- In the previous example, if the net force on the car at its highest
point is straight down, why doesn’t the car fall straight down?
Here’s How to Crack It
Remember that force tells an object how to accelerate. If the car had zero velocity
at this point, then it would fall straight down, but the car has a nonzero velocity (to
the left) at this point. The fact that the acceleration is downward means that, at the
next moment v will point down and to the left at a slight angle, ensuring that the car
remains on a circular path, in contact with the track. This contact force is the
normal force. You can think of the normal force as a measure of the strength of
contact between two surfaces. If it is a high value, they are pressing very hard
against one another. If it is a low value, they are not pressing very hard against one
another. It is easy to see, then, that if the normal force is zero, there is no contact
between the cart and the surface. So long as the normal force is not zero, the cart is
still in contact with the surface.
- In the previous examples, what is the minimum speed necessary
to keep the roller coaster on the track at all times?
Here’s How to Crack It
The position where the roller coaster is most in danger of leaving the track is at the
top.
The slower the object moves, the smaller FN gets (FW is a constant). Therefore, the
minimum speed occurs when FN =0. (For a moment, the rollercoaster is touching
the track but not pressed against it; it’s almost losing contact.)