Thermodynamics - Need help with inclined plane problem

In summary, the train engine climbs a hill with an incline of 20° from the horizontal. The engine is 30% efficient, so the work done is 3.53GJ.
  • #1
dlacombe13
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3

Homework Statement


A train engine climbs a hill. The engine is 30% efficient. The train has a mass of 12000kg. The hill is 750m in height, with an incline of 20° form the horizontal. Friction exerts a force opposing motion of 8000N throughout the climb. Find w, Qin and Qout for the engine.

Homework Equations


We have only covered the first law, and some equations for various types of work:
dE/dt = Qin - Qout - w
w = Fd

The Attempt at a Solution


I assume I have to first solve for the forces as if it were an inclined plane problem. I'm used to the friction being the opposite way, so I am a little confused about how to resolve these forces. I think the work will be:
F|| * hypotenuse of the incline.
But would this work be the work done by the engine? It just seems strange to me since I'm thinking that is due to the train itself, and not the engine within the train. Here's what I have so far:
IMAG1298_zpsg3xpibr7.jpg
 
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  • #2
Find the force that the train must exert to overcome friction and gravity. And the distance it must exert this force to get to the top of the hill. And thus the work it must do.

Note that the mass is 12000 kg, not 12000 x 103 kg.
 
  • #3
If its 12000kg that means its 12,000 kg * 1000g/1kg = 12,000,000g = 12000x103 g, right?
Also, I assumed that the force would be the F||. Now I am little confused, since I am used to having the friction force on the other side, and so F|| and F are just the components of the gravitation force. Would this still be so?
 
  • #4
dlacombe13 said:
dE/dt = Qin - Qout - w
This doesn't look dimensionally correct to me. You have power on the left side and energy on the right side.
 
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  • #5
dlacombe13 said:
If its 12000kg that means its 12,000 kg * 1000g/1kg = 12,000,000g = 12000x103 g, right?
Why in the world are you converting to grams?

dlacombe13 said:
Also, I assumed that the force would be the F||. Now I am little confused, since I am used to having the friction force on the other side
Not sure what you mean. Friction and gravity both act down the incline; the train must exert an equal force up the incline to climb the hill.
 
  • #6
For some reason I though I needed to. And gravity acts directly downwards, but I tilted the incline onto the x-axis to make it easier to work with. Is my diagram wrong? I am treating this as an incline plane problem, is that wrong?
 
  • #7
dlacombe13 said:
For some reason I though I needed to.
No, don't do that.

dlacombe13 said:
And gravity acts directly downwards, but I tilted the incline onto the x-axis to make it easier to work with. Is my diagram wrong? I am treating this as an incline plane problem, is that wrong?
It is an incline plane problem. So what's the component of gravity parallel to the incline?
 
  • #8
Oh I believe I know what I'm doing wrong. I didn't realize that the friction force is equal to the horizontal component of gravity. Here is a new FBD:
IMAG1300_zpssa1gvmfn.jpg

If this is correct, then Fpush (force required for the train to go up) is equal to 8kN?
Also, where would the 30% efficiency come in? Would be applied to the work I will find?
 
  • #9
dlacombe13 said:
I didn't realize that the friction force is equal to the horizontal component of gravity.
It's not. There are two downward forces that must be overcome by the engine: gravity & friction. Add them to find the total force. (The friction force is given; you need to calculate the component of gravity.)
 
  • #10
Oh oops, okay so there are two horizontal components acting to the left. Friction and mg*sin(20). So Fpush = 8kN + 40.26kN = 48.2kN.
For work, the distance would be:
d = 750/sin(20°) = 2192.85m
And so the work would be:
w = Fd = (48.2)*(2192.85) = 105.83kJ

Now where does the efficiency come in?
 
