Force question

[csx]

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an obj is acted upon by 2 forces (f1 and f2) and the obj has a non-zero acceleration. Therefore which of these is true?

A) The magnitude of F1=/=F2
B) The obj's speed will not be constant
C) F1+F2 cannot be zero

They all sound correct to me since the obj has a non-zero acceleration. Apparently C is the right answer.
How come???

 

[justadream]

@csx

I think A) can be true because:
Think about a box. You apply two forces, both of which are 10N. If you apply F1 to the right and F2 straight down, the box will go to the right. F2 is canceled out by normal force.

B) can be true. Let's say you have something in uniform circular motion. Acceleration is non-zero because velocity is changing. Speed is always the same.

 

[csx]

@csx

I think A) can be true because:
Think about a box. You apply two forces, both of which are 10N. If you apply F1 to the right and F2 straight down, the box will go to the right. F2 is canceled out by normal force.

B) can be true. Let's say you have something in uniform circular motion. Acceleration is non-zero because velocity is changing. Speed is always the same.

For B...since there is indeed a force, this would require an acceleration to be true. If acceleration was zero then there would be no force (F=m x zero = 0). That said since there is an acceleration the speed cannot be constant because it is slowly increasing no matter how small the magnitude. That was my rationale .

 

[justadream]

@csx

In UCM, use F = ma = mv^2 / r. So acceleration can be non-zero.