# B Reaction Force, Centripetal Force

1. Oct 14, 2016

### Kajan thana

Does reaction force only occur when it is in contact with another object?

And also if rollercoaster goes around in a circle, at the maxium height, why isn't there any reaction force (got this from a book). Personally, I think that rollercoaser have clamps attached between the path and the cubicle, so when it is at the top, shouldn't there be reaction force against weight point upward.

2. Oct 14, 2016

### Zypheros_Knight

Not really, think the system between the earth and sun, gravity provides the centripetal force and earth's mass pulling outward provides the reaction force (don't know if I was clear enough), basically, there is no direct contact between sun and earth.
But! I request you to please describe briefly the second part of your question (because I'm completely lost there!!!).

3. Oct 14, 2016

### PhanthomJay

The term "Reaction Force" is vague. It has different meanings depending on whether you are considering Newton's first law ( forces acting on the same object for equilibrium) or his 3rd law (equal and opposite forces acting on different objects). Generally, contact is required, unless action at a distance forces like gravity or magnetic forces are involved.

What kind of a roller coaster are you talking about, a 'loop-de- loop' coaster where at the top you are upside down under the rails, or a conventional coaster where at the top you are upright with the rails beneath you? In the first case there is a normal contact force (I wouldn't call it a reaction force) between you and the seat, acting downward on you. For the second case, any contact force between you and the seat acts upward on you, unless the coaster's speed is high enough to make you jump off the seat, in which case safety features like the safety bar and rail clamps provide the contact force.

4. Oct 14, 2016

### Zypheros_Knight

He's talking about the 3rd Law (I think) and in Newton's 1st Law the thing you were talking about is ''Resultant Force'' because resultant force decides whether the object travels with uniform motion or stays at rest.

5. Oct 14, 2016

### PhanthomJay

Often in engineering if you have say a loaded beam on simple supports, the forces from the supports on the beam are called reaction forces, determined from the 1st law, not the third.

6. Oct 14, 2016

### Kajan thana

Is there any reaction fore at maxium height in loop de loop rollercoaster.
And if there is, what direction will it act?

7. Oct 14, 2016

### Zypheros_Knight

If the roller-coaster is going in loop de loops or around any circular path or even anything!!! Even if the roller-coaster comes to rest at a max height or any height there will still be a reaction force....because the gravity is pulling the coaster down but the rail's provide reaction force so that the coaster remains at rest (or uniform motion) !!!

8. Oct 14, 2016

### Kajan thana

I always though reaction for acting in opposite direction to your weight. So with the loop de loop rollercoaster, at maxium height the there is centripetal force towards the centre so is the weight acting in opposite direction? From the concept of sun and earth.

Or in order for the people not to fall down when they are upside down, (from newtons third law) should there not be equal and opposite force acting in opposite direction to weight?

9. Oct 14, 2016

### Zypheros_Knight

I highly recommend that you draw a neat diagram to represent this, sorry!

Edit*: Your first statement is true!!!

10. Oct 14, 2016

### Kajan thana

Buddy,

Can we consider reaction force and contact force the same in this context?
And what direction will this act?

11. Oct 14, 2016

### PhanthomJay

In a ' loop de loop' coaster, at the top of the loop, there are 2 forces acting on you. One of these forces is your weight, W, which is the non-contact gravity force of the earth acting down on you. The other force acting on you is the normal contact force (N) of the seat, which also acts down on you. Together, these forces sum to provide the centripetal force and acceleration acting toward the center of the loop (W + N = mv^2/r). Neither of these forces are reaction forces from a Newton 3 viewpoint. I would not call the contact force a reaction force. The contact force is the normal force which acts on you just like the weight acts on you. I suppose, from a third law stand point, you could call the equal and opposite normal force that you exert on the seat a so-called 'reaction' force, but the term is confusing. Even more so when you consider forces from your frame of reference.

12. Oct 14, 2016

### A.T.

Depends on how fast you go through the loop.

13. Oct 14, 2016

### Kajan thana

Made so much sense. Thank you Jay.

14. Oct 14, 2016

### rcgldr

At the top of the loop, if the coaster is going fast enough, then the track is still exerting some downwards centripetal force onto the coaster, part of a Newton third law pair where the coaster exerts an equal in magnitude but upwards reaction force onto the track.

In the two body circular orbit case, both bodies circle around a common center of mass, and both experience only a centripetal force towards the common center of mass due to gravity. There's no reaction force when the only force involved is gravity (free fall). The Newton third pair of forces are the gravitational forces that each body exerts on the other.

15. Oct 14, 2016