chap 8
On a balanced seesaw, a boy three times as heavy as his partner sits
1/3 the distance from the fulcrum.
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.
Calculate the tension in a horizontal string that whirls a 1.9-kg toy in a circle of radius 2.9 m when it moves at 3.2 m/s on an icy surface.
6.7 N
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.
A ring and a disk both at rest roll down a hill together. Which rolls slower?
A ring and a disk both at rest roll down a hill together. Which rolls slower? depends on the masses disk ring both roll at the same speed need more information A RING
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.
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 required to change the angular momentum of a system?
External torque
How does the force needed to turn the wrench change if you increase the lever arm?
It decreases.
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.
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
Inertia depends on mass; rotational inertia depends on mass and something else. What?
The distribution of mass about the axis of rotation
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.
How do clockwise and counterclockwise torques compare when a system is balanced?
They are equal in magnitude and opposite in direction.
What does a torque tend to do to an object?
Torque tends to twist or change the state of rotation of the object.
Consider someone in a rotating space habitat. Rotational motion provides _______.
a support force
Multiple the equation for linear momentum by radial distance r and you have
angular momentum.
A vertically-held sledge hammer is easier to balance when the heavier end is
at the top, away from your hand.
Toss a baseball bat into the air and it wobbles about its
center of mass.
Centripetal force does no work on a circularly-moving object because
centripetal force has no component in the direction of motion.
The center of mass of a human body is located at a point that
changes as a person bends over.
The net force exerted on a car traveling in a circular path at constant speed is
directed toward the center of the curve.
Compared with a force, a torque involves
distance from an axis of rotation. rotation. leverage. all the above
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.
When the rotational speed of a rotating system doubles, its angular momentum _______.
doubles
For a system in mechanical equilibrium, the resultant
force must be zero. torque must be zero. both of these
Rotational inertia about the midpoint of an object becomes greater with _______.
increased mass and increased distance to mass concentration
Which of these animals has a quicker stride?
mouse
A ball that gains speed as it rolls down a hill experiences a
net force. net torque. both of these
When a rock tied to a string is whirled in a horizontal circle, doubling the speed _______.
quadruples the tension in the string
Which jar will roll down an incline in the shortest time, an empty one or one filled with peanut butter?
the filled jar
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
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.
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
To rotate a stubborn screw, it is best to use a screwdriver that has a
wide handle.
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
Calculate the torque produced by a 48-N force when a pipe extends the length of the wrench to 0.56
τ = 27 N⋅m
What is the law of inertia for rotating systems in terms of angular momentum?
Angular momentum does not change without an applied external torque.
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.
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.
How can gravity be simulated in an orbiting space station?
Spin a station shaped like a bicycle wheel Spin two pods connected by a cable Rotate a cylinder to create centrifugal force as viewed by a person on the inside of the curved outer wall All of the above
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.
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
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.
If the rotational speed of a rotating habitat in space increases, the weight experienced by occupants
increases.
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
What are the units of measurement for tangential speed? For rotational speed?
m/s for tangential, RPM for rotational
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.
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
As a huge rotating cloud of particles in space gravitate together forming an increasingly dense ball, it shrinks in size and
rotates faster.
Centrifugal forces are an apparent reality to observers in a reference frame that is
rotating
A torque acting on an object tends to produce
rotation.
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.
A ring, a disk, and a solid sphere begin rolling down a hill together. Which reaches the bottom first?
sphere
The rotational inertia of your leg is greater when your leg is
straight
