Resultant force on object

[monkeysay]

Hello everyone. :)

I encountered this question during my revision and can't remember how I got it correct.

"A force of 3.0N and a force of 4.0N act on an object at the same time. Which of the forces cannot be the resultant force on the object?"

There are 4 options (0.5N, 3.5N, 4.0N, 5.0N)

I chose 4.0N but the correct answer is 0.5N.


Can someone explain why my answer is wrong and why 0.5N is correct? Thanks in advance.

 

[johnj7]

Hello,
By resultant force you mean net force, right?

In that case, think of forces as vectors. Draw it out, pretend like the forces act on a box. Then think of triangles. If you know 2 sides of a triangle, what length can the other side be? What inequalities apply to the length of the unknown side of the triangle?

 

[RoryP]

do the forces act on the object horizontally or vertically or one of each? if its one of each you could construct a right angled triangle and use pythagoras' a^2 + b^2 = c^2

 

[monkeysay]

Hello John, thanks for the reply.

I still don't understand why 0.5N cannot be a possible net force. Can't the resultant arrow be small if you have the opposite angle small as well?

I don't understand how the other force of 3.0N can act to achieve 4.0N only. Where would it act upon?


I hope you won't find it too hard to explain, I am pretty bad at physics I admit. Thanks again!

 

[monkeysay]

Hello Rory, for the question the forces can act anywhere.

Thus I don't think Pythagoras theorem can be used since the vectors need not form a right angle.

 

[RoryP]

oh right, hmmm in that case I am not totally sure.sorry for the lack of help!

 

[Doc Al]

Hint: For the two forces to add up to the maximum value, how would they have to point with respect to each other? What about to get the minimum value? (Once you have max and min, the actual value can be anything in between, depending on the angle between the vectors.)

 

[monkeysay]

Thanks Doc Al.

I have gotten some pretty good replies here:
http://answers.yahoo.com/question/i...NDZLLjnsy6IX;_ylv=3?qid=20080901002011AAhD7oU


What I didn't realize was that the minimum value was when the two forces are parallel and in opposite direction to each other.



I guess this 'mystery' (for me at least) is solved now. Thanks again guys!

 

[Defennder]

I wouldn't trust Yahoo Answers. I have seen a lot of satirical and nonsensical answers get voted as the "Best".

 

[monkeysay]

Well I take answers there with a pinch of salt. Some of them frequent there to tout their products or give nonsensical and irresponsible comments.

But if you look at the discussion I linked to, I thought the replies I got were pretty decent.

And if you think there are no best answers, as an answerer, you have the choice to select No Best Answer.


But this isn't really the place to discuss the validity of Yahoo Answers is it? =p