Non-inertial Frames of Reference

[Balsam]

Homework Statement

You are in a car accelerating forwards. There is a baseball at your feet. Draw two FBDs showing the the ball's motion from the frame of reference of the car and the frame of reference of the sidewalk. Which frame of reference is non-inertial? In which frame do you observe the fictitious force?

Homework Equations

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The Attempt at a Solution

I don't know what to draw for the FBDs, but I'm confused because I think that both frames are non-inertial and there is a fictitious force in both frames since there is no external force acting on the ball, yet it accellerates backwards from the car and sidewalk's point of view. However, the wording of the question implies that only one frame is non-inertial and only one frame has a fictitious force.

 

[PeroK]

Homework Statement

You are in a car accelerating forwards. There is a baseball at your feet. Draw two FBDs showing the the ball's motion from the frame of reference of the car and the frame of reference of the sidewalk. Which frame of reference is non-inertial? In which frame do you observe the fictitious force?

Homework Equations

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The Attempt at a Solution

I don't know what to draw for the FBDs, but I'm confused because I think that both frames are non-inertial and there is a fictitious force in both frames since there is no external force acting on the ball, yet it accellerates backwards from the car and sidewalk's point of view. However, the wording of the question implies that only one frame is non-inertial and only one frame has a fictitious force.

Why would the sidewalk frame be non-inertial?

 

[Balsam]

Why would the sidewalk frame be non-inertial?

The ball is moving backwards, but no external force is acting on it?

 

[drvrm]

I don't know what to draw for the FBDs, but I'm confused because I think that both frames are non-inertial and there is a fictitious force in both frames since there is no external force acting on the ball, yet it accellerates backwards from the car and sidewalk's point of view. However, the wording of the question implies that only one frame is non-inertial and only one frame has a fictitious force.

its a homework problem so you must submit a template of your attempts-so that you can proceed further.
you might have learn't about inertia and inertial frames ,so go to textbook and see how free body diagrams are drawn.

 

[Balsam]

its a homework problem so you must submit a template of your attempts-so that you can proceed further.
you might have learn't about inertia and inertial frames ,so go to textbook and see how free body diagrams are drawn.

I know how to draw free body diagrams, but I don't think the ball would have any forces acting on it besides gravity and normal and friction- those are the only forces I would draw.

 

[drvrm]

you have two observers
1. in the car
2. on the sidewalk

so two sets of FBD will be there . now which one is non-inertial?
/

 

[PeroK]

The ball is moving backwards, but no external force is acting on it?

Do you think a person on the sidewalk would see the ball moving backwards?

 

[Balsam]

Do you think a person on the sidewalk would see the ball moving backwards?

Ok, it would be moving forwards from their point of view, but I still don't know which force causes it to move forwards other than just the car's acceleration/

 

[Balsam]

you have two observers
1. in the car
2. on the sidewalk

so two sets of FBD will be there . now which one is non-inertial?

I'm not sure. I don't think there's an external force acting on the ball, so wouldn't both be non-inertial? /

 

[PeroK]

Ok, it would be moving forwards from their point of view, but I still don't know which force causes it to move forwards other than just the car's acceleration/

You need to sort two things out in your mind. First: what is a non-inertial reference frame?

The answer to that is essentially a reference frame that is (itself) accelerating. The sidewalk will never be non-inertial unless something very strange happens. The car will vary between an inertial and non-inertial reference frame depending on whether it's accelerating or not.

One of the features of a non-inertial reference frame (e.g. an accelerating car) is that other things (buildings, people, sidewalks) appear to accelerate (to you from inside your car) without any real forces on them.

The second question is what happens to the ball when the car accelerates? Personally, I don't like these questions, because it's not at all clear what will happen to the ball. There is friction on the floor, there are things to bump into and the ball can't go very far before it is knocked forward by something.

