Ch. 8 Check Your Understanding Flashcards
In the drawing, the flat triangular sheet ABC is lying in the plane of the paper. This sheet is going to rotate about first one axis and then another axis. Both of these axes lie in the plane of the paper and pass through point A. For each of the axes the points B and C move on separate circular paths that have the same radii. Identify these two axes.
Both axes lie in the plane of the paper.
One passes through point A and is parallel to the line BC.
The other passes through point A and the midpoint of the line BC.
Three objects are visible in the night sky. They have the following diameters (in multiples of d) and subtend the following angles (in multiples of theta0) at the eye of the observer. Object A has a diameter of 4d and subtends at an angle of 2theta0. Object B has a diameter of 3d and subtends an angle of theta0/2. Object C has a diameter of d/2 and subtends an angle of theta0/8.
Rank them in descending order (greatest first) according to their distance from the observer.
B, C, A
A pair of scissors is being used to cut a piece of paper in half. Does each blade of the scissors have the same angular velocity (both magnitude and direction) at a given instant?
No, the instantaneous angular speed of each blade is the same, but the blades are rotating in opposite directions
An electric clock is hanging on the wall. As you are watching the second hand rotate, the clocks battery stops functioning, and the second hand comes to a halt over a brief period of time. Which of the following statements correctly describes the angular velocity w and angular acceleration alpha of the second hand as it slows down? A) w and alpha are both negative B) w is positive and alpha is negative C) w is negative and alpha is positive D) w and alpha are both positive
C
The blades of a ceiling fan start from rest and, after two revolutions, have an angular speed of 0.50 rev/s. The angular acceleration of the blades is constant. What is the angular speed after eight revolutions?
1.0 rev/s
Equation theta=(w0)(t)+(1/2)alpha(t^2) is being used to solve a problem in rotational kinematics. Which one of the following sets of values for the variables w0, alpha, and t cannot be substituted directly into this equation to calculate a value for theta?
A) w0=1.0rad/s, alpha=1.8rad/s^2, t=3.8s
B) w0=0.16rev/s, alpha=1.8rad/s^2, t=3.8s
C) w0=0.16rev/s, alpha=0.29rev/s^s, t=3.8s
B
A thin rod rotates at a constant angular speed. In case A the axis of rotation is perpendicular to the rod at its center. In case B the axis is perpendicular to the rod at one end. In which case, if either, are there points on the rod that have the same tangential speed?
Case A
It is possible to build a clock in which the tips of the hour hand and the second hand move with the same tangential speed. This is normally never done, however. Why?
The length of the hour hand would be 3600 times greater than the length of the second hand.
The earth rotates once per day about its axis, which is perpendicular to the plane of the equator and passes through the north geographic pole. Where on the earths surface should you stand in order to have the smallest possible tangential speed?
At the North Pole or the south pole
A building is located on the earths equator. As the earth rotates about its axis, which floor of the building has the greatest tangential speed?
A) the first floor
B) the tenth floor
C) the twentieth floor
C
The blade of a lawn mower is rotating at an angular speed of 17 rev/s. The tangential speed of the outer edge of the blade is 32 m/s. What is the radius of the blade?
0.30m
A car is up on a hydraulic lift at a garage. The wheels are free to rotate, and the drive wheels are rotating with a constant angular velocity. Which one of the following statements is true?
A) a point on the rim has no tangential and no centripetal acceleration
B) a point on the rim has both a nonzero acceleration and a nonzero centripetal acceleration
C) a point on the rim has a nonzero tangential acceleration but no centripetal acceleration
D) a point on the rim has no tangential acceleration but does have a nonzero centripetal acceleration
D
Section 5.6 discusses how the uniform circular motion of a space station can be used to create artificial gravity. This can be done by adjusting the angular speed of the space station, so the centripetal acceleration at an astronauts feet equals g, the magnitude of the acceleration due to the earths gravity. If such an adjustment is made, will the acceleration at the astronauts head due to the artificial gravity be a) greater than, b) equal to, or c) less than g?
C
A bicycle is turned upside down, the front wheel is spinning, and there is an angular acceleration. At the instant shown, there are six points on the wheel that have arrows associated with them. Which one of the following quantities could the arrows NOT represent?
A) tangential velocity
B) centripetal acceleration
C) tangential acceleration
B
A rotating object starts from rest and has a constant angular acceleration. Three seconds later the centripetal acceleration of a point in the object has a magnitude of 2.0m/s^2. What is the magnitude of the centripetal acceleration of this point six seconds after the motion begins?
8.0 m/s^2
The speedometer of a truck is set to read the linear speed of the truck, but uses a device that actually measures the angular speed of the rolling tires that came with the truck. However, the owner replaces the tires with larger-diameter versions. Does the reading on the speedometer after the replacement give a speed that is a) less than, b) equal to, or c) greater than the true linear speed of the truck?
A
Rolling motion is an example that involves rotation about an axis that is not fixed. Give three other examples of rotational motion about an axis that is not fixed.
- the motions of a frisbee through the air
- the earth in its orbit
- a twirling baton that has been thrown into the air
- the blades on a moving lawn mower cutting the grass
- an ice skater performing a quadruple jump
The moon is 3.85x10^8 m from earth and has a diameters of 3.48x10^6 m. You have a pea (diameter= 0.50cm) and a dime (diameter= 1.8cm). You close one eye and hold each object at arms length (71cm) between your open eye and the moon. Which objects, if any, completely cover your view of the moon? Assume that the moon and both objects are sufficiently far from your eye that the given diameters are equal arc lengths when calculating angles.
The dime does.
Theta = arc length/radius
A rotating object has an angular acceleration of alpha=0rad/s^2. Which one or more of the following three statements is consistent with a zero angular acceleration?
A) the angular velocity is w=0rad/s at all times
B) the angular velocity is w=10rad/s at all times
C) the angular displacement theta has the same value at all times.
A, B, and C
A wheel rotates with a constant angular speed w. Which one is true concerning the angular acceleration alpha of the wheel, the tangential acceleration aT of a point on the rim of the wheel, and the centripetal acceleration act of a point on the rim?
Alpha = 0 rad/s^2
AT = 0 m/s^2
Ac can’t equal 0 m/s^2
