Question

Dear sir/ma'am,

I was watching a video of chapter Force and Laws of Motion, in that chapter there was a video (Engine Problem one video) in that video question was the same as my ncert science textbook (question no. 7 of the book). The question was as follows,

A 8000 Kg engine pulls a train of 5 wagons, each of 2000 Kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate :

A. the net accelerating force;

B.the acceleration of the train ; and

C. the force of wagon 1non wagon 2.

So, my droughts are that

1.In the video he did find the acceleration of the train(3.5 m/s²)but he excluded the mass of the train, and when I find the acceleration of the train including the mass of the train my answer was same as in the book(1.944 m/s²) So, why did he excluded the mass of train?

2.The third part of the question was to find the force of wagon 1 on wagon 2, he did find that also but in the book answer is different (15556).Why?

Can I get a solution of this question? (all the parts)

Answer 1

Mass of the engine = 8000 kg
Mass of each wagon = 2000 kg,
Np. of wagons = 5
mass of total wagon=2000*5
=10000
Total mass of train = 8000+2000*5= 18000 kg
Force, F = 40000 N
Track resistance , R = 5000 N
Total accelerating force = F −R
=40000−5000
=35000 N
Let acceleration = y m/s²
Acceleration = Accelerating force / mass
y=35000/18000
= 35/18
=1.94 m/s²
Frictional resistance of 4 wagons = 5000*(2000*4)/18000 =2222 .22 N
Accelerating force on 4 wagons =2000*4*1.944 =15552 N
Hence total force exertred by wagon on wagon 2= Accelerating force +Frictional resistance
=2222 .22 +15552=17774.22 N

"I find the acceleration of the train including the mass of the train my answer was same as in the book(1.944 m/s²)" --- This is the correct approach and way of doing it.
Hope this clears your doubts.