Which object is moving to the right with a constant speed?

[Ineedhelpwithphysics]

Homework Statement

Which one(s) of the following force diagrams depict an object moving to the right with a constant speed? List all that apply.

Relevant Equations

Newtons first law

I need help with this question how in the world is it A and C. In A everything is cancelling out so nothing is movingfand nothing moving to the right. In C its not moving to the right. Isnt it suppose to be D since sits going right constant to 20N.

 

[PeroK]

What does Newton's first law tell you?

 

[Ineedhelpwithphysics]

What does Newton's first law tell you?

idk object keep going in straight line

 

[PeroK]

idk object keep going in straight line

Whatever!

 

[hmmm27]

The question says "constant speed". Is the object in 'd' moving at a constant speed ? what about 'b' ?

 

[DaveC426913]

idk object keep going in straight line

No really. Go read your lessons! What is Newtons First Law?

 

[Ineedhelpwithphysics]

Whatever!

huh i still don't understand none of them is moving right with a constant speed execpt for d. For a, the box is not moving since in all directions its canceled out. , b is not right, c the box isn't moving. d, is the only one moving right

 

[PeroK]

huh i still don't understand none of them is moving right with a constant speed execpt for d. For a, the box is not moving since in all directions its canceled out. , b is not right, c the box isn't moving. d, is the only one moving right

It seems to me that you are using Aristotle's laws of motion, rather than Newton's. Assuming you really don't know what Newton's laws say, then you ought to find out.

 

[Ineedhelpwithphysics]

It seems to me that you are using Aristotle's laws of motion, rather than Newton's. Assuming you really don't know what Newton's laws say, then you ought to find out.

im not the quesition is not saying where the object is moving. How in the world if everything is canceled out in both directiosn its going right. IIm pulling a box up and down how is the box moving right

 

[PeroK]

im not the quesition is not saying where the object is moving. How in the world if everything is canceled out in both directiosn its going right. IIm pulling a box up and down how is the box moving right

Because it was moving right before you started pulling on it?

Presumably you drive a car? If you take your foot off the accelerator, does the car keep moving or stop dead?

 

[Ineedhelpwithphysics]

Because it was moving right before you started pulling on it?

Presumably you drive a car? If you take your foot off the accelerator, does the car keep moving or stop dead?

Lol my teacher told me the same thing but the questions didnt say it was already pulled to the right and i still don't get how its pulled up and down it an move right

 

[sysprog]

huh i still don't understand none of them is moving right with a constant speed execpt for d. For a, the box is not moving since in all directions its canceled out. , b is not right, c the box isn't moving. d, is the only one moving right

D has a constant acceleration rightward; not a constant speed (the Ns in the diagram refer to 'Newton', a unit of force).

Where the accelerations in opposite directions are equal, the speed in either of those directions, whatever that speed may be, is constant (see the First Law of Motion).

It may be confusing to you that the question asks which objects are moving rightward at a constant speed when it could as easily have said moving leftward, but the direction of motion has nothing to do with the change or constancy of the speed.

If the accelerations in opposite directions are equal, then in whichever of those directions the object may be moving, in that direction its speed is constant.

 

[PeroK]

Lol my teacher told me the same thing but the questions didnt say it was already pulled to the right and i still don't get how its pulled up and down it an move right

You're in your car, gravity is pushing you down, the road is pushing you up; yet the car continues to move forwards.

That's Newton's first law: objects don't need a force to be moving.

If you throw a ball, what keeps it moving after it leaves your hand?

 

[Ineedhelpwithphysics]

You're in your car, gravity is pushing you down, the road is pushing you up; yet the car continues to move forwards.

That's Newton's first law: objects don't need a force to be moving.

If you throw a ball, what keeps it moving after it leaves your hand?

Ok but what about C its not moving right

 

[sysprog]

Ok but what about C its not moving right

The diagram doesn't show anything about that. It just shows balanced oppositional accelerations in the directions that it shows.

 

[PeroK]

Ok but what about C its not moving right

It may be moving right at constant speed. It's certainly moving at constant velocity.

 

[Ineedhelpwithphysics]

The diagram doesn't show anything about that. It just shows balanced oppositional accelerations in the directions that it shows.

i think i get it but the question needs to be more clearers i got confused because of the moving to the right thing

 

[sysprog]

i think i get it but the question needs to be more clearers i got confused because of the moving to the right thing

From my prior post (#12):

It may be confusing to you that the question asks which objects are moving rightward at a constant speed when it could as easily have said moving leftward, but the direction of motion has nothing to do with the change or constancy of the speed.​

 

[Lnewqban]

i think i get it but the question needs to be more clearers i got confused because of the moving to the right thing

The problem is not telling you that a force was applied (which induced the movement to the right that we are now seeing), and then removed (no additional acceleration or increasing velocity happened after that moment) from the object before the moment or condition they are showing.

http://www.physicsclassroom.com/Class/newtlaws/u2l1a1.gif

 

[Steve4Physics]

Hi @Ineedhelpwithphysics.

