# Loop-the-loop normal force

1. Mar 19, 2005

### metalmonkey

Can anyone provide an explanation as to what the normal force is reacting to in the case when a jet is flying a loop-the-loop?? I understand for a rollercoaster that at the top of the loop the normal force is due to the track exerting a force back on to the rollercoaster cart, contributing to the centripetal force, but for a plane, where is the normal coming from???

Only thing I can imagine is that it has something to do with the inertia of the plane?? But then why would the normal act downwards at the top of the loop?

Cheers

2. Mar 19, 2005

### Andrew Mason

The air.

AM

3. Mar 19, 2005

### Staff: Mentor

On the plane itself, consider the vertical forces acting at the top of the loop: The air pushing on the plane ("lift") and gravity. These forces must produce the (downward) centripetal acceleration of the plane if the plane executes circular motion.

The term "normal" force is usually reserved for the perpendicular force between two surfaces, such as between car and track in the roller coaster case. In the airplane example, one could ask for the normal force on the pilot (exerted by the plane) at the top of the loop.

4. Mar 19, 2005

### barney007

look at it, as whats exerting the normal force on the person in the plane, which would be the chair.

5. Mar 19, 2005

### mick5

Normal force

For the plane moving in a circle. If the magnitude of the velocity is constant, then by Newton's second law, the net force on the plane is towards the centre (centripetal force). The pilot in the plane (if tightly secured to the seat) will also move in a circle. At the top of the loop the weight acting on the pilot and force exerted by the seat on the pilot act downward. These two forces provide the size of the centripetal force.

n +mg = mv*v / r rearranging for n gives n = m(v*v /r - g)

6. Mar 19, 2005

### Andrew Mason

And what is it that provides the force on the seat that the pilot feels. It can't be gravity. It is the force of the plane against the air.

AM

7. Mar 19, 2005

### barney007

so if the track exerts a force on a car in loop to loop, what exerts the force on the track?

8. Mar 20, 2005

### Andrew Mason

The mass/inertia of the rest of the rollercoaster and the earth to which it is attached.

AM

9. Mar 21, 2005

### Antiphon

The forces are the aerodynamic forces of lift which (usually) point up from
the wings. When the plane is upside down, the wings point down.

The easy way to visualize this is that there is a vaccum condition just above
the wings of plane which always pull the wings toward the vacuum.