Physics Mechanics-force

[yingao88]

Can someone explain to me why the following choices are wrong?　ｐｌｅａｓｅ!😍

An object is being acted upon by two (and only two) external forces, F1 and F2. if the object has a nonzero acceleration, which one of the following must be true?

A. the object cannot move at constant speed.
(if acceleration is not zero then velocity cannot move at constant speed?)

B. the forces F1 and F2 have the same line of action.
(what does that mean by same line of action? )

C. the magnitude of F1 cannot equal the magnitude of F2.


D. the sum F1+F2 is not zero. (I understand this one)

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[MMa23]

An object is being acted upon by two (and only two) external forces, F1 and F2. if the object has a nonzero acceleration, which one of the following must be true?

A. the object cannot move at constant speed.
(if acceleration is not zero then velocity cannot move at constant speed?)
constant speed = no net force thus cannot have an acceleration, this is false

B. the forces F1 and F2 have the same line of action.
(what does that mean by same line of action? )

It means they point in the same direction. so this is false

C. the magnitude of F1 cannot equal the magnitude of F2.

If F1 is not equal to F2 then at the most extreme case they are opposite each other and there is a net force. this could possibly be true since the forces can each point in the same direction

D. the sum F1+F2 is not zero. (I understand this one)

this is the correct answer

 

[mehc012]

An object is being acted upon by two (and only two) external forces, F1 and F2. if the object has a nonzero acceleration, which one of the following must be true?

A. the object cannot move at constant speed.
(if acceleration is not zero then velocity cannot move at constant speed?)
constant speed = no net force thus cannot have an acceleration, this is false

I'm going to have to disagree with you on this part...first you state that "[the object] cannot move at a constant speed", and then you say that this means that choice A is false.

However, both conclusions are incorrect...if your first conclusion were correct, then A would not be false.

At any rate (no pun intended, lol) an object undergoing nonzero acceleration CAN move at a constant speed. Acceleration implies a change in velocity, but as velocity is a vector quantity, changes in direction and/or magnitude both count as acceleration. An acceleration which alters only the direction would leave the object with the same speed (magnitude of velocity). For example, think of an object moving in a circle: the velocity is constantly changing as it turns, but it could maintain a constant speed.

So, because an object with nonzero acceleration CAN maintain a constant speed, Option A is false!

 

[dellian101]

I think A is false also.


Velocity requires for a nonzero acceleration. Constant velocity means there is no acceleration. If you have acceleration, your velocity cannot be constant, because it will always be changing.

 

[Meredith92]

I think the answer is B If they act in the same line of action I take that to mean they're in the same line... Which doesn't have to be true for non zero accel.. They could act at angles to each other... Like in a pendulum the tension and force of gravity are not in the same "line of action" and therefore the object accelerates.

I may be interpreting " line of action" incorrectly but thats how I would have answered it

 

[Deadlifts]

I think the answer is B If they act in the same line of action I take that to mean they're in the same line... Which doesn't have to be true for non zero accel.. They could act at angles to each other... Like in a pendulum the tension and force of gravity are not in the same "line of action" and therefore the object accelerates.

I may be interpreting " line of action" incorrectly but thats how I would have answered it

Line of action implies the vector quality of the force is going through the same coordinates (aka overlapping).

The question is asking which MUST be true, not what might be true.

A. It can have a constant speed as long as the direction is changing (velocity is a vector quantity) and still have a non zero acceleration.
B. Possible but not necessary.
C. Possible but not necessary.
D. This is necessary for the condition of a non-zero acceleration if F1 and F2 are the only forces acting upon the object. sumF=ma

 

[mehc012]

Yeah, D is for certain the right answer...the first reply gives pretty good details as to why (with the exception of the explanation for A, which I tried to address), but in summary, according to Newtons laws, you cannot have a nonzero acceleration unless the net force on an object =/= 0.
Since F1 and F2 are the only forces acting on the object, the net force would be F1+F2, and therefore F1+F2 =/= 0 while the object is undergoing nonzero acceleration.

Sorry, just realized that I added nothing which Deadlifts hadn't already addressed...great answer, D, concise and thorough!

 

[MMa23]

I'm going to have to disagree with you on this part...first you state that "[the object] cannot move at a constant speed", and then you say that this means that choice A is false.

However, both conclusions are incorrect...if your first conclusion were correct, then A would not be false.

At any rate (no pun intended, lol) an object undergoing nonzero acceleration CAN move at a constant speed. Acceleration implies a change in velocity, but as velocity is a vector quantity, changes in direction and/or magnitude both count as acceleration. An acceleration which alters only the direction would leave the object with the same speed (magnitude of velocity). For example, think of an object moving in a circle: the velocity is constantly changing as it turns, but it could maintain a constant speed.

So, because an object with nonzero acceleration CAN maintain a constant speed, Option A is false!

Yeah, you're correct, sorry lol can't think straight right now!

 

[yingao88]

thanks everyone for the reply!