# Homework Help: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h after o

1. Jun 10, 2012

### manal950

Hi all

1 ) A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h after one min . if the frictional resistance of the track is 5N/KN of the train's weight . Find the tractive force developed by the engine ?

I don't understand what is mean "tractive force " ??

anyway I start solve the question

w = 3000 kN
mass = 305810
u = 0
v = 54 km/h

v = u + at
a = 0.25
F = m X a
F = 305810 X 0.25
76452.5 N

2. Jun 10, 2012

### rock.freak667

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

Right a = 0.25 m/s2 (Always remember to put your units).

In the formula F=ma, a is the resultant acceleration of the mass m (Train in this case). So F is the resultant force.

If you draw a free body diagram, what are the forces on the train? (The tractive force is essentially the force generated by the train itself).

3. Jun 10, 2012

### manal950

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

force on the train
1 weight
2 frictional resistance
3 tractive force

now How I can find the tractive force

4. Jun 12, 2012

### manal950

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

I don't know where is the other helper ..

5. Jun 12, 2012

### rock.freak667

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

In which direction do these forces act?

How would you find the resultant horizontal force?

6. Jun 12, 2012

### manal950

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

I don't know how to find resultant horizontal force?

anyway the direction for each one weight dawn , frictional resistance opposite of tractive force

7. Jun 12, 2012

### rock.freak667

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

Right so let's say frictional force is going left and the tractive force is going right. The train is moving to the right. How would you calculate the value of the force that the train is moving with from these two forces?

in other words, if we are in a tug of war competition and you are pulling with 10 N and I am pulling with 11 N, what would be the resultant force in this case?

8. Jun 14, 2012

### manal950

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

Resultant force = tractive force - frictional force

9. Jun 14, 2012

### rock.freak667

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

Right!

So you calculated the resultant force in your first post. And they told you what the frictional force was. So you can rearrange to get the tractive force.

10. Jun 15, 2012

### manal950

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

tractive force = Resultant force - frictional force
= 76452.5 - 3000
now this is correct ??

11. Jun 15, 2012

### rock.freak667

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

Check the signs in your equation.

12. Jun 15, 2012

### manal950

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

76452.5 + 3000 ... ?

13. Jun 15, 2012

### rock.freak667

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

That should work.

14. Jun 15, 2012

### HallsofIvy

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

Remember that the "tractive force", the force applied to the track by the locomotive, must be larger then the resultant force as it has to overcome the frictional force.

You can write it as "Resultant force- frictional force" if you remember to treat "frictional force" as negative.

15. Jun 16, 2012

### manal950

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

now is my answer if fully correct ... ?

16. Jun 16, 2012

### rock.freak667

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

It should be.

17. Jun 23, 2012

### manal950

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

my teacher said the answer is not correct

and he said why you don't multiply

5 X 3000

I don't know what he mean ?

and my other questions is the Resultant force
ma + tractive force

or only F = ma

Last edited: Jun 23, 2012
18. Jun 23, 2012

### azizlwl

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

frictional resistance of the track is 5N/KN of the train's weight

19. Jun 25, 2012

### manal950

Re: A train of 3000 KN weight starts from rest and reached a velocity of 54 km/h afte

5 X 3000

unit is Kn or N

and my other questions is the Resultant force
ma + tractive force

or only F = ma