Chapter 6 physics
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?
!!!!! Both particles will have the same kinetic energy at the bottom.
Immediately after lunch, for each case, rank from greatest to least the speed of the formerly hungry fish. Rank from greatest to least. To rank items as equivalent, overlap them.
(5x3=15)-(1x3=3)=12 Least (5x4=20)-(1x4=4)=16 Greatest (5x4=20)-(1x6=6)=14 Middle
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?
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?
0 watts
Suppose we repeat the experiment from the video, but this time we use a rocket three times as massive as the one in the video, and in place of water we use a fluid that is twice as massive (dense) as water. If the new fluid leaves the rocket at the same speed as the water in the video, what will be the ratio of the horizontal speed of our rocket to the horizontal speed of the rocket in the video after all the fluid has left the rocket? (Ignore air resistance.)
2/3
What is the momentum of an 8.0-kg bowling ball rolling at 2.6 m/s ? Express your answer to two significant figures and include the appropriate units.
20.8 (or 21 in significant figures) kg x m/s
A 3.6-kg chihuahua charges at a speed of 3.3m/s. What is the magnitude of the average force needed to bring the chihuahua to a stop in 0.50s? Express your answer to two significant figures.
24N
When the useful energy output of a simple machine is 100 J, and the total energy input is 200 J, the efficiency is _______.
50 %.
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?
500 watts
In a simple machine, how much work is done when an input of 10 N acts over a distance of 5 m?
50J
Shown below is a graph of a force applied to a small object as a function of time. If the object has a mass of 5.0kg and is at rest at t=0s, how fast is the object moving at t=4.0s?
7.2 m/s
In the opening scenes of the pre-lecture video, we outlined some of the key properties of momentum. Which of the following statements are consistent with these properties?
An object's momentum is equal to the product of its mass and its velocity. Momentum is a vector quantity.
A 1700kg rhino charges at a speed of 50.0km/h. What is the magnitude of the average force needed to bring the rhino to a stop in 0.50s? Express your answer to two significant figures.
Convert km/hr into seconds first. v=(50km/hr x 1hr/60min x 1min/60sec = 50km/3600s = 0.013889km/s) a=Average Velocity / average time a=0.0138km/s / 0.5s = 0.027m/s^2 F=ma F=(1700kg)(0.027m/s^2)= 47.2N I DO NOT know why here but multiply 47.2 by 1,000, giving you 47,200. 2 significant figures equates to two total, countable numbers. (After the first number is listed, all numbers count as significant figures. Move the decimal point over and multiply the new number by 10 if you have to use less significant figures than allowed. "multiplied by 10^8" does not count as any significant figures.) 47,200 can be written as "4.72 x 10^4", however, 4, 7, and 2 are three rather than two significant figures, so we must round the 7 up or down based on the 2 in 4.72. Two is closer to zero so we will leave the 7 alone. Now we have "4.7 x 10^4 N", which is our final answer.
Find the force acting on the egg when it hits the sheet. Express your answer in terms of the variables m, v, and t.
F=m(v/t)
The work that is done when twice the load is lifted twice the distance is _______.
Four times as much
Which of the following forces is considered a conservative force?
Gravity
Imagine that you replace the block in the video with a happy or sad ball identical to the one used as a pendulum, so that the sad ball strikes a sad ball and the happy ball strikes a happy ball. The target balls are free to move, and all the balls have the same mass. In the collision between the sad balls, how much of the balls' kinetic energy is dissipated?
Half of it
How does Dr. Hewitt break the piece of wood?
He causes a change in momentum of his hand over a short period of time.
How is Dr. Hewitt able to break a piece of wood in his demonstration?
He moves his hand very quickly.
What would Dr. Hewitt need to have done to exert an even greater force than he did in his karate demonstration?
Increase the change in momentum, and decrease the time duration.
In which is momentum conserved: an elastic collision or an inelastic collision?
Momentum is conserved in elastic and inelastic collisions.
Now, consider the collision between two happy balls described in Part A. How much of the balls' kinetic energy is dissipated?
None of it
Which energy production method does not ultimately depend on the Sun?
Nuclear fission
What impulse occurs when an average force of 15 N is exerted on a cart for 2.9 s ?
Overall, we are looking to solve for p (momentum) in p=mv. p's unit is kg x m/s We have a Force equal to 15N. F=15N. F=ma 15=ma We have an average time, 2.9 seconds. We can assume acceleration is equal to 10m/s^2 because we were not given a velocity or an acceleration and we were not told this was happening off of Earth's surface. Plug and chug for the mass. 15=m x 10 15/10=m 1.5=m We are still missing a velocity, but we have an acceleration and a time. So, using the acceleration formula, a=average v/ average t, we have a=10 and t=2.9 10=AVERAGE v/2.9 10 x 2.9 = AVERAGE v 29= Average v. now, we have mass and velocity. We can plug and chug for momentum. p=mv p=1.5 x 29 = 43.5 kg x m/s. Finally, we must put this into 2 significant figures. We currently have 3. 43.5 rounds up to 44, so our final answer is 44 kg x m/s
Bumper cars A and B undergo a collision during which the momentum of the combined system is conserved. Which equation(s) correctly states the principle of conservation of momentum? Check all that apply.
P⃗ A,i+P⃗ B,i=P⃗ A,f+P⃗ B,f ΔP⃗ A+ΔP⃗ B=0
Suppose a ball of putty moving horizontally with 1 kg·m/s of momentum collides with and sticks to an identical ball of putty moving vertically with 1 kg·m/s of momentum. What is the magnitude of their combined momentum?
Since they are same-magnitude vectors at right angles to each other, the combined momentum is √2 kg m/s, which is approximately equal to 1.41 m/s.
