Centripetal force

  • #1
Hello everyone
I know my question might be strange and stupid but I have to ask it!
When an object attached to a string and moved in a horizontal uniform circular motion, the tension of the string will point to the center of the circular motion, right? Well, I cant see why! I know that the acceleration points toward the center of the circle and F=ma, nevertheless, I dont find it reasonable that the force points toward the center because if the tension point toward the center, then the object would move toward the center!! What is the problem in my logic??
 

Answers and Replies

  • #2
970
3
The force always point in the direction of the acceleration, according to Newton's 2nd law.

Objects can't fall toward the center of your force if they have angular momentum about the center of your force. So basically if an object has any speed perpendicular to your force, then it'll never fall towards the center. It can come very close, but never fall in. In order to get the object to fall into the center, you have to slow it down in directions perpendicular to the force.
 
  • #3
rcgldr
Homework Helper
8,774
572
The tension in the string exerts an inwards force on the object but also an outwards force on whatever the string is attached to at the middle of the circular path.

The object is always accelerating towards the center, but it's velocity is just enough for the resultant path to be a circle as opposed to a spiral or other curved path. In the case of a string, ignoring the fact that the string can stretch by a small amount, the string will only generate enough tension to keep the object in a circular path.

In the case of two orbiting objects of equal size in space, the path could be circular or elliptical.
 
  • #4
306
1
Here are some other ideas as well.

Imagine you're roller skating down a street and approach a corner. You decide to turn at the corner by grabbing hold of a lamp post to your right. There is now a force on you, to the right, that deflects you from your previous motion.

The string is deflecting the motion of the mass from a straightline. So if the mass moves counterclockwise the force must be to its left and if it moves clockwise the force must be to its right.

So you might say "yeah but that doesn't mean the force points inwards" so let's imagine what would happen if it didn't. So imagine you're on a large open frozen lake and you're sliding along on your sneakers and you're heading north. Suddenly there is a strong wind blowing to the east (so coming out of the west) and you're deflected from your path, but you are still moving north, its just that now you are also moving east. Together you are moving north-east with an angle from north that depends on how strong the wind is.

The circular motion results from the force always being to the right or left of the mass...which means it points towards the center. If it didn't you wouldn't see circular motion, but some other deflection.

Is that any help? Sometimes I find it helps to imagine what would occur if it weren't the case rather than if it were.
 
  • #5
5,607
40
If you understand F = ma, you should realize that the vectors F and a are colinear and point in the same direction.

To have an acceleration in a directon you need a force in that SAME direction. If you push your feet against the floor (a force) to move a chair back ( an acceleration) , you don't expect the chair to move forwards, correct? ?
 
  • #6
Thx everyone for your help... You helped me to understand the subject
I appreciate it :)
 
  • #7
Hello everyone
I dont find it reasonable that the force points toward the center because if the tension point toward the center, then the object would move toward the center!! What is the problem in my logic??

The object does not move towards the center it moves around it.
 
  • #8
10
0
It doesn't move towards the center, because the direction of the force is constantly changing! And that is because it has an initial velocity.
 

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