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Mechanics - work done conservation of energy *Help needed*

by EddyH
Tags: conservation, energy, mechanics, work
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EddyH
#1
Feb26-12, 10:57 AM
P: 2
1. The problem statement, all variables and given/known data
A train with a mass of 250 tonnes starts from rest and accelerates up an incline of 1 in 100 attaining a speed of 45 Kph after traveling 200m. If the frictional resistance to motion is constant at 30KN calculate the work done by the engine using the principle of conservation of energy

2. Relevant equations
Possible relevant equations:

Work done= Force*Distance
Force=Mass*acceleration
Kinetic energy = 1/2*Mass*Velocity^2
Potential energy= Mass*Gravity (9.81)* Height

3. The attempt at a solution

Force= 250*103 Kg*12.5= 3 125 000 kg/m/s
Work done= 3 125 000*200= 625 000 000(J)
Not sure whether this is along the right lines or not. I am also unsure where the frictional resistance and gravity is used.

Thank you for your time.
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PhanthomJay
#2
Feb26-12, 10:48 PM
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Quote Quote by EddyH View Post
1. The problem statement, all variables and given/known data
A train with a mass of 250 tonnes starts from rest and accelerates up an incline of 1 in 100 attaining a speed of 45 Kph after traveling 200m. If the frictional resistance to motion is constant at 30KN calculate the work done by the engine using the principle of conservation of energy

2. Relevant equations
Possible relevant equations:

Work done= Force*Distance
Force=Mass*acceleration
Kinetic energy = 1/2*Mass*Velocity^2
Potential energy= Mass*Gravity (9.81)* Height

3. The attempt at a solution

Force= 250*103 Kg*12.5= 3 125 000 kg/m/s
net force is mass times acceleration. You are not using the net force or the acceleration.
Work done= 3 125 000*200= 625 000 000(J)
Your incorrect values are calculating the net total work done. You are looking for the work done by the engine only.
Not sure whether this is along the right lines or not. I am also unsure where the frictional resistance and gravity is used.

Thank you for your time.
the problem is asking you to use conservation of energy, not force and kinematic equations. What is the conservation of energy equation that relates work and energy?
EddyH
#3
Feb27-12, 09:39 AM
P: 2
Sorry to be a pain, but I do not know how to go about this, please can you explain?
Thank you

PhanthomJay
#4
Feb27-12, 09:58 AM
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Mechanics - work done conservation of energy *Help needed*

Say, Eddy, if you are asked to solve the problem using energy methods, you should know about the possible energy equations to use, for example, you should know that the work done by non conservative forces (like the engine force and friction force in this example) must equal the change in PE plus the change in KE of the system. Give it a try.


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