Query 112

course phy201

I had many question on this I am a little unclear so far.

011. `query 11

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Question: `q set 3 problems 15-19. Explain the difference between a conservative and a nonconservative force, and give an example of each.

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Your solution:

Conservative force would be the one that has forces that hinder

nonconservative has other forces working against it

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Given Solution:

`a** A conservative force conserves energy--you can get your energy back.

For example:

Push something massive up a hill, then climb back down the hill. The object, by virtue of its position, has the potential to return most of your energy to you, after regaining it as it rolls back down. You will have done work against gravity as you move along a path up the hill, and gravity can return the energy as it follows its path back down the hill. In this sense gravity conserves energy, and we call it a conservative force.

However, there is some friction involved--you do extra work against friction, which doesn't come back to you. And some of the energy returned by gravity also gets lost to friction as the object rolls back down the hill. This energy isn't conserved--it's nonconservative. **

Another more rigorous definition of a conservative force is that a force is conservative if the work done to get from one point to another independent of the path taken between those two points.

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Self-critique (if necessary):

Ok, so conservative force is a force that conserves energy or gains its energy babck, ie rolling down a hill.

nonconservative is not conserved or the friction that the object loose energy

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Question: `qIf a system does work W1 against a nonconservative force while conservative forces do work W2 on the system, what are the change in the KE and PE of the system? Explain your reasoning from a commonsense point of view, and include a simple example involving a rubber band, a weight, an incline and friction.

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Your solution:

Potential energy is energy stored ready to do work, so this would be conservative. KE would be doing work so this would be the nonconservative actingagainst the system or friction.

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Given Solution:

`a** `dKE is equal to the NET work done ON the system.

The KE of a system changes by an amount equal to the net work done on a system.

If work W1 is done BY the system against a nonconservative force then work -W1 is done ON the system by that force.

`dPE is the work done BY the system AGAINST conservative forces, and so is the negative of the work done ON the system BY nonconservative forces. In this case then `dPE = - W2. PE decreases, thereby tending to increase KE.

If work -W1 is done ON the system by a nonconservative force and W2 is done ON the system by a conservative force, the NET work done ON the system is -W1 + W2.

The KE of the system therefore changes by `dKE = -W1 + W2.

If the nonconservative force is friction and the conservative force is gravity, then since the system must do positive work against friction, W1 must be positive and hence the -W1 contribution to `dKE tends to decrease the KE.

e.g., if the system does 50 J of work against friction, then there is 50 J less KE increase than if there was no friction.

If the work done by the nonconservative force on the system is positive, e.g., gravity acting on an object which is falling downward (force and displacement in the same direction implies positive work), the tendency will be to increase the KE of the system and W2 would be positive.

If W2 is 150 J and W1 is 50 J, this means that gravity tends to increase the KE by 150 J but friction dissipates 50 J of that energy, so the change in KE will be only 100 J.

If the object was rising, displacement and gravitational force would be in opposite directions, and the work done by gravity would be negative. In this case W2 might be, say, -150 J. Then `dKE would be -150 J - 50 J = -200 J. The object would lose 200 J of KE (which would only be possible if it had at least 200 J of KE to lose--think of an object with considerable velocity sliding up a hill). **

STUDENT COMMENT

I find this really confusing. Could this be laid out in another way?

INSTRUCTOR RESPONSE

If you find this confusing at this point, you will have a lot of company. This is a challenge for most students, and these ideas will occupy us for a number of assignments. There is light at the end of the tunnel: It takes awhile, but once you understand these ideas, the basic ideas become pretty simple and even obvious, and once understood they are usually (but not always) fairly easy to apply

This could be laid out differently, but would probably be equally confusing to any given student. Different students will require clarification of different aspects of the situation.

If you tell me what you do and do not understand about the given solution, then I can clarify in a way that will make sense to you.

I also expect that in the process of answering subsequent questions, these ideas will become increasingly clear to you.

In any case feel free to insert your own interpretations, questions, etc. into a copy of this document (mark insertions with &&&& so I can locate them), and submit a copy.

STUDENT QUESTION

If the system goes against the force will this always make it negative?

INSTRUCTOR COMMENT

If a force and the displacement are in opposite directions, then the work done by that force is negative.

If the system moves in a direction opposite the force exerted BY the system, the work done BY the system is negative.

Note, however, that if this is the case then any equal and opposite force exerted ON the system will be in the direction of motion, so the force will do positive work ON the system.

A separate document related to this problem is located in the document

work_on_vs_by_dKE_dPE_etc_questions_answers.htm

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Self-critique (if necessary):

Wk1 is against a nonconserative force

WK2is conservative forces on the sytem

WK1= done by system against nonconservative force, -W1 is done on system, so it would be moving down a hillin positive direction. W1 is negative because the kinetic energy is decreased. W1 is the conservative force for example gravity, but negative because kinetic energy is decreased. If the WK1 is not negative, or positive, then kinetic energy would increase making WK! positive, (not the case here).

