Friction and fictitious forces

• ergonomics
In summary, if you are in a non-inertial frame of reference, and you fall back due to a fictitious force, an observer outside of your frame of reference would see the paper clip moving backwards.
ergonomics
i have a question regarding friction and fictitious forces.

if two crates are lying on a flat surface, while the smaller one is on top of the bigger one, and a force of 10N is acted upon the bigger one

will the smaller one be pushed backwards because of fictitous forces?

and because there is friction between the surfaces of the upper side of
the bigger crate and the bottom side of the smaller crate, the smaller one should be slowed down accordingly by kinetic friction?

Why would "fictitious" forces be involved? They appear when you view things from an accelerated reference frame.

In this case, the only horizontal force acting on the smaller crate is due to the friction between the surfaces. If the 10N force acts to the right on the bigger crate and accelerates the crates to the right, the friction force (between the crates) would also act to the right on the smaller crate, dragging it along for the ride.

yes but what happens if you push it a little bit stronger?

intuitively it seems very right to me, that if you put an object on another object and you push the one on the bottom to the right, the upper object seems to be pushed to the left, and then the friction gets in motion, and attempts to stop it.

what is the force which pushes the object to the left?
because the same thing does not seem to apply when the speed is constant.

when the speed is constant the only chance it would ever slip back, is if you tried to increase the speed, but an increase in speed also leads to a small time of acceleration.

ergonomics said:
yes but what happens if you push it a little bit stronger?

intuitively it seems very right to me, that if you put an object on another object and you push the one on the bottom to the right, the upper object seems to be pushed to the left, and then the friction gets in motion, and attempts to stop it.
If for some reason you choose to view things from the frame of the accelerating lower crate, then yes the upper crate will seem to be pushed to the left with respect to the lower crate.

But you can also--more simply--view things from the inertial frame of the floor. In that case, there are no fictitious forces to worry about. The only force acting on the upper crate is friction--a real force. So, with respect to the floor, the upper crate will accelerate to the right, never to the left. (Viewed from the floor, the upper crate never moves to the left, only to the right.)

what is the force which pushes the object to the left?
because the same thing does not seem to apply when the speed is constant.
That "force" pushing to the left is just an artifact of viewing things from an accelerating frame. If the speed is constant, there is no accelerating frame and no fictitious forces.

i tried conducting a little experiment testing this phenomenas. i took a paper clip and placed it on my notebook, and as i pushed the notebook to the right, from my frame of reference, which is the chair or the floor, the paper clip seemed to move backwards

am i getting something wrong here?
your explanation of the frames of reference really clears up a lot of things to me, but something still does not seem to sit right.

and i have another question if we are already discussing this.
an hypothetical situation -
if i am in a non inertial frame of reference, and i fall back due to a fictitious force, how would it look to an outside observer? if fictitious forces only take place in my frame.

Last edited:
ergonomics said:
i tried conducting a little experiment testing this phenomenas. i took a paper clip and placed it on my notebook, and as i pushed the notebook to the right, from my frame of reference, which is the chair or the floor, the paper clip seemed to move backwards

am i getting something wrong here?
That simply isn't possible. Make a mark on the table and actually look to see if the paper clip moves to the left or right of the mark. The paperclip will move to the right.

yes i think i understand where i got it wrong now.

i went through my physics book, and if i understand it right, the lower object is dragging the upper object along with it due to electromagnetism, and reasons which supposedly will be explained later referring to other units of the course.

what i do not understand though, why would the upper object deattached from the lower one in the first place, is the electromagnetic force not strong to keep up?

and how can the electromagnetic force(friction) be affected by external forces that are pushing the lower object forward?

the way i see it if this is all true, then everytime i touch something i change its internal composition.

ergonomics said:
what i do not understand though, why would the upper object deattached from the lower one in the first place, is the electromagnetic force not strong to keep up?
Exactly. The static friction force has a maximum strength:
$$F_{static} \leq \mu mg$$

If the acceleration of the lower block is too great, the static friction will not be enough to give the upper block the same acceleration. In that case, the upper block will start slipping, being dragged along by kinetic friction but not enough to keep up with the lower block.

1. What is friction?

Friction is a force that resists the relative motion between two surfaces in contact. It is caused by the microscopic irregularities on the surfaces, which interact with each other when they come into contact.

2. How does friction affect motion?

Friction can either increase or decrease the motion of an object, depending on the direction of the force. When the force of friction is opposite to the direction of motion, it will slow down the object. When the force of friction is in the same direction as the motion, it can increase the object's speed.

3. What is the difference between static and kinetic friction?

Static friction is the force that resists the motion of an object that is at rest. It is typically greater than kinetic friction, which is the force that opposes the motion of an object that is already in motion.

4. What are fictitious forces?

Fictitious forces are forces that appear to act on an object but are not caused by an actual physical interaction. These forces are typically observed in non-inertial reference frames, such as an accelerating or rotating frame of reference.

5. How do fictitious forces relate to friction?

Fictitious forces are often used to account for the effects of friction in non-inertial reference frames. For example, in a car turning around a curve, the centripetal force is a fictitious force that is used to account for the friction between the tires and the road that allows the car to make the turn.

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