The video shows an animated billiards experiment in which a cue ball strikes a glued-in-place eight-ball. Which of the following explains why the momentum of the eight-ball is conserved?
The "glue force" cancels the collision force.
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?
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?
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?
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?
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?
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?
The frictional force does negative work on the block and decreases its kinetic energy.
Which statement must be true for the momentum of a system to be conserved?
The net external force on the system is zero.
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? Figure (a) shows a particle of mass m on the edge of a vertical cliff of height h. Figure (b) shows a particle of mass m at the top of a straight, shallow slope of height h.
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?
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?
The potential energy in the spring increases because the spring does negative work on the block.
While goofing off at the ice skating rink, a student takes off her shoes and places each of them on the ice. Her friend, a hockey player, then shoots a hockey puck at each shoe. The first puck immediately comes to rest after it collides with the left shoe. The second puck rebounds after it collides with the right shoe. If each hockey puck has the same incoming speed, which shoe has greater speed after the collision?
The right shoe
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?
The spring has zero potential energy when the block is at x = 0, where the spring is neither stretched nor compressed.
Predict what will happen if you reduce the magnitude of the velocity of the green truck to a magnitude smaller than that of the red truck.
The trucks will stick together after the collision and move to the left.
Click "Reset." Now, predict what will happen if you change the velocity of the red truck so that it has a magnitude smaller than that of the green truck but is negative.
The trucks will stick together after the collision and move to the right.
Click "Reset." Now, predict what will happen if you change the velocity of the red truck so that it has a magnitude smaller than that of the green truck but is positive.
The trucks will stick together after the collision and move to the right.
Click "Reset." Now, predict what will happen if you increase the mass of the green truck so that it is greater than the mass of the red truck.
The trucks will stick together after the collision and move to the right.
Before you click "Play," predict what will happen if the red and green truck with equal mass and equal magnitude of velocity collide.
The trucks will stick together after the collision and not move.
What is true about the work done by a conservative force?
The work done by a conservative force is always path independent.
What is true about the work done by a non-conservative force?
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?
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?
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?
They have twice the kinetic energy of either ball by itself.
In which of these processes is the total energy of the system conserved?
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?
When the total work done on the object is positive, the object's speed will increase.
Suppose we replace both hover pucks with pucks that are the same size as the originals but twice as massive. Otherwise, we keep the experiment the same. Compared to the pucks in the video, this pair of pucks will rotate
at the same rate
When traveling twice as fast your kinetic energy is increased _______.
by a factor of 4
Energy cannot be _______.
destroyed
A boxer rides with the punch so as to reduce _______.
force
When you lift twice the load twice as high, in half the time, the increase in potential energy is _______.
four times
Cassy can get more force on the bricks she breaks with a blow of her bare hand when _______.
her hand is made to bounce from the bricks.
A slowly moving ship can have a greater momentum than a fast-moving racing car when _______.
its mass times velocity is greater than that of the car.
A 5.8-kg fish swimming 1.4 m/s swallows an absentminded 1.0-kg fish swimming toward it at a speed that brings both fish to a halt immediately after lunch. Find the speed of the approaching smaller fish before lunch. Express your answer to two significant figures and include the appropriate units.
m1=5.8kg v1=1.4m/s m2=1kg v2=? both speeds = 0m/s when combined. 2 sig figs 5.8x1.4=1x? 8.12=1? 8.12=1x8.12 8.12=8.12 2 SIG FIGS 8.1m/s=8.1m/s 8.1m/s The answer is POSITIVE 8.1m/s instead of negative because the negative number would only be showing a different direction, but because neither direction implies a negative or positive value, both sides of the equation should represent two values that equal one another. Additionally, if the two values are equal, then when subtracting one from the other it equals 0m/s, whereas a negative and a positive value would create a doubled value, meaning the fish would have sped up upon impact.
If the mass of the egg is 1.5 kg , its initial speed is 1.7 m/s , and the time to stop is 0.20 s, find the average force on the egg. Express your answer to two significant figures and include the appropriate units.
m=1.5kg speed=velocity=1.7m/s t=0.2s F=? 2 sig figs F=m(v/t) F=(1.5)(1.7/0.2)=1.5(8.5)=12.75N Round 12.75 up to 13N for your final answer.
Object A has a mass m and a speed v, object B has a mass m/2 and a speed 4v, and object C has a mass 3m and a speed v/3. Rank the objects according to the magnitude of their momentum.
momentum equation: p=mv when p=1, m=q, and v=1 A: p=mxv= 1x1=1 B: p=(m/2)x4v=0.5x4=2 C: p=3mx(v/3)=3x(1/3)=1 B has the largest momentum (p=2), and A and C have equally momentums (p=1) that are smaller than B's. Answer: A&C<B
The mechanical energy of a system is conserved during a certain process only if __________.
non-conservative forces do zero total work on the system during that process
How much impulse stops a 60-kg carton sliding at 4.8 m/s when it meets a rough surface? Express your answer to two significant figures and include the appropriate units.
p=mv = Ft=mv, but with Newtons instead of kg x m. p=? m=60kg v=4.8m/s p=60x4.8= 288N/s Significant figures are at 3, we need to bring them to 2. 288 can be rounded to 290N/s, which is our final answer.
The quantity that is called impulse can be measured by the _______.
product of force and time.
If Quantity X is conserved for a certain system and during a certain process, that means__________.
quantity X for that system does not change in any way during that process
When a big fish swims into an oncoming smaller fish and swallows it, the momentum of the two-fish system _______.
remains the same
When one does twice the work in twice the time, the power expended is _______.
the same
Fossil fuels, hydroelectric power, and wind power ultimately get their energy from _______.
the sun?
Recycled energy is mainly _______.
use of energy otherwise wasted
When two vehicles collide, momentum is conserved _______.
whether the collision is elastic or inelastic.
