Thread: Help with physics

During takeoff, you accidentally leave your tray table down. You find that your car keys slide off of the tray. The explanation for this, in your reference system, is that

a. a forward friction force was exerted on the keys.
b. a backward friction force was exerted on the keys.
c. a forward “fictitious” force was exerted on the keys
d. a backward “fictitious” force was exerted on the keys

i really think its b

What reason do you have for thinking that?

the forward friction of the takeoff will move the keys backwards. I think it should be A now

no ... it's choice (d)

To expand on this a bit...

In your reference frame we ignore what the plane is doing and simple focus on the keys. The keys are moving toward you, so you have to assume a force on the keys. But no force is available. Neither the normal force nor the weight have a component toward you. In your coordinate system these forces are both perpendicular to the tray. Friction cannot cause motion, it can only oppose it. So we have to assume a new kind of force is at work. Since this force has no rational explanation in your coordinate system, we call this force a "fictitious force."

-Dan