# Need Help Understanding Concepts of Centripetal Force.

1. Mar 1, 2009

### ha9981

Now as I understand, a force must be present to cause centripetal acceleration. So as a car goes around a circular arc it is friction between the tires which causes centripetal acceleration. So since the car is turning around the curve the friction will be perpendicular to the instantaneous velocity and therefore towards the center of the arc.

So with this I am guessing Ff must be equal to mv2/r or any other form of the centripetal force eqn. But as I see it friction force and centripetal force are the same in this situation and it is directed to the center, now I ask what is holding the car in its path. To me it seems its friction, but isn't friction causing centripetal force. So what is the force in the opposite direction? I know it has to be equal in magnitude. I was thinking centrifugal but not to sure how it works. From what i remember that centrifugal originates from inertia, so if there was no centripetal force able to be exerted the object in uniform circular motion will go off on a tangent.

Then I read this: "Caution: In doing problems with uniform circular motion, you may be tempted to include an extra outward force of magnitude mv2/r to keep the body "out there" or to "keep it in equilibrium". This outward force is called the centrifugal force (fleeing from the center). Resist this temptation, because this approach is simply wrong. In an inertial frame of reference there is no such thing as centrifugal force."

2. Mar 1, 2009

### Staff: Mentor

You don't need anything to "hold the car in its path". You do need a net force to change the path--to make it go in a circle. In your example, that net force is provided by friction.

3. Mar 1, 2009

### ha9981

I am not to sure what you are saying.

I know there is a centripetal force, and because of that there must be another force or the car will go into the center of the circle, or am i seeing this all wrong?

4. Mar 1, 2009

### Staff: Mentor

You are seeing it wrong, but it's a bit tricky. The acceleration is toward the center, not the velocity.

A force doesn't necessarily make something move in the direction of the force. What it does is provide an acceleration (a change of velocity) in the direction of the force. If the acceleration is perpendicular to the object's direction of motion, the force just makes it turn sideways in a circle, not move toward the center of the circle.

Also: If there were another force acting opposite to the centripetal force, canceling it out, then the net force on the object would be zero. It would just keep moving in a straight line--not around in a circle.

5. Mar 1, 2009

### tiny-tim

Welcome to PF!

Hi ha9981! Welcome to PF!
The centripetal force is the friction force.

Because of that force, the car does go into the centre …

only its tangential velocity prevents it from "falling in".

This is what good ol' Newton discovered about gravity …

if you throw an apple horizontally, it falls … if you throw it hard enough, it still falls at the same rate, but it goes so fast sideways that it stays the same distance from the centre of the Earth … in other words it goes into (very-near-Earth!) orbit.

6. Mar 1, 2009

### ha9981

Re: Welcome to PF!

When you refer to "tangential velocity" is this basically inertia? The inertia the car is feeling because of tangential velocity at each point in the curve keeps it from falling in?

Is this like :
Also is the inertia based force centrifugal force? Because I read that it is fictitious since its based on inertia which doesn't exactly make it a force but just following newtons laws.

7. Mar 1, 2009

### Staff: Mentor

Re: Welcome to PF!

If you wish to view things from the rotating frame of reference, you would add a centrifugal force (not a "real" force, just an artifact of using an accelerating frame of reference) in order to apply Newton's laws.

But I recommend that you try to understand things from the usual inertial frame of reference and don't rely on the "crutch" of the centrifugal force. (At least until you get to more advanced courses, where you'll need to use rotating frames.)

8. Mar 1, 2009

### tiny-tim

Yeah, basically … Newton's first law means it wants to keep going in a straight line.
There's no "inertia based forces" … inertia is inertia, and force is force, and they're more-or-less opposites.

(there are so-called "inertial forces", also called "fictitious forces", but that only means forces whose strength doesn't depend on charge or velocity but only on mass, which is another word for inertia)