Can you accurately state Newton’s first law? (You’ve already been asked a couple of times.)

Unless you know, understand and can apply the law, you are limiting your ability to answer the original question and similar ones. So what is the law?

 

[sysprog]

Hi @Ineedhelpwithphysics.

Can you accurately state Newton’s first law? (You’ve already been asked a couple of times.)

Unless you know, understand and can apply the law, you are limiting your ability to answer the original question and similar ones. So what is the law?

The illustration linked to by @Lnewqban might be helpful here:

 

[haruspex]

i think i get it but the question needs to be more clearers i got confused because of the moving to the right thing

I agree the question is poorly worded. It should ask which could be moving to the right at constant speed.
It could also have been made a bit more interesting by removing one of the 25N forces in C, say. Do you see how that could still be an answer?

 

[Ineedhelpwithphysics]

Hi @Ineedhelpwithphysics.

Can you accurately state Newton’s first law? (You’ve already been asked a couple of times.)

Unless you know, understand and can apply the law, you are limiting your ability to answer the original question and similar ones. So what is the law?

object at rest stay at rest unless actec upon by external force, object go in straight line forever with constant speed unless acted upon by external force

 

[Steve4Physics]

object at rest stay at rest unless actec upon by external force, object go in straight line forever with constant speed unless acted upon by external force

That's good. A slightly more useful version might be:

"An object at rest stays at rest unless acted upon by a resultant external force; an object goes in a straight line forever with constant speed unless acted upon by a resultant external force"

Resultant force means total force,.

You can now go through the four diagrams in Post #1 and find the resultant force for each. Can you tell us what the four resultant forces are (magnitude and direction)?

 

[DaveC426913]

Well, you don't want total force; you want net non-zero force.

 

[sysprog]

@DaveC426913, I think that @Steve4Physics meant total in the sense of what remains after positives and negatives are tallied, which is the same thing as net; however, I too think that that "resultant force means net force" would be more clear than "resultant force means total force" is.

 

[kuruman]

@DaveC426913, I think that @Steve4Physics meant total in the sense of what remains after positives and negatives are tallied, which is the same thing as net; however, I too think that that "resultant force means net force" would be more clear than "resultant force means total force" is.

I think that using "resultant force" to denote "net force" could lead to ambiguity. I understand "resultant force" to mean "vector sum of any number of forces". Suppose you have three forces acting on a mass. You can add vectorially $\vec F_1$ and $\vec F_2$ to get the resultant $\vec F_{12}$. Likewise, you can get two more resultants, $\vec F_{13}$ and $\vec F_{23}$. However, you can get only one net force and that is $\vec F_1+\vec F_2+\vec F_3.$ Of course it so happens that the net force is also the resultant of all the forces acting on the mass, but it is not the only resultant that you can have acting on the mass. That is why I avoid using "resultant force" when I mean "net force." Conversely, when I use "resultant", I specify which forces are to be added vectorially to form it.

 

[sysprog]

@kuruman, yes, I think that it would have been clearer if Newton had said 'net force' to begin with.

 

[Steve4Physics]

I will forever deeply regret using the term 'resultant force' rather than 'net force'.

However, we still don't know if @Ineedhelpwithphysics understands how to find each net force(zero or otherwise) and then use it to answer the original question.

 

[jbriggs444]

@kuruman, yes, I think that it would have been clearer if Newton had said 'net force' to begin with.

Since "net" is not a Latin word, this would have been difficult, of course. One would also not have wanted Newton to have used "nitere".

net (adj.)

"remaining after deductions," early 15c., from earlier sense of "trim, elegant, clean, neat" (c. 1300), from Old French net, nette "clean, pure, unadulterated," from Latin nitere "to shine, look bright, glitter" (see neat (adj.)). Meaning influenced by Italian netto "remaining after deductions." As a noun, "what remains after deductions," by 1910. The notion is "clear of anything extraneous."

Net profit is "what remains as the clear gain of any business adventure, after deducting the capital invested in the business, the expenses incurred in its management, and the losses sustained by its operation" [Century Dictionary]. Net weight is the weight of merchandise after allowance has been made for casks, bags, cases, or other containers.