I W2 is nonconservative forces it is postive because it is increasing kinetic energy.

So, if W2= 150J and is increasing kinetic energy it is the friction or nonconservative force. If 50J is decreasing kinetic energy it is the conservative force and it is negative. Therefore 150j-50j=100j

????Is this the right idea??????????

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Question: `qIf the KE of an object changes by `dKE while the total nonconservative force does work W_nc on the object, by how much does the PE of the object change?

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Your solution:

W_nc= nonconservative force and works against and increases kinetice energy because it is doing work on the oject, potential energy is decreased.

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Given Solution:

`a** We have `dKE + `dPE + `dWbyNoncons = 0: The total of KE change of the system, PE change of the system and work done by the system against nonconservative forces is zero.

Regarding the object at the system, if W_nc is the work done ON the object by nonconservative forces then work -W_nc is done BY the object against nonconservative forces, and therefore `dWnoncons = -W_nc.

We therefore have `dKE + `dPE - W_nc = 0 so that `dPE = -`dKE + W_nc. **

Equivalently, the work-energy theorem can be stated

`dW_ON_nc = `dKE + `dPE

In this example the work done on the system by nonconservative forces is labeled W_nc, without the subscript ON and without the `d in front. However it means the same thing, so the above becomes

W_nc = `dKE + `dPE

and we solve for `dPE to get

`dPE = -`dKE + W_nc

STUDENT COMMENT

I’m still confused on how to understand when the energy is done on the object and when the energy is done against the object.

INSTRUCTOR RESPONSE

In an application, that can be the difficult question.

However in this case it is stated that W_nc is the work done by nonconservative forces ON the object.

STUDENT COMMENT:

I had the same logic as the given solution, however I got ‘dPE = -‘dKE – W_nc as the answer. I some how got an extra negative. Maybe Work can only be positive….??

INSTRUCTOR RESPONSE:

In this problem W_nc was specified as the work done on the object by nonconservative forces.

You have to be careful about whether W_nc is ON the system or BY the system.

You used the equation `dKE + `dPE + W_nc = 0; however that equation applies to the work done BY the system against nonconservative forces. Written more specifically the equation you used would be

‘dKE + ‘dPE + W_nc_BY = 0 so

`dPE = - `dKE - `W_nc_BY.

W_nc_BY = - W_nc_ON so `dPE = - `dKE + W_nc_ON.

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Self-critique (if necessary):

ok, so dke + dPe - W_nc = 0

W_nc is total nonconservative forces doing work on the object,

Right up to here

this increases kinetic energy and decreases potential energy.

there is no assumption about the sign of any of these quantities; any quantity could be positive or negative, as long as `dKE + `dPE - `dW_nc_on = 0

If `dW_nc_ON is positive then `dPE + `dKE is positive, but this could occur with positive `dKE and `dPE, or with a negative `dPE with lesser magnitude than a positive `dKE, or with a negative `dKE with lesser magnitude than a positive `dPE. All you would know is that `dKE + `dPE would be positive.

If `dW_nc_ON is negative then `dPE + `dKE is negative, but this doesn't tell you anything about the sign of either of the two quantities.

All we can say is that `dPE = `dW_nc_on - `dKE.

Since it is decreasing the potential energy it is negative. dKE is the kinetic energy which is positive since the potential energy is increased.

If `dW_nc_on = 0, for example, an increase in either KE or PE implies a decrease in the other. KE would increase due to a decrease in PE (e.g., if you drop an object), while an increase in PE would be associated with a decrease in KE (e.g., an object thrown upward gains PE as it loses KE).

So an increase in KE tends to decrease PE, though `dW_nc_on can be such that KE and PE both increase.

In this problem we solve for PE. So that dPE= - dKE + W_nc.

As potential energy increasess kinetic energy decreases and the non conservative work is positive because it is going with the direction of force more so than against it.

An increase in PE could be the result of loss of KE and/or positive work done by nonconservative forces.

PE could also increase along with KE as long as `dW_nc_on is positive and large enough (e.g., a rocket increases both PE and KE due to nonconservative forces (the nonconservative forces result from ejecting fuel at high speed, i.e., from the rocket engines).

???????????????????Is this reasoning correct?????????????????

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Question: `qGive a specific example of such a process.

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Your solution:

dPe= -dKE + W_nc

A ball going down a smooth frictionless incline, maybe with a greased surface???????

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Given Solution:

`a** For example suppose I lift an object weighing 50 N and in the process the total nonconservative force (my force and friction) does +300 J of work on the object while its KE changes by +200 J.

