Hello everyone.

Im 14 years old from Israel and I had a test about the Newton's 3 Laws of Motion.

I have one question for you.

 

Ruth is pushing a stroller.

Which sentence exlplains the situation?

A. The force that Ruth is pushing the stroller is bigger than the force that used by the stroller.

B. The 2 are equal.

 

I think that B is the right answer because of the third law of Newton.

Please help me, it worth 3% of the grade.

Thanks a lot,

Etgar.

Edited December 31, 201213 yr by EtgarSH

Yes, that is correct. The force Ruth exerts on the stroller must be equal in magnitude and opposite in direction to the force it exerts on her.

Ooh that brings me back many years ago. Thank you Etgar.

 

If the force is equal then we have equilibrium, don't we?

Edited December 31, 201213 yr by michel123456

Thanks a lot!

Happy New Year

Ooh that brings me back many years ago. Thank you Etgar.

 

If the force is equal then we have equilibrium, don't we?

Not necessarily. The force will be equal but opposite in any case. Whether the stroller accelerates or not is dependant on whether the external forces on it balance.

Key word in JCM's post is external. The acceleration of an abject is due to the forces acting on it. A force exerted by an object does not go into that calculation.

Not necessarily. The force will be equal but opposite in any case. Whether the stroller accelerates or not is dependant on whether the external forces on it balance.

In this case, the force acting point is important. I draw a detail picture about the problem.

If the 2 forces are equal and in opposite direction, what is that makes the stroller move in one direction and not the other?

Two forces are always the same.

The difference is that dF/dt is 0 or other t function.

If dF/dt is zero, It might be a constant acceleration moving or not moving condition.

If dF/dt is not zero, it might be a nonconstant acceleration. This is a general case when we move something.

Edited January 1, 201313 yr by alpha2cen

If the 2 forces are equal and in opposite direction, what is that makes the stroller move in one direction and not the other?

 

Read my previous post.

 

Read my previous post.

 

i don't understand your previous post.

 

This is what i understand:

When Ruth is at a distance the force she exerts on the stroller is zero and is equal to the force the stroller exerts upon Ruth: that is zero.

When Ruth pushes a little bit on the stroller, the force Ruth exerts on the stroller is the same with the force the stroller exerts upon Ruth.

When Ruth pushes more, again the 2 forces are equal and in opposite direction. IOW the forces are always equal and in opposite direction.

 

At some time, the force Ruth exerts on the stroller is enough to put the stroller in motion. What is that? Does Ruth exerts an external force? the change in motion means acceleration, no?

 

i don't understand your previous post.

 

This is what i understand:

When Ruth is at a distance the force she exerts on the stroller is zero and is equal to the force the stroller exerts upon Ruth: that is zero.

When Ruth pushes a little bit on the stroller, the force Ruth exerts on the stroller is the same with the force the stroller exerts upon Ruth.

When Ruth pushes more, again the 2 forces are equal and in opposite direction. IOW the forces are always equal and in opposite direction.

 

At some time, the force Ruth exerts on the stroller is enough to put the stroller in motion. What is that? Does Ruth exerts an external force? the change in motion means acceleration, no?

 

Ruth exerts a force on the stroller. The net force exerted on the stroller dictates its motion. The force exerted by the stroller is not part of that analysis. That the forces are equal and opposite is not pertinent to the discussion of the motion, because they do not cancel each other in any way — they are not exerted on the same object.

 

Ruth exerts a force on the stroller. The net force exerted on the stroller dictates its motion. The force exerted by the stroller is not part of that analysis. That the forces are equal and opposite is not pertinent to the discussion of the motion, because they do not cancel each other in any way — they are not exerted on the same object.

 

So there is a net force on the stroller that dictates its motion.

 

So there is a net force on the stroller that dictates its motion.

Yes

The question is hard to interpret as stated, we seem to have a translation or paraphrase issue.

 

It looks like one of those questions designed to evaluate the clarity of the student's understanding of accelleration - that friction etc are supposed to be encompassed in the "force used by the stroller", and the student is supposed to demonstrate that they know accelleration, not motion itself, characterizes unbalanced forces.

Forgot to say: we in Minnesota have an issue with the standardized math exam intended to be a high school graduation requirement, in that too many kids are flunking it and will not graduate if the intention is enforced.

 

And when the newspaper printed some sample questions, most (an actual majority of those printed in the paper) of them read more or less like the OP here - the sharper the attention paid, the more confusing the question became.

 

This is getting to be a fairly serious problem, I think.