TPR In-class compendium physics question

[amalwa12]

Hi!

 

I'm wondering if anyone who has last year's set of TPR books can help me with Passage 1 of physics (page 2-3). Question 3. I don't know why the answer is D.

 

For anyone willing to help but who doesn't have the book, I'll try and explain the scenario, hopefully you'll understand:

 

So we have the usual block on a table top attached to a pulley from which another mass hangs. This is an experiment concerning static friction, etc.

 

The question specifically asks "as the block slides across the tabletop, the force exerted by the table on the block...is...". The way I thought about it, if there's a block on a table that is moved, it has to overcome the friction that is caused by the tabletop and the block touching. So I would say that the force exerted by the table on the block is parallel to the tabletop. But the answer is "neither perpendicular nor parallel to the tabletop"

 

Any help would be appreciated

[Grey's Anatomy]

I think the reason is that you can think of the table as exerting two forces on the block: normal force (perpendicular to the surface) and the friction force (parallel to the surface). The vector sum of these forces means that the net force exerted by the table on the block is neither perpendicular nor parallel to the tabletop. That's my reasoning. Please correct me if I'm wrong.

[amalwa12]

Thanks! I think that makes sense but I'd never have thought about it that way unless you mentioned it

[Leon]

I think the reason is that you can think of the table as exerting two forces on the block: normal force (perpendicular to the surface) and the friction force (parallel to the surface). The vector sum of these forces means that the net force exerted by the table on the block is neither perpendicular nor parallel to the tabletop. That's my reasoning. Please correct me if I'm wrong.

 

+1 though the question should really have said resultant force. Nevertheless, we have two forces there so the fact that the question uses the singular "force" suggests that they're asking you about the resultant.

[amalwa12]

Thanks to both of you for your help.

 

I have another question, if anyone could please give me some insight:

 

Why does lift force have no effect on the speed of a projectile? (lift force being the one perpendicular to drag and acting against the Y-component of velocity)

 

Thanks!!

[Grey's Anatomy]

Thanks to both of you for your help.

 

I have another question, if anyone could please give me some insight:

 

Why does lift force have no effect on the speed of a projectile? (lift force being the one perpendicular to drag and acting against the Y-component of velocity)

 

Thanks!!

 

Are you sure lift force doesn't affect the speed of a projectile? I would imagine it would have some sort of effect...but if I had to reason why it wouldn't, it would probably be because the force is perpendicular to the tangential direction of motion (almost like centripetal force where the force doesn't change the speed but does result in change in velocity due to change in direction).

[Femto]

Are you sure lift force doesn't affect the speed of a projectile? I would imagine it would have some sort of effect...but if I had to reason why it wouldn't' date=' it would probably be because the force is perpendicular to the tangential direction of motion[/b'] (almost like centripetal force where the force doesn't change the speed but does result in change in velocity due to change in direction).

 

This^

 

Any force perpendicular to the velocity direction doesn't change the speed. Notice the two italicized words. A force perpendicualr to velocity direction does change the velocity, but not the speed (because speed is a scalar, not a vector)

[Grey's Anatomy]

This^

 

Any force perpendicular to the velocity direction doesn't change the speed. Notice the two italicized words. A force perpendicualr to velocity direction does change the velocity, but not the speed (because speed is a scalar, not a vector)

 

My confusion is that for a projectile motion, the only time the velocity vector is purely horizontal is at the top of its flight, where velocity in the y-direction is zero. However, at other times in the flight, the projectile has velocity vectors both in the x- and y-directions (due to force of gravity) so if the lift force is acting against the force of gravity, I would think that the lift force should have some sort of impact on the speed.

[Grey's Anatomy]

To clarify, " Lift is the component of this force that is perpendicular to the oncoming flow direction. While the common meaning of the word "lift" assumes that lift opposes gravity, lift in its technical sense can be in any direction since it is defined with respect to the direction of flow rather than to the direction of gravity. When an aircraft is flying straight and level (cruise) most of the lift opposes gravity. However, when an aircraft is climbing, descending, or banking in a turn, for example, the lift is tilted with respect to the vertical." So given that the lift force is always perpendicular to the flow direction (direction of motion) it has no influence on the speed, but does change the direction and thus affects velocity.

 

http://en.wikipedia.org/wiki/Lift_(force)

[amalwa12]

Thanks Grey's and Femto. Yeah my biggest confusion was that I thought the Y-component of velocity would be affected. But what Femto said and the wiki link shows that the magnitude of velocity, i.e. speed, will not be affected because the force is acting perpendicular to the flow. However, the direction will be affected.

 

Thanks again! Some of these questions are really hard!!

[Femto]

To clarify' date=' " Lift is the component of this force that is perpendicular to the oncoming flow direction. [b']While the common meaning of the word "lift" assumes that lift opposes gravity, lift in its technical sense can be in any direction since it is defined with respect to the direction of flow rather than to the direction of gravity[/b]. When an aircraft is flying straight and level (cruise) most of the lift opposes gravity. However, when an aircraft is climbing, descending, or banking in a turn, for example, the lift is tilted with respect to the vertical." So given that the lift force is always perpendicular to the flow direction (direction of motion) it has no influence on the speed, but does change the direction and thus affects velocity.

 

http://en.wikipedia.org/wiki/Lift_(force)

 

The bolded part is why this seems so counter-intuitive. As with so much of the trickier MCAT stuff, I think the best idea is to somehow try and disregard common assumptions.

[amalwa12]

Yeah definitely !