Force

[Aakash Pandita]

There is something that I not able to understand about force. I would like to tell it to you with an example.

There is a train consisting of 1 engine of 8000kg and 5 wagons of 2000 kg each

The acceleration of the train is 2 m/s2

 

How would you tell what force is exerted by wagon 1 on wagon 2 and why?

[swansont]

F=ma

 

The net force on an object is the mass times its acceleration. The net force is made up of the vector sums of all the individual forces.

 

Each car will have a net force on it, because they are all accelerating. It's probably best to draw a diagram and identify and label the individual forces to investigate them. Each car will feel a force from whatever is pulling on them, from both the front and back (as applicable)

[Aakash Pandita]

I am just not able to understand that when we want to find the force exerted by wagon1 on wagon2..........what mass should we multiply with the acceleration and why?

[swansont]

I am just not able to understand that when we want to find the force exerted by wagon1 on wagon2..........what mass should we multiply with the acceleration and why?

 

The net force is the mass of the wagon* the acceleration; in this case 2000kg * 2 m/s^2 = 4000N

 

Now, what are the individual forces on wagon 2? There's one from wagon 1, but are there any others?

[Aakash Pandita]

wagon 1 is joined to the engine .....and we are asked the force which it exerts on wagon 2 which is connected to wagon 3,4,5.

will the force exerted by the wagon 1= mass of engine+mass of wagon 1 x a?

[swansont]

So wagon 1 has to pull wagons 2, 3, 4 and 5, and that force has to be present where it connects with wagon 2. What force is required to do that?

[Aakash Pandita]

f=ma

so what mass should we take to multiply with acceleration...................mass of engine+mass of wagon1..........or.............mass of wagon2+3+4+5

[swansont]

f=ma

so what mass should we take to multiply with acceleration...................mass of engine+mass of wagon1..........or.............mass of wagon2+3+4+5

 

What is wagon 1 pulling?

[Aakash Pandita]

it is pulling wagons2,3,4,5

[Sriman Dutta]

Suppose it's like this:

F <--- |eng|= |wg1|=|wg2|=|wg3|=|wg4|=|wg5|

So this is the train (or wagons).

Assuming that each wagon weighs m and the engine is of mass M, then

net acceleration a = F/(M+5m)

So force exerted by wg2 on wg3 is = 3ma

[Country Boy]

The entire train has mass 8000+ 5(2000)= 18000 kg.  Since it is accelerating at 2 m/s^2, the force on the entire train is 2(18000)= 36000 N.  The force necessary to accelerate the engine alone would be 2(8000)= 16000 N so the cars must be exerting a retarding force on the engine of 36000- 16000= 20000 N.  That, of course, means that the engine is pulling the cars with a force of 20000N.  The force necessary to pull only the first car at that speed is 2(2000)= 4000 N so the first car must be pulling the second car with force 20000- 4000= 16000 N.  Similarly, that second car must be pulling the third car with force 16000- 4000= 12000 N, the third car must be pulling the fourth car with force 12000- 8000= 4000 N and the fourth car must be pulling the fifth car with force 8000- 4000= 4000 N.