PHYS 207 Exam 3 iClicker & Concept Questions
A water skier is pulled by a boat traveling with a constant velocity. Which one of the following statements is FALSE concerning this situation?
There is a net horizontal force on the skier in the direction of the boat's velocity.
Which one of the following descriptions indicates that the object is in translational equilibrium?
Translational equilibrium occurs if the object is at rest or moving with constant velocity.
A ball is thrown vertically upward in the air. Which one of the following quantities is equal to zero at the highest point of the ball's trajectory?
kinetic energy
Is it correct to conclude that one engine is doing twice the work that another is doing just because it is generating twice the power
no
For the ballistic bullet problem from last time, what are the values for the initial and final velocities of the system, vo1 and vf ? vo1 is the velocity of the bullet and vf is the velocity of block and bullet together
vo1 > vf ; vf ≈ 4 m/s
A multiple-stage rocket is moving in outer space with a momentum of +150,000 kg · m/s. There are no external forces acting on the rocket. The rocket then separates into two stages, each of which moves along the same direction (either forward or backward) as the initial rocket. The table lists five possibilities for the momenta of the two stages. There is only one possibility that can be correct. Which is it?
+250,000 kgm/s, -100,000 kgm/s
Find the work done if the force is 45.0-N, the angle is 50.0 degrees, and the displacement is 75.0 m.
2170 Joules
As the name implies, the impulse-momentum theorem provides a relationship between impulse and momentum. Which one of the following statements correctly describes that relationship?
An impulse is equal to the change in an object's momentum that occurs when a net force acts on it.
Under what condition(s) is the angular momentum of a rotating body, such as a spinning ice skater, conserved?
Each external torque may be non-zero, but the sum of the torques must be zero.
What happens when a spinning ice skater draws in her outstretched arms?
Her moment of inertia decreases causing her to speed up
Two blocks are placed at the ends of a horizontal massless board, as in the drawing. The board is kept from rotating and rests on a support that serves as an axis of rotation. The block on the right has a mass of 2.6 kg. Determine the magnitude of the angular acceleration when the system is allowed to rotate (inertia)
I = 9.4 kg.m2
Which one of the following statements most accurately describes the center of gravity of an object?
It is the point from which the torque produced by the weight of the object can be calculated.
Which one of the following statements is true concerning momentum and impulse?
Momentum and impulse are measured in the same units.
A suitcase is hanging straight down from your hand as you ride an escalator. Your hand exerts a force on the suitcase, and this force does work. This work is
Positive when you ride up; negative when you ride down
During a certain process, the linear momentum of a system is conserved. Which one of the following statements concerning this system is correct?
The vector sum of the average external forces acting on the system is equal to zero newtons.
Which one of the following quantities is equal to the change in momentum of an object during a collision?
impulse
Air resistance is a non-conservative force. It always opposes the motion of an object. An airplane flies from New York to Atlanta and then returns to its point of departure. The net work done by air resistance during this round trip ___________.
is negative
A man stands on a platform that is rotating (without friction) with an angular speed of 7.5 rad/s; his arms are outstretched and he holds a brick in each hand. The rotational inertia of the system consisting of the man, bricks, and platform about the central vertical axis of the platform is 6.0 kgm2 . By moving the bricks the man decreases the rotational inertia of the system to 2.0 kgm2 . Part A When the man moves the bricks inward, the angular momentum of the system
remains the same
The Work-Energy Theorem and Kinetic Energy
- If work done by NET force is positive: KE of object INCREASES - If work done by NET force is negative: KE of object DECREASES - If work done by NET force is zero: KE remains the SAME
Blocks A and B are moving along the x axis and are heading toward each other. Block A has a mass of 2.0 kg and a velocity of +32 m/s, while block B has a mass of 4 kg and a velocity of −28 m/s, where the plus and minus sign indicate the direction of motion. They suffer a completely inelastic, head-on collision and no external forces act on them. Part A What is the total linear momentum of the system after the collision?
-48 kgm/s
A canoe with two people aboard is coasting with an initial momentum of +110 kg · m/s. Then, one of the people (person 1) dives off the back of the canoe. During this time, the net average external force acting on the system (the canoe and the two people) is zero. The table lists four possibilities for the final momentum of person 1 and the final momentum of person 2 plus the canoe, immediately after person 1 dives off. Only one possibility could be correct. Which one is it?
