boy three times as heavy as his partner sits
1/3 the distance from the fulcrum
A solitary boy cannot balance on a seesaw with its fulcrum at its midpoint. If the fulcrum is moved to one-quarter the distance from the boy, balance is achieved when the weight of the boy is
more than the weight of the seesaw
Toss a baseball bat into the air and it wobbles about its
center of mass
The famous Leaning Tower of Pisa doesn’t topple over because its center of gravity is
above a place of support
The center of mass of a human body is located at a point that
changes as a person bends over
To kick a football so it won’t topple end over end, kick it so the force of impact extends
through its center of gravity
A 1-kg rock is suspended from the tip of a horizontal meterstick at the 0-cm mark so that the meterstick barely balances like a seesaw when its fulcrum is at the 25-cm mark. From this information, the mass of the meterstick is
1 kg
You can safely stand on the overhanging end of a heavy plank that rests on a table. How much overhang depends on your mass and the plank’s mass. If you can stand on the end of a plank that overhangs the edge of the supporting table 1/4 its total length, how massive is the plank compared to your mass?
the same
Centripetal force does no work on a circularly-moving object because
centripetal force has no component in the direction of motion
Centrifugal forces are an apparent reality to observers in a reference frame that is
rotating
Multiple the equation for linear momentum by radial distance r and you have
angular momentum
If the rotational speed of a rotating habitat in space increases, the weight experienced by occupants
increases
When a twirling ice skater brings her arms inward, her rotational speed
increases
The chef at the infamous Fattening Tower of Pizza tosses a spinning disk of uncooked pizza dough into the air. The disk becomes wider during its flight, while its rotational speed
slows
As a huge rotating cloud of particles in space gravitate together forming an increasingly dense ball, it shrinks in size and
rotates faster
As the polar icecaps melt, the resulting water spreads over the entire Earth. This new mass distribution tends to make the length of a day
longer
Calculate the tension in a horizontal string that whirls a 1.5- kg toy in a circle of radius 2.5 m when it moves at 3.5 m/s on an icy surface.
7.4 N
Calculate the torque produced by a 48-
N force when a pipe extends the length of the wrench to 0.60 m
29 Nm
Why can’t you stand with your heels and back to a wall and then bend over to touch your toes and return to your stand-up position?
A vertical line through the center of gravity of your body is in front of the tips of your toes, which is outside your support base.
If a turntable’s rotational speed is doubled, then the linear speed of a pet hamster sitting on the edge of the record will
double
Which of these animals has a quicker stride?
mouse
Which jar will roll down an incline in the shortest time, an empty one or one filled with peanut butter?
filled jar