  • #11
dlacombe13 said:
Oh oops, okay so there are two horizontal components acting to the left. Friction and mg*sin(20). So Fpush = 8kN + 40.26kN = 48.2kN.
For work, the distance would be:
d = 750/sin(20°) = 2192.85m
And so the work would be:
w = Fd = (48.2)*(2192.85) = 105.83kJ
Good, but be careful with units. (I think you dropped a factor of 103.)

dlacombe13 said:
Now where does the efficiency come in?
What's the definition of efficiency?
 
  • #12
Oh okay so it should be 105.83MJ.
Efficiency of a heat engine is Wout/Qin
So would it be:
.30 = Wout/Qin
?
 
  • #13
dlacombe13 said:
Oh okay so it should be 105.83MJ.
Efficiency of a heat engine is Wout/Qin
So would it be:
.30 = Wout/Qin
?
Good. Now apply the 1st law to find Qout.
 
  • #14
So Qin = Wout/0.30 = 3.53GJ
dE/dt = Qin - Qout - Wout
0 = 3.53GJ - Qout - 105.83MJ
Qout = 3.53GJ - 105.83MJ = 3.42GJ
 
  • #15
dlacombe13 said:
So Qin = Wout/0.30 = 3.53GJ
Right idea, but careful with the decimal point.

dlacombe13 said:
dE/dt = Qin - Qout - Wout
As kuruman pointed out, that equation is dimensionally inconsistent. Better to use:
Qin = Qout + Wout
 
  • #16
Oops, should be:
Qout = 352.77MJ - 105.83MJ = 246.94MJ
And that equation came from my notes. Why is it plus and not minus?
 
  • #17
dlacombe13 said:
Oops, should be:
Qout = 352.77MJ - 105.83MJ = 246.94MJ
That looks better.
dlacombe13 said:
And that equation came from my notes. Why is it plus and not minus?
We're using the same equation. (Just not equal to dE/dt, but dE alone.)
 
  • #18
Right, so the equation is the same, and we get the same answer. So why exactly are you guys pointing out my equation and saying it is dimensionally inconsistent?. What does that mean?
 
  • #19
dlacombe13 said:
Right, so the equation is the same, and we get the same answer. So why exactly are you guys pointing out my equation and saying it is dimensionally inconsistent?. What does that mean?
The equation you quoted:
dlacombe13 said:
dE/dt = Qin - Qout - Wout
Cannot be true as is, since the left side expresses power (J/sec) and the right expresses energy (J). Equations must have the same dimensions (units) on both sides.

But the equation you actually used was this one:
Qin = Qout + Wout
Which is fine.
 
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  • #20
Oh okay that makes sense, thank you both for your help. I appreciate it!
 

FAQ: Thermodynamics - Need help with inclined plane problem

How do I calculate the work done on an object on an inclined plane?

The work done on an object on an inclined plane is calculated by multiplying the force applied to the object by the distance that the object is moved in the direction of the force. This can be represented by the equation W = F*d*cos(theta), where W is the work done, F is the force applied, d is the distance, and theta is the angle of the incline.

What is the equation for calculating the mechanical advantage of an inclined plane?

The mechanical advantage of an inclined plane is calculated by dividing the length of the incline by the height of the incline. This can be represented by the equation MA = L/H, where MA is the mechanical advantage, L is the length of the incline, and H is the height of the incline.

How does friction affect the efficiency of an inclined plane?

Friction can decrease the efficiency of an inclined plane by causing some of the applied force to be lost as heat. This means that the actual work done on the object will be less than the theoretical work calculated without taking friction into account.

Can an inclined plane have a mechanical advantage greater than 1?

Yes, an inclined plane can have a mechanical advantage greater than 1. This means that the force required to move an object up the incline is less than the weight of the object, making it easier to lift the object.

How does the angle of the incline affect the work done on an object?

The angle of the incline affects the work done on an object by changing the amount of force needed to move the object a certain distance. A steeper incline will require more force to move the object the same distance, while a shallower incline will require less force.

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