Physics Problem

[MajesticMoose]

Here's the question straight off my review sheet:

What coefficient of friction is needed to prevent a person from sliding when standing on the train which is accelerating at 7 m/s^2?

The way I think i'm supposed to do it to find the force exerted on the person and then use that to solve for coefficient of friction. My problem is that i don't know how to do this w/o the mass of the person.

Also according to my calculations, this train is accelerating faster than a porsche 911 turbo(0-60: porsche - 4.2s, train - 3.83s). Even if a train could accelerate that fast, it would matter what the coefficient of friction is seeing as how anyone standing would be thrown on their @ss

Thanks
Moose

 

[StageLeft]

Since their mass should not matter I used to solve these problems by throwing in a random mass. Say the person weighs 50 kg, then check with 100 kg to make sure it works. It was a few years ago so I can't help ya

60 miles/hour ~ 312000 feet/hour ~ 87 feet/sec ~ 27 meters/sec.

Divided by 4 sec = about 6.5-7m/sec acceleration for your porsche. Those are my loose calculations

Lets see if I can get this:

gravity is 9.8m/sec right? this train is going at 7m/sec acceleration...so their normal force on the train * coeficient of static friction is the amount of force holding them from slipping...anyway I think it might be as simple as 7/9.8 = .714 as your coefficient of friction.

Yeah thats gotta be it. Check it out:
100 kg * 9.8m/sec = 980 Newtons.
980 * .714 = 700 N "holding" the person on the train.
train's 7 m/sec * 100 kg = 700 N "pulling" the guy off the train as it accelerates under him. So the coeficient is .714

 

[cpars]

Im too old to remember those formulas, but it did remind me of one i heard and asked my physics teacher. If you were in your car traveling at the speed of light and turned on your headlights what would happen?

 

[Passions]

Turn on your headlights at the speed of lights and you would see yourself!

 

[MajesticMoose]

actually it doesn't matter because you would burn up long b4 you got to the speed of light and so you don't exist to turn on your nonexisting headlights. Hypothetically if it were possible the light from your headlights would travel at twice the speed of light, I think.

man this problem really sucks, the more i think about it the more confused i get about it

Moose

 

[The Stigenator]

this is a simple problem.

YOu are given a lateral directional movement of the train heading. Its going at 7m/s^2.
Now the weight of the body can be 'X'
Since the intertia acting on the body is also 7m/s^2 there should be an equal amount of Friction on the legs (newtons theroy.. for every action there is an equal and opposite reation).
Since the body is not falling down.. accelration due to gravity = 0 m/s.

If this thing was going vertical then it would be inverse of the speed and driection travelling in. Now we know Teta = 0 and we know S = 7m/s^2.. plug them in. the coeff friction (assuming the train doesnt have any drag coeff on it assuming its travelling in vacume) then friction = weight of the body acting downwards * 9.8 / accelration of the train

I am not sure how to explain it to you..but you can get a visual idea.

 

[MajesticMoose]

Thanks skoorb, that looks like it could be right, but frankly i don't know(that's why i asked). Can somebody check that and see if its right. I don't remeber ever doing that b4, i must have learned it some other way or just completely forgot. I definitly gonna ask the teacher tomorrow. I hate midterms :|

thanks for the help,
Moose

 

[Napalm381]

F=ma. m= mass of human, a=7 m/s^2. So you have a force of (7m) newtons.

F(friction)=(coef of friction)*(normal force). You need a force of 7m in the opposite direction. The normal force is the force exerted by gravity.

So you have: (7m)=(coef)*(mg).

Or: (coef of friction)=7/g.

Get it?

 

[DonaldDuck82]

i agree with skoorb

Sum of Fx=0 (since the man can not move), we know that f(k)=u(k)*n=u(k)*m*g
Newton's second law lets us
Sum of Fx=-f(k)=ma
-u(m)g=(m)-7.00 m/s^2
-u*g=-7.00 m/s^2
u=7/9.8
u=.714286

 

[MajesticMoose]

ok um since when is 7 meter/sec^2 a force?

oh crap all these letters , essentially the idea is that
(rate of acceleration)/(acceleration due to gravity)= (coefficient of friction)

that right?

Moose

 

[Napalm381]

7 m/s^2 is not a force. 7*m (where m is the mass of the human) is the force.

 

[StageLeft]

Its a force because F = MA, so M * 7m/sec = Force

So, since you weren't given the mass, and the mass doesn't matter, then that is why he called it a force.

donald duck Looks like you've not been away from the physics books as long as some of us. Your "syntax" looks good ! hehe

 

[MajesticMoose]

Ah i see, i assumed that the m was for meters which was shortened from meters/sec^2

Moose

 

[iamwiz82]

Majestic, light would not travel at 2x speed of light. The speed of light is a barrier, at that point, according to Einstein, you become nothing, since weight goes to infinity time goes to 0, and length goes to infinity.

That is why time moves faster when you are going somewhere =].

 

[RossGr]

If you could get to the speed of light, you would observe the light from your flashlight as moveing at the speed of light. You would also measure the speed of objects that you passed as near light speed. It's all relative.

 

[Unsickle]

<< and length goes to infinity >>



It's called the Lorentz Contraction for a reason.