-60 kgm/s, +170 kgm/s
Blocks A and B are moving along the x axis and are heading toward each other. Block A has a mass of 2.0 kg and a velocity of +32 m/s, while block B has a mass of 4 kg and a velocity of −28 m/s, where the plus and minus sign indicate the direction of motion. They suffer a completely inelastic, head-on collision and no external forces act on them. Part B What is the velocity of the two blocks after the collision?
-8 m/s
In which one of the following situations will there be an increase in kinetic energy?
A fan is switched from "low speed" to "high speed."
Alice and Bill, who happen to have the same mass, both want to climb to the top of a mountain. Alice wants to take the steep path straight up, but Bill wants to take the path that gently winds around the mountain, even though it is 8 times longer than the steep path. They both eventually reach the top of the mountain, but Alice reaches the top in 1/3 the time that Bill takes using the steep route. How does the work that Alice did in climbing the mountain compare with the amount that Bill did? Consider only gravitational work.
Alice and Bill did the same amount of work.
Engine A has a greater power rating that engine B. Which one of the following statements correctly describes the abilities of these engines to do work
Engines A and B can do the same amount of work, but engine A can do it more quickly.
The diver's weight is 530 N and F2 was calculated in the previous slide to be 1480 N. Calculate magnitude of F1?
F1 = 950 N
The figure shows three forces applied to a trunk that moves leftward by 3 m over a frictionless floor. The indicated angle theta=60 degrees. Which force(s) do negative work during the 3 m displacement?
F2
A motorcyclist is trying to leap across the canyon by driving horizontally off a cliff at 38.0 m/s. Ignoring air resistance and only considering gravitational force, find the speed with which the cycle strikes the ground on the other side.
46 m/s
Blocks A and B are moving along the x axis and are heading toward each other. Block A has a mass of 2.0 kg and a velocity of +32 m/s, while block B has a mass of 4 kg and a velocity of −28 m/s, where the plus and minus sign indicate the direction of motion. They suffer a completely inelastic, head-on collision and no external forces act on them. Part C Let KE0 be the total kinetic energy of the two-block system before the collision and KEf be the total kinetic energy of the two-block system after the collision. Which statement is true?
KE0 > KEf
Which one of the following statements concerning the momentum of a system when the net force acting on the system has a positive value is true?
The momentum of the system is increasing.
The same force F is applied to the edge of two hoops. The hoops have the same mass, although the radius of the larger hoop is twice that of the smaller one. The entire mass of each hoop is concentrated at its rim, so the moment of inertia is I = Mr2, where M is the mass and r is the radius. Which hoop has the greater angular acceleration, and how many times as great is it compared to that of the other hoop?
The smaller hoop; two times as great.
Which one of the following statements describes the condition(s) for which a rigid body is in equilibrium?
The sum of the externally applied forces must be zero, and the sum of the externally applied torques must be zero
A collision between two objects is elastic. Which one of the following statements concerning this situation is true?
The total kinetic energy of the objects is the same before and after the collision
Two identical cars are traveling at the same speed. One is heading due east and the other due north, as the drawing shows. Which statement is true regarding the kinetic energies and momenta of the cars?
They have the same kinetic energies, but different momenta
Sometimes, even with a wrench, you 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 it at the other end of the pipe. With the aid of the pipe, the applied force produces a
a greater torque
Complete the following statement: In an isolated system, the total mechanical energy remains constant if
all exerted forces are conservative
Two hoops, starting from rest, roll down identical inclined planes. The work done by nonconservative forces, such as air resistance, is zero (Wnc = 0 J). Both have the same mass M, but, as the drawing shows, one hoop has twice the radius as the other. The moment of inertia for each hoop is I = Mr2, where r is its radius. Which, if either, has the greater total kinetic energy (translational plus rotational) at the bottom of the incline?
both have the same total kinetic energy
A car is traveling at a velocity of 20 m/s. Consider two cases: (1) the car crashes into a brick wall and comes to rest very quickly and (2) the driver applies the brakes and the car gradually comes to a stop. In which case is the AVERAGE FORCE on the car greater?
case 1
A car is traveling at a velocity of 20 m/s. Consider two cases: (1) the car crashes into a brick wall and comes to rest very quickly and (2) the driver applies the brakes and the car gradually comes to a stop. In which case is the impulse on the car greater?
case 1 & 2 tie
Which one of the following provides a basis to distinguish different types of collisions?
conservation of kinetic energy
A thin rod rotates in a circle on a frictionless tabletop. The axis is perpendicular to the length of the rod at one of its ends. A bug standing on the axis decides to crawl out to the other end of the rod. Complete the sentence: As the bug walks along the rod away from the axis of rotation, the angular speed
decreases
Complete the following statement: The net work done by a conservative force acting on an object
depends only on the starting and ending points.