But, ideally, the ball stays precisely where it was (unmoved by the car's accleration) until ... it hits something. So what you are being told theorectically will happen is clearly not what happens in reality. The baseball may very well just move with the car - assuming you don't accelerate too fast and it finds a bit of something on the floor to rest against!

 

[Balsam]

You need to sort two things out in your mind. First: what is a non-inertial reference frame?

The answer to that is essentially a reference frame that is (itself) accelerating. The sidewalk will never be non-inertial unless something very strange happens. The car will vary between an inertial and non-inertial reference frame depending on whether it's accelerating or not.

One of the features of a non-inertial reference frame (e.g. an accelerating car) is that other things (buildings, people, sidewalks) appear to accelerate (to you from inside your car) without any real forces on them.

The second question is what happens to the ball when the car accelerates? Personally, I don't like these questions, because it's not at all clear what will happen to the ball. There is friction on the floor, there are things to bump into and the ball can't go very far before it is knocked forward by something.

But, ideally, the ball stays precisely where it was (unmoved by the car's accleration) until ... it hits something. So what you are being told theorectically will happen is clearly not what happens in reality. The baseball may very well just move with the car - assuming you don't accelerate too fast and it finds a bit of something on the floor to rest against!

How would the ball stay where it was? Wouldn't it move backwards relative to the car?

 

[drvrm]

'm not sure. I don't think there's an external force acting on the ball, so wouldn't both be non-inertial?

non inertial frames have accelerations therefore a fictitious force operate in those frames so one can expect the ball in the car to move opposite to the car movement /acceleration .just like you are in a lift going up with acceleration a and you feel heavier /or if you are standing on a scale the wt shown is m.(g+a) . so it acts opposite to the frame's acceleration. and equal to the magnitude of frame acceleration.

 

[PeroK]

How would the ball stay where it was? Wouldn't it move backwards relative to the car?

Not if it's stuck on a bit of carpet! Maybe it would and maybe it wouldn't. I don't know whether you drive, but sometimes things do move about what you accelerate and sometimes they don't.

Also, the ball can only move a few inches perhaps, which is negligible compared to the motion of the car.

I'm just saying that if you did an experiment, it may not match the simplistic theory.

 

[Balsam]

Not if it's stuck on a bit of carpet! Maybe it would and maybe it wouldn't. I don't know whether you drive, but sometimes things do move about what you accelerate and sometimes they don't.

Also, the ball can only move a few inches perhaps, which is negligible compared to the motion of the car.

I'm just saying that if you did an experiment, it may not match the simplistic theory.

The whole lesson we did in class was based on the idea that if you have an object inside an accelerating vehicle, it'll move opposite the direction of acceleration

 

[Balsam]

'm not sure. I don't think there's an external force acting on the ball, so wouldn't both be non-inertial?

non inertial frames have accelerations therefore a fictitious force operate in those frames so one can expect the ball in the car to move opposite to the car movement /acceleration .just like you are in a lift going up with acceleration a and you feel heavier /or if you are standing on a scale the wt shown is m.(g+a) . so it acts opposite to the frame's acceleration. and equal to the magnitude of frame acceleration.

So the non-inertial frame is from the car's point of view, but is there still a fictitious force from the sidewalk's point of view

 

[jbriggs444]

The whole lesson we did in class was based on the idea that if you have an object inside an accelerating vehicle, it'll move opposite the direction of acceleration

If that is what you took away from the experiment, it was an incorrect conclusion. Objects inside an accelerating vehicle seem to move opposite the direction of acceleration only when measured against the accelerating vehicle.

 

[Balsam]

If that is what you took away from the experiment, it was an incorrect conclusion. Objects inside an accelerating vehicle move opposite the direction of acceleration only when measured against the accelerating vehicle.

So relative to the accelerating vehicle, the object moves in the opposite direction?

 

[jbriggs444]

So relative to the accelerating vehicle, the object moves in the opposite direction?

Yes.