The 300 J of work done by my force and friction is used to increase the KE by 200 J, leaving 100 J to be accounted for.

More formally, `dW_noncons_ON = +300 J and `dKE = +200 J. Since `dW_noncons_ON = `dKE + `dPE,

So +300 J = +200 J + `dPE, and it follows that `dPE = +100 J.

This 100 J goes into the PE of the object. **

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Self-critique (if necessary):

dW_nc (work on systme)= 'dKE + 'dPE

dW_nc =300J

dke= 200j

300J(on system)= 200J + 'dPE

300J-200J= 'dPE

'dPE= -dKE + W_nc

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Question: `qClass notes #10.

Why does it make sense that the work done by gravity on a set of identical hanging washers should be proportional to the product of the number of washers and the distance through which they fall? Why is this consistent with the idea that the work done on a given cart on an incline is proportional to the vertical distance through which the cart is raised?

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Your solution:

Work done by gravity on a set of identical hanging washers is proportioanl to the product of the number of washers and distance through which they fall because the number of washers determines the mass of what is hanging. The mass of the hanging washers determines the rate or speedthat hte object falls or travels witht he force of gravity through a certain distanceat a specific amount of time. The height of the incline determines the speed that the object at a certain weight will travel through a distance at a certain amount of time.

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Given Solution:

`a** Informally:

The more clips, the more gravitational force, and the more the clips descend the more work is done by that force.

The amount of work depends on how many clips, and on how far they descend.

The number of clips required is proportional to the slope (as long as the slope is small).

More formally, the force exerted by gravity is the same on each clip, so the total gravitational force on the hanging clips is proportional to the number of clips. The work done is the product of the force and the displacement in the direction of the force, so the work done is proportional to product of the number of washers and the vertical displacement.

To pull the cart up a slope at constant velocity the number of washers required is proportional to the slope (for small slopes), and the vertical distance through which the cart is raised by a given distance of descent is proportional to the slope, to the work done is proportional to the vertical distance thru which the cart is raised. **

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Self-critique (if necessary):

ok, so the more clips the more weight and the more gravitational force does work on those clips. The amopunt of work depends also on the distance and how many clips because more clips adds more weight.

Number of clips is the slope.

I think I got this

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Question: `qHow does the work done against friction of the cart-incline-pulley-washer system compare with the work done by gravity on the washers and the work done to raise the cart? Which is greatest? What is the relationship among the three?

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Your solution:

If the cart is being raised up the inlcine it is going against gravity. The hanging washers would aslo be going against gravity, but the work done by gravity on the washers would be the greatest. The cart would be going with the frictional force of the pulley system.

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Given Solution:

`a** The force exerted by gravity on the hanging weights tends to move the system up the incline. The force exerted by gravity on the cart has a component perpendicular to the incline and a component down the incline, and the force exerted by friction is opposed to the motion of the system.

In order for the cart to move with constant velocity up the incline the net force must be zero (constant velocity implies zero accel implies zero net force) so the force exerted by gravity in the positive direction must be equal and opposite to the sum of the other two forces. So the force exerted by gravity on the hanging weights is greater than either of the opposing forces.

So the force exerted by friction is less than that exerted by gravity on the washers, and since these forces act through the same distance the work done against friction is less than the work done by gravity on the washers.

The work done against gravity to raise the cart is also less than the work done by gravity on the washers.

Work done against friction + work against gravity to raise cart = work by gravity on the hanging weights. **

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Self-critique (if necessary):

Force exerted by gravity on hanging weights is greater than forces of friction from the pulley and cart being pulled up the incline. Force of friction is alos less that the gravity of the hanging washers. And work done against gravity to raise the cart is less than the work done by gravity on washers.

?????????????????????????????????????Is this ok??????????????????????????????????????????????

Yes. Your first and seconds sentences might be redundant: there is only one total frictional force, and it is less than the force exerted by gravity on the hanging objects. You seem to have said this twice, once in the first sentence and once in the second.

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Question: `qWhat is our evidence that the acceleration of the cart is proportional to the net force on the cart?

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Your solution:

a=F/m

So acceleration is proprtional to forces being exerted on the cart divided by the mass of the cart.

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Given Solution:

`a** the graph of acceleration vs. number of washers should be linear **

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Self-critique (if necessary):

ok

????????????????????????????????would this be acceleration increases as number of washers increases????????????????????????????????

The net force is the sum of the gravitational force on the weights, and the frictional force (one force being positive, the other negative).

The acceleration is the net force divided by total mass (mass of cart plus hanger plus washers).

Washers are progressively transferred from the cart to the hanger, which keeps the mass of the system constant while increasing the net force. So the acceleration increases with the number of washers on the hanger.

The gravitational force on the weights is therefore proportional to the number of washers on the hanger. With each added washer we get the same additional force, so we get the same additional acceleration. So the graph is linear.

