# Understanding centripetal force

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1. Sep 26, 2015

### SamC

I don't have a specific homework assignment, but rather my problem is with the general concept. I don't understand the forces acting in uniform circular motion. As I understand it, the force is directed inward, and there is no other force in the same dimension (in an idealized model). But in my mind, that would mean you would be flung towards the center, or at least slowly move towards the center in a spiral.

If I imagine a Gravitron that moves really fast and causes you to be pushed back against the wall, there should be an acceleration towards the center, but also a tangential force "trying to leave" the circle in a straight line, in the direction of the velocity vector. But in the free-body diagram in my book, there is only a centripetal force in the direction of the radius (the normal force in this case). I can understand that perhaps it doesn't show because it is not a force acted on me, but by me. But I still feel like there should be some force that causes the tangential velocity in the first place, and that causes me to speed up enough to "lift" from the floor. After all, both me and the Gravitron would be standing still to begin with.

Please, I am not questioning the validity of the idea that there is only a force inward. I just need help understanding in a way that helps change my intuition.

2. Sep 26, 2015

### nuuskur

The way I think about the Centripetal force is, paraphrasing, "center seeking" force. Without the presence of the centripetal force you would continue on in a straight line, but as you go round in a circular motion, the only logical explanation is there is a force that acts towards the centerpoint. The feeling you feel, commonly referred to as "centrifugal" force", the imaginary force that supposedly pushes you away from the center is a result of inertia (Newton's first law).

You are always speeding up, because you are constantly accelerating.

Last edited: Sep 26, 2015
3. Sep 26, 2015

### SamC

So, is it kind of like if you were sitting in a resting car, and it suddenly gets hit by a truck and you crash into the window? That is to say, my mass is naturally trying to stand still (or move along with earth), and the car is trying to move "through" me, but my inertia resists (in a tangential direction)?

4. Sep 26, 2015

### jbriggs444

You should not think of inertia as resisting applied forces. Rather you should think in terms of forces causing acceleration. The magnitude of that acceleration is inversely proportional to an object's mass. The direction of that acceleration is identical to the direction of the net applied force.

If you have a radial force, you have a radial acceleration. Not a tangential acceleration.

In uniform circular motion, the direction of the acceleration is exactly radial.

5. Sep 27, 2015

### CWatters

Velocity has components speed and direction. Change either component and you have changing velocity which means acceleration. Newton says that to have acceleration a net force is needed in the direction of the acceleration. That force has to be provided by something like gravity, tension in a string or increased lift on a wing depending on the situation. There might be lots of forces in different directions acting on the object but in uniform circular motion in the end its the net force acting at right angles to the direction of motion (eg towards the centre) that provides the necessary centripetal force.

6. Sep 27, 2015

### CWatters

Just to repeat.. Newton says that a net force implies an acceleration. An acceleration can involve a change to either the speed or direction component of velocity. In uniform circular motion you have acceleration towards the centre without movement towards the centre. That might seem odd but its not the only place something like this occurs. Perhaps take a look at the velocity and acceleration of a piston in a car engine at the very instant it passes the top.

7. Sep 29, 2015

### SamC

Thank you!
I did understand it fine when we were just dealing with the motion, because I could see a tangential velocity vector, and a radial acceleration vector. I just got confused when we did the same thing with forces, because now there was no longer a tangential vector to "counteract" the radial one, and it didn't make any sense at first.
I do think i get it now though, thank you for your explanation!