Help with Physics!!!

[yawho]

Second part of the same question: Now the 0.5 kg book is falling off the table. Assuming there is no air resistance, name the reaction force to the weight force on the book in Newton's third law.

[rife168]

Second part of the same question: Now the 0.5 kg book is falling off the table. Assuming there is no air resistance, name the reaction force to the weight force on the book in Newton's third law.

edit: oops, I was incorrect, see queendaisy's response -.-

[queendaisy]

Another question: A 0.5 kg book resting on a horizontal table has a weight force acting on it. Name the reaction force to this weight force in Newton's third law.

Action reaction forces always act on different bodies. Therefore, the gravitational force of the earth on the book (I.e this weight force you mentioned) has a reaction pair that is the gravitational force of the book on the earth: the earth is pulled towards the book as well, but it's not noticeable cos the mass of the book is so small in comparison to the mass of the earth!
Normal reaction and weight are not action reaction pairs cos they both act on the book.

In response to your second q (with book falling) the reaction force is the same as the first question. There should always be a reaction force.. Otherwise newtons third law would not work!

[yawho]

What is your opinion on the inclined plane question a few posts ago?

[queendaisy]

What is your opinion on the inclined plane question a few posts ago?

I agree with what has been said, you can't get a numerical answer for the magnitude of the normal force without the angle (N=mgcosx) or the coefficient of static friction (you can determine the angle from there). The direction is perpendicular to the inclined plane. Sorry I couldn't help you further, but I am pretty sure there is no way of working it out without one of those two!

[yawho]

I'll ask my tutor she finished 12 last year and let you know in 10 min.

[yawho]

no response

[paulsterio]

I'll ask my tutor she finished 12 last year and let you know in 10 min.

Mind posting a scan of the question or a reference as to where the question could be found? Maybe you didn't pick up something crucial.

[yawho]

What is your opinion on the inclined plane question a few posts ago?

I agree with what has been said, you can't get a numerical answer for the magnitude of the normal force without the angle (N=mgcosx) or the coefficient of static friction (you can determine the angle from there). The direction is perpendicular to the inclined plane. Sorry I couldn't help you further, but I am pretty sure there is no way of working it out without one of those two!

My teacher said the reaction force of the plane on the object is 10 N vertically upward when the angle is less than 90 deg and the object is at rest.

[queendaisy]

What is your opinion on the inclined plane question a few posts ago?

I agree with what has been said, you can't get a numerical answer for the magnitude of the normal force without the angle (N=mgcosx) or the coefficient of static friction (you can determine the angle from there). The direction is perpendicular to the inclined plane. Sorry I couldn't help you further, but I am pretty sure there is no way of working it out without one of those two!

My teacher said the reaction force of the plane on the object is 10 N vertically upward when the angle is less than 90 deg and the object is at rest.

.. I guess that makes sense, but isn't that really only the vertical component of the normal force?

[Phy124]

.. I guess that makes sense, but isn't that really only the vertical component of the normal force?

I would have thought so. The actual normal reaction force is perpendicular to the plane as far as I know.

[queendaisy]

What is your opinion on the inclined plane question a few posts ago?

I agree with what has been said, you can't get a numerical answer for the magnitude of the normal force without the angle (N=mgcosx) or the coefficient of static friction (you can determine the angle from there). The direction is perpendicular to the inclined plane. Sorry I couldn't help you further, but I am pretty sure there is no way of working it out without one of those two!

My teacher said the reaction force of the plane on the object is 10 N vertically upward when the angle is less than 90 deg and the object is at rest.

.. I guess that makes sense, but isn't that really only the vertical component of the normal force?

Actually no, I don't think that works, because that doesn't take into account the friction, which there obviously must be if the object is at rest. If you are going to resolve the forces into vertical and horizontal (which would be must more time consuming than resolving along and perpendicular to plane btw) then there is a vertical component to the friction, so vertical comp. of normal cannot equal weight.

[paulsterio]

How is the reaction force 10N? Its mass is 1kg, giving it a weight of 10N (g = 10N/kg), if it is on an incline, the reaction force will be LESS than 10N. As the incline gets greater (i.e. higher angle) the reaction force will decrease. Think about it logically. As the slope gets higher, less force is pushing upward perpendicular to the surface. You can also look at it mathematically.



Look at your cos graph, it is greatest when theta = 0 and will gradually decrease to 0 when theta = 90 degrees.

[yawho]

How is the reaction force 10N? Its mass is 1kg, giving it a weight of 10N (g = 10N/kg), if it is on an incline, the reaction force will be LESS than 10N. As the incline gets greater (i.e. higher angle) the reaction force will decrease. Think about it logically. As the slope gets higher, less force is pushing upward perpendicular to the surface. You can also look at it mathematically.



Look at your cos graph, it is greatest when theta = 0 and will gradually decrease to 0 when theta = 90 degrees.

My teacher is smart. She is right 99.9...% time. May be you got the wrong idea.

[paulsterio]

Maybe she's got the wrong idea.

I can assure you, there's a misunderstanding somewhere along the lines. I guarantee you that the Normal Reaction Force is less than the Weight force.