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

[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?

 

[SammyS]

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]

It is not said in the problem.

 

[SammyS]

It is not said in the problem.

Of course you must figure it out from the given information.

 

[Laura0901]

And how would you do that? Using the kinematic equations?

 

[Laura0901]

Why would we need to find the acceleration? I need the friction force.

 

[SammyS]

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]

Either gravity or velocity..I think.

 

[SammyS]

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.

 

[stephen8686]

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)

 

[Laura0901]

Thanks for trying to help me understand

 

[Laura0901]

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.

 

[SammyS]

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]

The Positive direction

 

[Laura0901]

Is it 2N?

 

[SammyS]

The Positive direction

Positive in what sense?

Positive x-direction?
or
Positive as in upward?
​

 

[Laura0901]

The x direction, it is horizontal

 

[stephen8686]

Okay... I hope you are asking if f_k is 2N because M is not in Newtons. I don't think f_k=2N either though...

 

[SammyS]

Okay... I hope you are asking if f_k is 2N because M is not in Newtons. I don't think f_k=2N either though...

Right. It would take very long time to stop.

 

[Laura0901]

Ughh I'm so bad at everything sorry guys.

 

[Laura0901]

I don't understand how you'd even get a number just because it's a horizontal force?

 

[stephen8686]

Laura0901, It seems like you are getting things very confused, more than a few messages can help with. I recommend you watch this video, it really helped me learn friction: https://www.khanacademy.org/science...riction/v/static-and-kinetic-friction-example.

Yeah, it would take a reeeaaallly long time to stop if f_k=2N

 

[Laura0901]

I think I understand friction now thanks for the video.

 

[Laura0901]

So is air resistance part of it?

 

[SammyS]

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.

 

[Laura0901]

I got one by using v=at

 

[Laura0901]

I'm so bad at this :/

 

[stephen8686]

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.

 

[stephen8686]

I agree a=1 (or -1 depending on your positive x direction).

 

[Laura0901]

Thank you for your time. If I get this problem wrong on the quiz, I'll get a 75%. I'll take that..

 

[SammyS]

I got one by using v=at

Well what did you get?

 

[Laura0901]

I used F=ma so: F=(10,000kg)(1)= 10,000

 

[Laura0901]

Which I think is definitely wrong! I don't see how to use acceleration in the equation F=Mmg..

 

[SammyS]

I used F=ma so: F=(10,000kg)(1)= 10,000

Why is that wrong? (Except of course you left the units off.) 10,000 N

Now use you "friction" equation: F_f = μN = μmg.

 

[Laura0901]

10,000 N is the force?