I think I understand, but where do you find force?

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Laura0901

Homework Statement


Find the coefficient of kinetic friction that is used to stop a train engine along its track of the trains velocity is 30 m/s when the brakes are applied, and it takes 30 sec for the 10,000 kg train to stop

Homework Equations


Ffric=MN=Mmg
(I think that's the equation to use.

The Attempt at a Solution


Ffric=M(10000kg)(9.8m/s)
Ffric=M(98,000)
What do you use for the force? 30m/s?
 

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Laura0901 said:

Homework Statement


Find the coefficient of kinetic friction that is used to stop a train engine along its track of the trains velocity is 30 m/s when the brakes are applied, and it takes 30 sec for the 10,000 kg train to stop

Homework Equations


Ffric=MN=Mmg
(I think that's the equation to use.

The Attempt at a Solution


Ffric=M(10000kg)(9.8m/s)
Ffric=M(98,000)
What do you use for the force? 30m/s?
Hello Laura0901. Welcome to PF !

What is the acceleration of the train ?
 
Laura0901 said:
It is not said in the problem.
Of course you must figure it out from the given information.
 
Laura0901 said:
Why would we need to find the acceleration? I need the friction force.
And, just what force is it that is slowing the train?
 
Laura0901 said:
Either gravity or velocity..I think.
The train slows to a stop. That's an acceleration. That requires a force. (Ask Mr. Newton.)

Gravity is a force, but it acts in a vertical, the train's motion and its acceleration are in the horizontal direction. Gravity is not the force which slows the train.

Velocity is not a force.
 
1) Figure out the force in the x-direction that SammyS is talking about, the force that causes the train to stop.
2) Use kinematics to find a
3)Use Fx=max to find the magnitude of the force mentioned in step one
Then you should have enough data to solve Ffric=M(98,000)
 
There aren't any forces in the x direction. I don't think length is a force. I don't know how to find friction.
 
Laura0901 said:
There aren't any forces in the x direction. .
Then the train's going to keep going in the x direction.

By the way: What direction is the friction force ?
 
Laura0901 said:
The Positive direction

Positive in what sense?
Positive x-direction?
or
Positive as in upward?
​
 
Okay... I hope you are asking if fk is 2N because M is not in Newtons. I don't think fk=2N either though...
 
stephen8686 said:
Okay... I hope you are asking if fk is 2N because M is not in Newtons. I don't think fk=2N either though...
Right. It would take very long time to stop.
 
I don't understand how you'd even get a number just because it's a horizontal force?
 
Laura0901 said:
So is air resistance part of it?
Probably not much. The friction force is stopping a train.

Calculate the acceleration as has been suggested previously.
 
Air resistance is a totally different thing. You'll learn that later in physics. Until then, assume there is no air resistance. Kinetic friction is between two objects sliding on each other.
 
I agree a=1 (or -1 depending on your positive x direction).
 
Thank you for your time. If I get this problem wrong on the quiz, I'll get a 75%. I'll take that..