unit 10
I swing a ball around my head at constant speed in a circle with circumference 3 m. What is the work done on the ball by the 10 N tension force in the string during one revolution of the ball? 30 J 20 J 10 J 0 J
0 J
Ball A has half the mass and eight times the kinetic energy of ball B. What is the speed ratio vA/vB? 16 4 2 1/4 1/16
4
Which of the following forces is considered a conservative force?
CORRECT ANSWER: Gravity
The unit of work is The watt. The poise. The hertz. The pascal. The joule.
The joule.
A spring-loaded gun shoots a plastic ball with a launch speed of 2.0 m/s. If the spring is replaced with a new spring having twice the spring constant (but still compressed the same distance), the ball's launch speed will be 1.0 m/s 2.0 m/s 2.8 m/s 4.0 m/s 16.0 m/s
2.8 m/s
Rank in order, from largest to smallest, the gravitational potential energies of the balls. 1 > 2 = 4 > 3 1 > 2 > 3 > 4 3 > 2 > 4 > 1 3 > 2 = 4 > 1
3 > 2 = 4 > 1
A spring-loaded gun shoots a plastic ball with a launch speed of 2.0 m/s. If the spring is compressed twice as far, the ball's launch speed will be 1.0 m/s 2.0 m/s 2.8 m/s 4.0 m/s 16.0 m/s
4.0 m/s
Three balls are thrown from a cliff with the same speed but at different angles. Which ball has the greatest speed just before it hits the ground? Ball A. Ball B. Ball C. All balls have the same speed.
All balls have the same speed.
A tow rope pulls a skier up the slope at constant speed. What energy transfer (or transfers) is taking place? W → Ug W → K W → Eth Both A and B. Both A and C.
Both A and C.
A weightlifter brings a 400-N barbell upward from his shoulders to a point 50 cm higher at a steady speed. During this process, what is the total work done on the barbell?
CORRECT ANSWER: 0 joules
A weightlifter exerts an upward force on a 1000-N barbell and holds it at a height of 1 meter for 2 seconds. Approximately how much power does the weightlifter exert on the barbell during this time?
CORRECT ANSWER: 0 watts
A weightlifter exerts an upward force on a 1000-N barbell and lifts the barbell 1 meter upward in 2 seconds. Approximately how much power does the weightlifter exert on the barbell during this time?
CORRECT ANSWER: 500 watts
In both figures, a particle of mass m is released from rest at a height, h. In figure (a), the particle is dropped straight downward and in figure (b) the particle is released from rest down a frictionless ramp. Which particle, the one in figure (a) or (b), will have more kinetic energy at the bottom?
CORRECT ANSWER: Both particles will have the same kinetic energy at the bottom.
A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. If you push on the block in the negative x-direction to compress the spring and then release the block, what happens to the energy in the system immediately after the block is released?
CORRECT ANSWER: The elastic potential energy in the spring decreases while the kinetic energy of the block increases.
Which of the following forces is considered a conservative force?
CORRECT ANSWER: The force due to a spring
When helping a friend move into a new home, you push a chair across the room. What do you know about the force of gravity applied to the chair?
CORRECT ANSWER: The force of gravity applied to the chair does not change the energy of the system (the chair) and therefore does no work on the system.
When helping a friend move into a new home, you push a chair across the room. What do you know about the force that you exert on the chair?
CORRECT ANSWER: The force you exert on the chair contributes to the overall change in kinetic energy of the system (the chair) with a positive amount and therefore does positive work on the system.
When helping a friend move into a new home, you push a chair across the room. What do you know about the force of friction applied to the chair by the floor?
CORRECT ANSWER: The frictional force applied to the chair contributes to the overall change in kinetic energy of the system (the chair) with a negative amount and therefore does negative work on the system.
A block slides along a rough surface and comes to a stop. What can you conclude about the frictional force exerted on the block?
CORRECT ANSWER: The frictional force does negative work on the block and decreases its kinetic energy.
In both figures, a particle of mass m, is released from rest at a height, h. In figure (a), the particle is dropped straight downward and in figure (b) the particle is released from rest and slides down a ramp with a rough surface. Which particle, the one in figure (a) or (b), will have more kinetic energy at the bottom?
CORRECT ANSWER: The particle in figure (a) will have more kinetic energy than the particle in figure (b) at the bottom.
In figure (b), a particle of mass m is released from rest at the top of a frictionless ramp of height h and in figure (c), a particle of mass 2m is released from rest at the top of the same frictionless ramp. Which particle, the one in figure (b) or (c), will have more kinetic energy at the bottom?
CORRECT ANSWER: The particle in figure (c) will have more kinetic energy than the particle in figure (b) at the bottom.
