Because of your physics background, you have been able to get a job with a company devising stunts for an upcoming adventure movie being shot in Minnesota. In the script, the hero has been fighting the villain on the top of the locomotive of a train going down a straight horizontal track at 20 mph. He has just snuck on the train as it passed over a lake so he is wearing his rubber wet suit. During the fight, the hero slips and hangs by his fingers on the top edge of the front of the locomotive. The locomotive has a smooth steel vertical front face. Now the villain stomps on the hero's fingers so he will be forced to let go and slip down the front of the locomotive and be crushed under its wheels. Meanwhile, the hero's partner is at the controls of the locomotive trying to stop the train. To add to the suspense, the brakes have been locked by the villain. It will take her 10 seconds to open the lock. To her horror, she sees the hero's fingers give way before she can get the lock off. Since she is the brains of the outfit, she immediately opens the throttle causing the train to accelerate forward. This causes the hero to stay on the front face of the locomotive without slipping down giving her time to save the hero's life. The movie company wants to know what minimum acceleration is necessary to perform this stunt. The hero weighs 180 lbs in his wet suit. The locomotive weighs 100 tons. You look in a book giving the properties of materials and find that the coefficient of kinetic friction for rubber on steel is 0.50 and its coefficient of static friction is 0.60.

this is the question. I have found the kinetic friction force is 90lbs and the static friction force is 108 lbs. I know that F=ma so a=F/m but I'm not sure how to find the force or where to go from here

Hmmm. Let me think out loud. This problem is complicated by its verbiage.

The frictional force will be an upward force. The downward force is only gravity. Since the front face is vertical, the only effect of acceleration will be to increase the upward frictional force. At a minimum, that force has to exactly counteract the downward force of gravity, You know the guy weighs 180 lbs (presumably that's his mass). With the accelration from gravity, you can calculate the downward force.

The upward frictional force without acceleration is zero because there is no force normal to the front of the train (except from acceleration). You'd assume static frictional force (we're trying to find the minimum force) because the guy isn't supposed to slide down.



The normal force, N = ma where m is the guy's mass and a is the acceleration of the train



It looks like the mass falls out, g is the acceleration due to gravity:



That looks too simple. I'm sure I did something wrong but maybe it's enough for you to go from there.

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Originally Posted by Perito

Hmmm. Let me think out loud. This problem is complicated by its verbiage.

The frictional force will be an upward force. The downward force is only gravity. Since the front face is vertical, the only effect of acceleration will be to increase the upward frictional force. At a minimum, that force has to exactly counteract the downward force of gravity, You know the guy weighs 180 lbs (presumably that's his mass). With the accelration from gravity, you can calculate the downward force.

The upward frictional force without acceleration is zero because there is no force normal to the front of the train (except from acceleration). You'd assume static frictional force (we're trying to find the minimum force) because the guy isn't supposed to slide down.



The normal force, N = ma where m is the guy's mass and a is the acceleration of the train



It looks like the mass falls out, g is the acceleration due to gravity:



That looks too simple. I'm sure I did something wrong but maybe it's enough for you to go from there.

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Actually we went over it in class today and you were perfectly right! Thank you so much for your help

Sorry I couldn't get to it earlier.

Could you explain how u got
F = mg = 0.6ma

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Originally Posted by snigi

Could you explain how u got
F = mg = 0.6ma

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Have you read the previous post at least? There is the answer, plain and clear. :rolleyes:

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Originally Posted by snigi

Could you explain how u got
F = mg = 0.6ma

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mg is the downward force.

0.6ma is the upward force. They have to be equal or the guy would slide down.