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 05-27-2012, 12:12 AM #1 Senior Member     Join Date: May 2010 Location: Canada Posts: 569 Question regarding Mechanics SDN Members don't see this ad. (About Ads) This is a discrete and not from a passage. ----- A woman is driving a car along a road when she realizes, almost too late, that she needs to make a left hand turn. She quickly turns the wheel, and the books which were in the passenger seat go crashing against the passenger door. Consider the following statements: I. The books were pushed against the door by a centrifugal force. II.The books were pushed against the door by a centripetal force. III. The forces acting on the books while they are crashing against the door are gravity, the normal force, and a force toward the right. Which is (are) true? A. I and III are true. B. I1 and IIIare true. C. 111only is true. D. None is true. D is correct. Is it because only 'I' applies and 'II & III' are wrong? The books have a centrifugal force because they want to keep moving in a straight line. Or does the car door have a centrifugal force which makes it looks like the books have a force on the car? Thanks, Lunasly.
 05-27-2012, 12:52 AM #2 New Member   Status Pre-Medical Join Date: Apr 2012 Posts: 4 Centrifugal forces don't exist. The book wants to keep moving in a straight line, but the door accelerates centripetally and pushes on the book, keeping it in the car. II is wrong because no force moves the book against the door - the door moves to get in the way of the book. III is wrong because the centripetal force acts towards the radius, which is to the left.
 05-27-2012, 09:02 AM #3 zzzz     Status: Pre-Medical Join Date: Jul 2011 Location: chillville Posts: 2,454 To add to what the above poster said; think of newtons law. The law of inertia; an object in motion tends to stay in motion unless acted upon by an outside force. The books are traveling in a straight path and will continue their straight path but the car is experiencing a force causing the car door to collide with the books that are still traveling forward.
 05-27-2012, 01:46 PM #4 Senior Member     Join Date: May 2010 Location: Canada Posts: 569 Okay so in essence all choices are wrong except #1. The centrifugal force which is the tendency for the books to want to keep moving in a straight line is there, but the centripetal force from the door (or the road?) is providing a force which moves towards the centre and thus the books smash into the door. So I is correct and II & III are wrong?
05-27-2012, 04:15 PM   #5
zzzz

Status: Pre-Medical
Join Date: Jul 2011
Location: chillville
Posts: 2,454

Quote:
 Originally Posted by Lunasly Okay so in essence all choices are wrong except #1. The centrifugal force which is the tendency for the books to want to keep moving in a straight line is there, but the centripetal force from the door (or the road?) is providing a force which moves towards the centre and thus the books smash into the door. So I is correct and II & III are wrong?
you're a little confused. all of them are wrong; there is no centrifugal force in this case the books are moving in a straight line and they want to keep moving in that straight line according to newtons laws. the car is experience a force though, so it collides with the books that still want to move straight.

 05-27-2012, 06:23 PM #6 Senior Member     Join Date: May 2010 Location: Canada Posts: 569 Ok, let me see if I got this: I understand that that there is a centripetal force on the car that is exerted by the friction of the road on the tires (thus allowing you to turn). Therefore, because the books want to continue moving along their tangental path (straight line), the door smashes into the books. The definition of a centrifugal force (which is not a real force) is the tendency for an object to continue in a straight line. If this is the definition of centrifugal force, then it complies with newtons first law which states that an object will stay in motion unless acted upon by a force. In this case, the centripetal force pointing perpendicular to the tangental force is causing the collision between the books (which want to move in a straight path) and the car door. ---- EDIT: I think I understand where you are coming from. What I wrote above is correct (or at least I think its correct), but #1 doesn't make sense because like you said A) centrifugal force is not a real force and B) the books don't push towards the door, its the door that pushes towards the books. Am I correct?

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