A person is riding on a Ferris wheel. When the wheel makes one complete turn, the net work done on the person by the gravitational force ____________.
is zero
A ball has a speed of 15 m/s. Only one external force acts on the ball. After this force acts, the speed of the ball is 7 m/s. The force does
negative work
Complete the following statement: The center of mass is
the point within an object that moves as if all of the object's mass where located there
Complete the following statement: If negative work is done on a moving object,
the velocity of the object will decrease
Complete the following statement: If positive work is done on a moving object,
the velocity of the object will increase.
Complete the following statement: A force that acts on an object is said to be conservative if
the work it does on the object is independent of the path of the motion.
Two identical automobiles have the same speed, one traveling east and one traveling west. Which of the following statements is correct?
The cars have different momenta, but the same kinetic energy
Which one of the following statements concerning kinetic energy is true?
The kinetic energy of an object always has a positive value
An object, which is considered a rigid body, is in equilibrium. Which one of the following statements is FALSE when determining the forces and torques acting on the object?
The linear acceleration or the angular acceleration of the object may not be equal to zero
Two solid spheres have the same mass, but one is made from lead and the other from wood. How do the moments of inertia of the two spheres compare? Moment of inertia of a solid sphere is given by I = 2/5×MR2
The moment of inertia of the wood sphere is greater than that of the one made of lead.
Momentum may be expressed using which of the following units?
kg x m/s
A sailboat is moving at a constant velocity. Is work being done by a net external force acting on the boat?
no
A hoop is rolling (without slipping) on a horizontal surface so it has two types of kinetic energy: translational kinetic energy and rotational kinetic energy. The entire mass M of the hoop is concentrated at its rim, so its moment of inertia is I = MR2, where R is the radius. What is the ratio of the translational kinetic energy to the rotational kinetic energy?
1
A 1.2 kg ball drops vertically onto a floor, hitting with a speed of 25 m/s. Immediately after the collision, the ball is moving upward with a speed of 10 m/s. The ball is in contact with the floor for 0.02 s. PART B: What is the magnitude of the average force that acts on the ball during the contact?
2100 N
A man stands on a platform that is rotating (without friction) with an angular speed of 7.5 rad/s; his arms are outstretched and he holds a brick in each hand. The rotational inertia of the system consisting of the man, bricks, and platform about the central vertical axis of the platform is 6.0 kgm2 . By moving the bricks the man decreases the rotational inertia of the system to 2.0 kgm2 . Part B What is the resulting angular speed of the platform?
22.5 rad/s
A 1.2 kg ball drops vertically onto a floor, hitting with a speed of 25 m/s. Immediately after the collision, the ball is moving upward with a speed of 10 m/s. The ball is in contact with the floor for 0.02 s. PART A: What is the magnitude of the impulse that acts on the ball during the contact?
42 N.s.
A disk with a rotational inertia of 0.6 kgm2 is initially at rest. A net torque is then applied to the disk, and the disk begins to rotate. What is the work done by the net torque when the disk's speed changes from 0 rad/s to 5 rad/s
7.5 J
The gymnast leaves the trampoline at an initial height of 1.20 m and reaches a maximum height of 4.80 m before falling back down. What was the initial speed of the gymnast?
8.4 m/s
In which one of the following systems is there a decrease in gravitational potential energy?
A girl jumps down from a bed and lands on her feet
Two balls collide in a one-dimensional elastic collision. The two balls constitute a system, and the net external force acting on them is zero. The table shows four possible sets of values for the initial and final momenta of the two balls, as well as their initial and final kinetic energies. Only one set of values is correct. Which set is it?
Ball 1: +9 kgm/s 25 J, +6 kgm/s 18 J Ball 2: +4 kgm/s 15 J, +7 kgm/s 22 J
An object, which is considered a rigid body, is NOT in equilibrium. Which one of the following statements must be true concerning the angular acceleration a and translational acceleration a of the object?
