# Uniform Circular Motion and Centripetal Acceleration (non-intuitive)

A.T.
...person exerts backwards force on seat.
Yeah, we must not forget how the seat feels about it.

CWatters
PeterO
Homework Helper
When a car is accelerating, you have a Newton 3rd law pair of forces, the seat exerts a forwards force on the person, the person exerts a reaction force on the seat. The reaction force is a real force in any frame of reference.

Using the car as an accelerating frame of reference, the fictitious force is the force that would appear to accelerate a body in "free fall" with respect to the accelerating frame of reference, and would not be part of a Newton 3rd law pair of forces. The Newton 3rd law pair of forces are still the same, seat exerts forward force on person, person exerts backwards force on seat.
Nothing wrong with those Newtons 3rd law Forces, other than why they are happening.
I contend a person only expects to apply a force on the seat, if they decide to push, or can simply explain why they are doing it.
In a stationary car, If a third person pushes an occupant, the occupant is not surprised that they therefore apply/transfer a push to the seat, and they will feel the seat pushing back. That person does NOT feel themselves pushing back on the third person - only the third person feels that force.
When the occupant is in the accelerating car, they feel the seat pushing forward, but may be at a loss to explain why the seat is doing that - because in that situation there is no third person instigating the action/reaction. Unfortunately, after just a few seconds, the car ceases to accelerate and the occupant quickly loses interest in the situation except to think "wow, something really pushed me into that seat just then", and perhaps include "it must have been the engine". Remember, we are not talking about a physics person explaining what is happening, we are talking about an "innocent" wondering what just happened.

rcgldr
Homework Helper
When the occupant is in the accelerating car, they feel the seat pushing forward, but may be at a loss to explain why the seat is doing that
A person undergoing acceleration is going to feel real forces internal to their body and feel compression forces at the points of contact with the seat. This is more clear in the case of greater acceleration such as a graviton / rotor spinning carnival ride that pins the persons (victims) against the wall while the floor drops, or in a vehicle capable of pulling more than 1 g.

PeterO
Homework Helper
A person undergoing acceleration is going to feel real forces internal to their body and feel compression forces at the points of contact with the seat. This is more clear in the case of greater acceleration such as a graviton / rotor spinning carnival ride that pins the persons (victims) against the wall while the floor drops, or in a vehicle capable of pulling more than 1 g.
Yes they will feel the force, but if they are a non-physics person (as are our students when they begin to stud the subject) they won't necessarily understand why that force suddenly came about. Seats and walls don't usually start pushing you unless you start pushing them - in these cases, the seat/wall seems to instigate the entire action/re-action.

rcgldr
Homework Helper
Yes they will feel the force, but if they are a non-physics person (as are our students when they begin to stud the subject) they won't necessarily understand why that force suddenly came about. Seats and walls don't usually start pushing you unless you start pushing them - in these cases, the seat/wall seems to instigate the entire action/re-action.
I think most non-physics persons get a sense of these forces when riding roller coasters that include high g (accelerating) turns.