Ch. 9 Check Your Understanding Flashcards
The drawing shows an overhead view of a horizontal bar that is free to rotate about an axis perpendicular to the page. Two forces act on the bar, and they have the same magnitude. However, one force is perpendicular to the bar, and the other makes an angle theta with respect to it. The angle can be 90, 45, or 0. Rank the values of theta according to the magnitude of the net torque (the sum of torques) that the two forces produce, largest net torque first.
0, 45, 90
Sometimes, even with a wrench, one cannot loosen a nut that is frozen tightly to a bolt. It is often possible to loosen the nut by slipping one end of a long pipe over the wrench handle and pushing at the other end of the pipe. With the aid of the pipe, does the applied force produce a smaller torque, a greater torque, or the same torque on the nut?
Greater torque
Is it possible a) for a large force to produce a small, or even zero, torque, and b) for a small force to produce a large torque?
A) yes, if the lever arm is very small
B) yes, of the lever arm is very large
The photograph shows a woman struggling to keep a stack of boxes balanced on a dolly. The woman’s left foot is in the axle of the dolly. Assuming that the boxes are identical, which one creates the greatest torque with respect to the axle?
The box at the far right
Three forces (magnitudes either F or 2F) act on each of the thin, square sheets shown in the drawing. In parts A and B of the drawing, the force labeled 2F acts at the center of the sheet. When considering angular acceleration, use an axis of rotation that is perpendicular to the plane of a sheet at its center. Determine in which drawing
A) the translational acceleration is equal to zero, but the angular acceleration is not equal to zero
B) the translational acceleration is not equal to zero, but the angular acceleration is equal to zero
C) both the translational and angular accelerations are zero
A. C
B. A
C. B
The free-body diagram in the drawing shows the forces that act on a thin rod. The three forces are drawn to scale and lie in the plane of the paper. Are these forces sufficient to keep the rod in equilibrium, or are additional forces necessary?
Additional forces are necessary
Starting in the spring, fruit begins to grow on the outer end of a branch on a pear tree. As the fruit grows, does the center of gravity of the pear-growing branch a) move toward the pears at the end of the branch, b) move away from the pears, or c) not move at all?
A
The drawing shows a wine rack for a single bottle of wine that seems to defy common sense as it balances on a tabletop. Where is the center of gravity of the combined wine rack and bottle of wine located?
Directly above the point where the wine rack touches the tabletop
Bob and bill have the same weight and wear identical shoes. When they both keep their feet flat on the floor and their bodies straight, bob can lean forward father than bill can before falling. Other things being equal, whose center of gravity is closer to the ground when both are standing erect?
Bob
Three massless rods (A, B, and C) are free to rotate about an axis at their left end. The same force F is applied to the right end of each rod. Objects with different masses are attached to the rods, but the total mass (3m) of the objects is the same for each rod. Rank the angular acceleration of the rods, largest to smallest.
A, B, C
A flat triangular sheet of uniform material is shown in the drawing. There are three possible axes of rotation, each perpendicular to the sheet and passing through one corner, A, B, or C. For which axis is the greatest net external torque required to bring the triangle up to an angular speed of 10.0rad/s in 10.0s, starting from rest? Assume that the net torque is kept constant while it is being applied.
Axis B
At a given instant an object has an angular velocity. It also has an angular acceleration due to torques that are present. Therefore, the angular velocity is changing. Does the angular velocity at this instant increase, decrease, or remain the same a) if additional torques ads applied so as to make the net torque suddenly equal to zero and B) of all the torques are suddenly removed?
A) remains the same
B) remains the same
The space probe in the drawing is initially moving with a constant translational velocity and zero angular velocity.
A) when the two engines are fired, each generating a thrust of magnitude T, does the translational velocity increase, decrease, or remain the same?
B) does the angular velocity increase, decrease, or remain the same?
A) remains the same
B) increases
Two uniform solid balls are placed side by side at the top of an incline plane, and starting from rest, are allowed to roll down the incline. Which ball, if either, has the greater translational speed at the bottom if a) they have the same radii, but one is more massive than the other and B) they have the same mass, but one has a larger radius?
A) both have the same translational speed
B) both have the same translational speed
A thin sheet of plastic is uniform and has the shape of an equilateral triangle. Consider two axes for rotation. Both are perpendicular to the plane of the triangle, axis A passing through the center of the triangle and axis B passing through one corner. If the angular speed w about each axis is the same, for which axis does the triangle have the greater rotational kinetic energy?
Axis B
A hoop, a small cylinder, a spherical shell, and a solid sphere are placed at rest at the top of an incline. All the objects have the same radius. They are then released at the same time. What is the order in which they will reach the bottom?
Solid sphere, solid cylinder, spherical shell, hoop
A woman is sitting on the spinning seat of a piano stool with her arms folded. Ignore any friction in the spinning stool. What happens to her a) angular velocity and b) angular momentum when she extends her arms outward?
A) decreases
B) remains the same
Suppose the ice cap at the South Pole were to melt and the water were distributed uniformly over the earths oceans. Would the earths angular velocity increase, decrease, or remain the same?
Decrease
A cloud of interstellar gas is rotating. Because the gravitational force pulls the gas particles together, the cloud shrinks, and, under the right conditions, a star may ultimately be formed. Would the angular velocity of the star be less than, equal to, or greater than the angular velocity of the rotating gas?
Greater than
A person is hanging motionless from a vertical rope over a swimming pool. She lets go of the rope and drops straight down. After letting go, is it possible for her to curl into a ball and start spinning?
No
The wheels on a moving bicycle have both translational (or linear) and rotational motions. What is meant by the phrase “a rigid body, such as a bicycle wheel, is in equilibrium.”?
The body can have translational and rotational motions, as kind as it’s translational acceleration and angular acceleration are zero
The drawing shows a top view of a square box lying on a frictionless floor. Three forces act on the box. Consider an angular acceleration with respect to an axis through the center of the box.
The box will have an angular acceleration but not a translational acceleration
The same force F is applied to the edge of two hoops. The hoops have the same mass, whereas the radius of the larger hoop is twice the radius of the smaller one. The entire mass of each hooo is concentrated at its rim, so the moment of inertia Jus I= Mr^2, where M is the mass and r is the radius. Which hooo has the greater angular acceleration, and how many times as great is it compared to the angular acceleration of the other hoop?
The smaller hoop; two times as great
Two hoops, starting from rest, roll down identical inclined planes. The work done by nonconservative forces, such as air resistance, is zero. Both have the same mass M, but one hoop has twice the radius of the other. The moment for inertia is I=Mr^2. Which hoop, if either, has the greater total kinetic energy at the bottom of the incline?
Both have the same total kinetic energy