Either a > 0 rad/s2 or a > 0 m/s2 , or both are greater than 0
A satellite explodes in outer space, far from any other body, sending thousands of pieces in all directions. Is the linear momentum of the satellite before the explosion less than, equal to, or greater than the total linear momentum of all the pieces after the explosion?
Equal to total linear momentum of all pieces after the explosion
Two particles collide. If the net external force on the two particle system is zero, the momentum of the two particle system will be conserved
In all types of collisions as long as the system is isolated
In a simulation on earth, an astronaut in her space suit climbs up a vertical ladder. On the moon, the same astronaut makes the exact same climb. Which one of the following statements correctly describes how the gravitational potential energy of the astronaut changes during the climb?
It changes by a greater amount on earth
Which one of the following choices represents the largest kinetic energy?
Mars is moving with a speed of 24.0 km/s in its orbit around the Sun
Which one of the following statements concerning the principle of the conservation of mechanical energy is true?
Mechanical energy is always conserved in situations where external non-conservative forces do no work.
Our solar system was once a cloud of interstellar gas rotating in space. Because gravitational force pulled the gas particles together, the cloud shrank, and ultimately, our Sun (a star) was formed. The angular velocity of the Sun will be
More than the angular velocity of the rotating cloud
Starting in the spring, fruit begins to grow on the outer end of a branch on a pear tree. As the fruit grows, the center of gravity of the peargrowing branch
Moves toward the pears at the end of the branch
Sara carries a heavy box of books down a flight of stairs from the third floor to the first floor. Which one of the following statements best describes the work done on the box by Sara?
Sara does a large amount of negative work on the heavy box of books
Which one of the following is an example of an object with a non-zero kinetic energy
The International Space Station orbits the Earth
A high-jumper, having just cleared the bar, lands on an air mattress and comes to rest. Had she landed directly on the hard ground, her stopping time would have been much shorter. Using the impulse-momentum theorem as your guide, determine which one of the following statements is correct.
The air mattress exerts the same impulse, but a smaller net average force, on the high-jumper than does the hard ground.
A child standing on the edge of a freely spinning merry-go-round moves quickly to the center. Which one of the following statements is necessarily true concerning this event and why?
The angular speed of the system increases because the moment of inertia of the system has decreased.
The wheels on a moving bicycle wheel 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 long as its translational acceleration and angular acceleration are zero.
A force is applied to a doorknob. This force will be most effective in causing the door to rotate when which of the following is true?
The direction of the force is at an angle of 90 degrees with respect to the door.
In which one of the following circumstances does the force do negative work on the object?
The direction of the force is in the opposite direction to the object's displacement.
In which one of the following circumstances does the force do positive work on the object?
The direction of the force is in the same direction as the object's displacement
An arrow is launched straight up from the surface of the Earth. Which one of the following statements describes energy transformation of the arrow as it rises? Neglect air resistance.
The kinetic energy of the arrow decreases as the potential energy increases.
The drawing shows two blocks that are placed at the ends of a massless board. The board is balanced on a support that serves as an axis of rotation. The block on the left has a mass of 5.1 kg. What is the moment of inertia of this system about the axis of rotation?
𝐼 = 1.4 kg.m2
Two blocks are placed at the ends of a horizontal massless board, as in the drawing. The board is kept from rotating and rests on a support that serves as an axis of rotation. The block on the right has a mass of 2.6 kg. Determine the magnitude of the angular acceleration when the system is allowed to rotate (acceleration)
𝛼 = 3.7 rad/s2
The tendon exerts a force of magnitude 720 N. Determine the torque (magnitude and direction) of this force about the ankle joint
𝜏 = 15 N.m.
Is it possible for a large force to produce a small, or even zero, torque?
yes
A 5.0-kg ball on the end of a chain is whirled at a constant speed of 1.0 m/s in a horizontal circle of radius 3.0 m. What is the work done by the centripetal force during one revolution?
zero J
A stone is thrown with a speed v0 and returns to earth, as the drawing shows. Ignore friction and air resistance, and consider the initial and final locations of the stone. Which one of the following correctly describes the change ΔPE in the gravitational potential energy and the change ΔKE in the kinetic energy of the stone as it moves from its initial to its final location?
ΔPE = 0 J and ΔKE = 0 J