A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. If you push on the block in the negative x-direction and compress the spring, what is true about the potential energy stored in the spring during this motion?
CORRECT ANSWER: The potential energy in the spring increases because the spring does negative work on the block.
A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. If you push on the block in the negative x-direction and compress the spring, what is true about the work done by the spring on the block during this motion?
CORRECT ANSWER: The spring does negative work on the block because the spring force is in the opposite direction of the block's displacement.
A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. The block can be pushed in the negative x-direction to compress the spring or pulled in the positive x-direction to stretch the spring. Where along the x-axis does the block have to be for the spring to have zero potential energy?
CORRECT ANSWER: The spring has zero potential energy when the block is at x = 0, where the spring is neither stretched nor compressed.
What is true about the work done by a conservative force?
CORRECT ANSWER: The work done by a conservative force is always path independent.
What is true about the work done by a non-conservative force?
CORRECT ANSWER: The work done by a non-conservative force will always change the total mechanical energy of a system.
The graph shows the x-component of a force applied to an object versus the position of that object in the x-direction. How is the work done by this force determined from the data in this graph?
CORRECT ANSWER: The work done by the force is equal to the area under the force versus position function.
When you lift a book upward off of a table, what is true about the work done on the book by the force of gravity?
CORRECT ANSWER: The work done by the force of gravity is negative and proportional to the upward displacement of the book.
You observe two identical balls of putty heading directly toward each other at equal speeds. What can you say about their total kinetic energy?
CORRECT ANSWER: They have twice the kinetic energy of either ball by itself.
In which of these processes is the total energy of the system conserved?
CORRECT ANSWER: Two balls of putty (the system) collide in midair and stick together.
A variety of forces are applied to an object such that the net force does positive work on that object. What can you conclude about the speed of the object?
CORRECT ANSWER: When the total work done on the object is positive, the object's speed will increase.
The mechanical energy of a system is conserved during a certain process only if __________.
CORRECT ANSWER: non-conservative forces do zero total work on the system during that process
If Quantity X is conserved for a certain system and during a certain process, that means__________.
CORRECT ANSWER: quantity X for that system does not change in any way during that process
If a system is isolated, the total energy of the system Increases constantly. Decreases constantly. Is constant. Depends on the work into the system. Depends on the work out of the system.
Is constant.
If you hold a heavy weight over your head, the work you do Is greater than zero. Is zero. Is less than zero. Is converted into chemical energy. Is converted into potential energy.
Is zero.
A hockey puck sliding on smooth ice at 4 m/s comes to a 1-m-high hill. Will it make it to the top of the hill? Yes. No. Can't answer without knowing the mass of the puck. Can't say without knowing the angle of the hill.
No
If you raise an object to a greater height, you are increasing Kinetic energy. Heat. Potential energy. Chemical energy. Thermal energy.
Potential energy.
Starting from rest, a marble first rolls down a steeper hill, then down a less steep hill of the same height. For which is it going faster at the bottom? Faster at the bottom of the steeper hill. Faster at the bottom of the less steep hill. Same speed at the bottom of both hills. Can't say without knowing the mass of the marble.
Same speed at the bottom of both hills.
Which force below does the most work? All three displacements are the same. The 10 N force. The 8 N force The 6 N force. They all do the same work.
The 6 N force.
A skier is gliding down a gentle slope at a constant speed. What energy transformation is taking place? K → Ug Ug → K Eth → K Ug → Eth K → Eth
Ug → Eth
A child is on a playground swing, motionless at the highest point of his arc. What energy transformation takes place as he swings back down to the lowest point of his motion? K → Ug Ug → K Eth → K Ug → Eth K → Eth
Ug → K
A crane lowers a girder into place at constant speed. Consider the work Wg done by gravity and the work WT done by the tension in the cable. Which is true? Wg > 0 and WT > 0 Wg > 0 and WT < 0 Wg < 0 and WT > 0 Wg < 0 and WT < 0 Wg = 0 and WT = 0
Wg > 0 and WT < 0
Which of the following is an energy transfer? Kinetic energy Work Potential energy Chemical energy Thermal energy
Work
A light plastic cart and a heavy steel cart are both pushed with the same force for a distance of 1.0 m, starting from rest. After the force is removed, the kinetic energy of the light plastic cart is ________ that of the heavy steel cart. greater than equal to less than Can't say. It depends on how big the force is.
equal to
Robert pushes the box to the left at constant speed. In doing so, Robert does ______ work on the box. positive negative zero
positive
A small child slides down the four frictionless slides A-D. Rank in order, from largest to smallest, her speeds at the bottom. vD > vA > vB > vC vD > vA = vB > vC vC > vA > vB > vD vA = vB = vC = vD
vA = vB = vC = vD
