Centripital Acceleration

[TheoPetro]

alright i wouldnt normaly do this but i just cant figure it out. this isnt homework but theres a test tomm and i cant for the life of me get this. theres a car on a road going around a conical turn. the road is on an angle, theta, from the ground. the car has mg acting downward, a normal force acting perpendicular to the road, and a centripital acceleration pointing to the center of the "circle" its truning around. theres a friction force opposing movement. how the heck do i know which way movement is? and intuitively i think the car will move outwards from the center of the circle but when i put the friction force this way the net force points to teh center of the circle. im so confused right now.

 

[MrDudeMan]

the friction force is toward the center of the circle. it is what is pushing the car toward the center while the car is pushing to the outside.

its like the friction force when you are walking. the force you exert on the ground is backward and the ground is exerting a force on you in the direction you are going.

 

[TheoPetro]

what force would ever make a car fly off the road then?

 

[DrPizza]

If the car lost it's friction with the road surface, there isn't a force that "makes the car fly off the road"; the car simply continues in the direction of its motion. i.e. if it hit a patch of ice, it will cease rounding the bend and will continue to travel in a straight line. It does not "slide off the road in a direction perpendicular to its motion" as many 1st year physics students often think (intuitively.) The reason for this misconception is because they're used to feeling themselves push against the door. This is merely the equal and opposite force because the door is pushing against them, keeping them moving in a circle. If the door were to open, yes, they would move away from the car, but in a straight line in the direction of the cars motion at the moment the door opened. This is often one of those "gotta see it to believe it" types of demonstrations that's necessary in class. (But, instead of a door, I use a piece of metal attached to an electromagnet which I can spin in a circle)

 

[TuxDave]

One mistake is that frictional force isn't always (coefficient of friction)*normal force. That is the maximum force it will exert opposing the direction of motion. So in this case, let me make up some numbers.

Let's say for the given angular velocity and radius and mass you needed a centripetal force of 50N to keep in the circular motion.

The force contributed by gravity is 40N
The maximum force contributed by friction is 40N

Frictional force will exert only 10N since that is the minimum needed to prevent motion. (motion being away from the center of rotation). It wil never exert a force to support a motion.

On the other hand, if you needed 100N of centripetal force, the frictional force will contribute all 40N and since it doesn't have enough force to maintain the circular motion, the car will eventually fall off the track.

 

[CycloWizard]

Originally posted by: DrPizza
If the car lost it's friction with the road surface, there isn't a force that "makes the car fly off the road"; the car simply continues in the direction of its motion. i.e. if it hit a patch of ice, it will cease rounding the bend and will continue to travel in a straight line. It does not "slide off the road in a direction perpendicular to its motion" as many 1st year physics students often think (intuitively.) The reason for this misconception is because they're used to feeling themselves push against the door. This is merely the equal and opposite force because the door is pushing against them, keeping them moving in a circle. If the door were to open, yes, they would move away from the car, but in a straight line in the direction of the cars motion at the moment the door opened. This is often one of those "gotta see it to believe it" types of demonstrations that's necessary in class. (But, instead of a door, I use a piece of metal attached to an electromagnet which I can spin in a circle)

:thumbsup:
I'm sure Dr.Pizza will correct me if I'm wrong here. At any given time, your velocity is tangential to the circle you're travelling in. Thus, your momentum is also in the tangential direction.