However the acceleration is not proportional to the number of weights. The net force on the system is equal to the gravitational force on the weights, plus the frictional force (which is of opposite sign, so while we are in fact adding quantities of opposite signs, it 'feels' like we're subtracting frictional force from gravitational force). The gravitational force on the weights is proportional to the number of washers, but when we add in the effect of the frictional force, our force is no longer proportional to the number of weights. Still linear, but not proportional to ... .

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Question: `qprin phy and gen phy prob 34: Car rolls off edge of cliff; how long to reach 85 km/hr?

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Your solution:

a=9.8m/s^2

vf=85km/h =

85km/h= 85km/h * 1000meters/3600seconds

=23.6m/s

Vf= v0 + a * 'dt

23.6m/s= 0 + 9.8m/s^2 * 'dt

'dt = 2.41 seconds

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Given Solution:

`aWe know that the acceleration of gravity is 9.8 m/s^2, and this is the rate at which the velocity of the car changes. The units of 85 km/hr are not compatible with the units m/s^2, so we convert this velocity to m/s, obtaining velocity

85 km/hr ( 1000 m/km) ( 1 hr / 3600 sec) = 23.6 m/s.

Common sense tells us that with velocity changing at 9.8 m/s every second, it will take between 2 and 3 seconds to reach 23.6 m/s.

More precisely, the car's initial vertical velocity is zero, so using the downward direction as positive, its change in velocity is `dv = 23.6 m/s.

Its acceleration is a = `dv / `dt, so

`dt = `dv / a = 23.6 m/s / (9.8 m/s^2) = 2.4 sec, approx..

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Self-critique (if necessary):

ok, I got it

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Question: `q**** prin phy and gen phy problem 2.52 car 0-50 m/s in 50 s by graph

How far did the car travel while in 4 th gear and how did you get the result?

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Your solution:

v0=0

vf= 50m/s

'dt = 50s

ds= vave *'dt

vave= (50m/s + 0m/s)/2 * 50s

ds= 25m/s * 50s

'ds= 1250meters

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Given Solution:

`a** In 4th gear the car's velocity goes from about 36.5 m/s to 45 m/s, between clock times 16 s and 27.5 s.

Its average velocity on that interval will therefore be

vAve = (36.5 m/s + 45 m/s) / 2 = 40.75 m/s and the time interval is

'dt = (27.5s - 16s) = 11.5 s.

We therefore have

'ds = vAve * `dt = 40.75 m/s * 11.5 s = 468.63 m.

The area under the curve is the displacement of the car, since vAve is represented by the average height of the graph and `dt by its width. It follows that the area is vAve*'dt, which is the displacement `ds.

The slope of the graph is the acceleration of the car. This is because slope is rise/run, in this case that is 'dv/'dt, which is the ave rate of change of velocity or acceleration.

We already know `dt, and we have `dv = 45 m/s - 36.5 m/s = 8.5 m/s.

The acceleration is therefore

a = `dv / `dt = (8.5 m/s) / (11.5 s) = .77 m/s^2, approx. **

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Self-critique (if necessary):

?????????????????????????????????????????????????????I do not understand when the car is in 4th gear is goed between 36.5 and 45 m/s, in between 16 - 27 seconds, is this general knowledge, or is hat information to be obtained from a graph that we make?????????????????????

?????????????????Would the graph be velocity vs cloctime and how would we know where 4th gear is?????????? This confused me.

The problem as stated in the text includes the graph, with the motion corresponding to different gears clearly labeled.

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Question: `q **** Gen phy what is the meaning of the slope of the graph and why should it have this meaning?

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Your solution:

slope is the rise/run or change in vertical axis divided by the change in horizontal axis

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Given Solution:

`a** The graph is of velocity vs. clock time, so the rise will be change in velocity and the run will be change in clock time. So the slope = rise/run represents change in vel / change in clock time, which is acceleration. **

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Self-critique (if necessary):

Ok, I understand that slope = acceleration for this graph.

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Question: `qGen phy what is the meaning of the area under the curve, and why does it have this meaning?

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Your solution:

area = distance

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Given Solution:

`a** The area under the curve is the distance traveled. This is so because 'ds = vAve*'dt.

'dt is equal to the width of the section under the curve and vAve is equal to the average height of the curve. The area of a trapezoid is width times average height. Although this is not a trapezoid it's close enough that we for the purpose of estimation can analyze it as such.

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Self-critique (if necessary):

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Question: `qGen phy what is the area of a rectangle on the graph and what does it represent?

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Your solution:

Area = distance

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Given Solution:

`a** The area of a rectangle on the graph represents a distance. **

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Self-critique (if necessary):

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&#Good responses. See my notes and let me know if you have questions. &#