 

[PeroK]

The whole lesson we did in class was based on the idea that if you have an object inside an accelerating vehicle, it'll move opposite the direction of acceleration

What about the driver? That used to happen on the Wacky Races (you're probably too young to remember that cartoon). The car would shoot off and the driver and the steering wheel would be left at the starting gate!

What I'm saying is that's a bad example, because all sorts of practical aspects complicate the matter. Do an experiment next time you're in a car.

A better example is when a car crashes: then anything that isn't strapped down (including passengers) goes flying.

 

[Balsam]

Yes.

That's what I meant

 

[Balsam]

What about the driver? That used to happen on the Wacky Races (you're probably too young to remember that cartoon). The car would shoot off and the driver and the steering wheel would be left at the starting gate!

What I'm saying is that's a bad example, because all sorts of practical aspects complicate the matter. Do an experiment next time you're in a car.

A better example is when a car crashes: then anything that isn't strapped down (including passengers) goes flying.

The experiment we did in class had a ball inside a dynamics cart. It wasn't strapped down, so when the cart accelerated forwards, the ball accelerated backwards even though no external forces acted on it

 

[PeroK]

The experiment we did in class had a ball inside a dynamics cart. It wasn't strapped down, so when the cart accelerated forwards, the ball accelerated backwards even though no external forces acted on it

Okay. Sorry if I confused the issue.

 

[Balsam]

Okay. Sorry if I confused the issue.

I'm still not sure how to solve this problem. I think the car's frame is non-inertial since it's an accelerating frame and the sidewalk's frame is not. But, there's no force that explains the motion of the ball in either frame

 

[PeroK]

I'm still not sure how to solve this problem. I think the car's frame is non-inertial since it's an accelerating frame and the sidewalk's frame is not. But, there's no force that explains the motion of the ball in either frame

If I said the ball doesn't accelerate backwards would you believe me?

 

[Balsam]

If I said the ball doesn't accelerate backwards would you believe me?

Why wouldn't it?

 

[jbriggs444]

Why wouldn't it?

Did you measure against the ground to see whether it did or did not?

Edit: To answer your question ("Why wouldn't it"), because Newton's First Law says it will not.

 

[Balsam]

Did you measure against the ground to see whether it did or did not?

Edit: To answer your question ("Why wouldn't it"), because Newton's First Law says it will not.

But this whole lesson is about how objects can move even if there is no net force acting on them

 

[jbriggs444]

But this whole lesson is about how objects can move even if there is no net force acting on them

On the other hand, objects can stay in place when there is no force acting on them. The lesson should also be about knowing when that is the case as well as knowing when it is not.

 

[Balsam]

On the other hand, objects can stay in place when there is no force acting on them. The lesson should also be about knowing when that is the case as well as knowing when it is not.

My teacher says Newton's first law applies when the vehicle is going at a constant velocity or is at rest, but not when the vehicle is accelerating

 

[jbriggs444]

My teacher says Newton's first law applies when the vehicle is going at a constant velocity or is at rest, but not when the vehicle is accelerating

Either the teacher is incorrect or you are interpreting that statement incorrectly. Newton's first law always applies if the frame of reference is inertial. The fact that the car is accelerating does not mean that the road no longer defines an inertial frame of reference.

 

[Balsam]

Either the teacher is incorrect or you are interpreting that statement incorrectly. Newton's first law always applies if the frame of reference is inertial. The fact that the car is accelerating does not mean that the road no longer defines an inertial frame of reference.

The frame of reference is the car and the sidewalk. So, from the car's frame of reference is non-inertial, while the sidewalk's is not.

 

[jbriggs444]

Could you please tell us what a frame of reference is?

 

[Balsam]

Could you please tell us what a frame of reference is?

The coordinate plane from which motion is observed.

 

[jbriggs444]

You spoke as if the car and the sidewalk together define a single frame of reference. Was that your intent?

 

[Balsam]

You spoke as if the car and the sidewalk together define a single frame of reference. Was that your intent?

No, I meant to say they are two different frames of reference