From: David Fairman (thefairman@yahoo.com)
Date: Tue Nov 04 2003 - 16:40:39 PST
Message-ID: <20031105004039.42384.qmail@web11207.mail.yahoo.com> Date: Tue, 4 Nov 2003 16:40:39 -0800 (PST) From: David Fairman <thefairman@yahoo.com> Subject: Re: pinhole Re: forces/friction
Ron-
I loved your response to Jennie's questions, and it brought up a point that I've
always wondered about. Let's say that you are driving a car and turn off the engine
so that there is no forward force due to the engine, and the only force at work is
the force of friction which acts in the backwards direction.
Now... what if this "coasting" car hit a brick wall. Certainly we would describe a
force being applied to the brick wall by the car, yet we just said that there is no
force in the forward direction.
I imagine my confusion arises from a misunderstanding of the relationship between
force and momentum. Could someone help me out?
-Dave
--- Ronald Wong <ronwong@inreach.com> wrote:
> Recently Jennie said:
>
> >...i'd love some help.
> >...if i am driving a car at a constant velocity, and the engine
> > is exerting a force of 100N, then the force of friction balances
> > that out with 100N the other way.
>
> Right.
>
>
>
>
> > now, let's say, i suddenly stop the car mid-motion and turn
> > off the engine. the car will slow down and eventually
> > stop because of the force of friction, which is counteracting
> > the car's "desire" to keep moving because of its inertia, right?.
>
> Right.
>
>
>
>
> > But how do we describe the forces on the car during its slow-down?
> > My student asked me if the only force on the car were friction
> > in the opposite direction, why the car wouldn't move backwards.
>
> That was a good question on the part of the student. It brought out the
> fact that the student didn't understand the nature of friction.
>
> My approach to physics teaching was to determine what the students knew
> about a topic BEFORE I introduced it to them. In this way they could
> integrate what they already knew about nature with the things that they
> were about to learn. The material seemed to stick with them a lot longer
> than if I simply introduced them to the topics of kinematics, dynamics,
> etc. as they are so often found in physics textbooks.
>
> I would frequently do this by means of a simple demonstration or two.
>
> For instance: I would place a book on a table and ask the students, "What
> forces are acting on the book as it lies on the table?". Some would say
> none because it wasn't moving and I'd give them an initial nod of approval
> because, in one sense, they were absolutely right (a point we return to
> later in the presentation). Inevitably, someone would mention the force of
> gravity (if not, I'd do a second demonstration that quickly got their grey
> cells going - I'd drop something on the floor).
>
> Now that they had a force acting down on the object, the next question was:
> "Why doesn't it fall down?". They quickly drew the conclusion that the
> table was pushing up on the book (i.e. "Holding it up"). This lead to a
> series of activities where the students pushed one of their fingers down on
> their desks and noticed the effect that this had on the tip of their
> finger. They then pushed the finger of one hand against that of the other.
> This was followed by other activities which, with time and patience,
> revealed to them the properties of Newton's 3rd Law.
>
> When we took these observations and applied them to the book sitting on the
> table, the students suddenly realized that it sat there NOT because there
> was no forces acting on the book but because the forces acting on the book
> were balanced (Newton's 1st law had already been covered earlier through a
> process similar to the one they were presently engaged in so this served as
> a review in one sense). The nice part is that I didn't have to tell them
> that and all I had to do thereafter was to remind them about these
> discoveries whenever they forgot.
>
> The next question, of course, was, "How do we set this book in motion along
> the table?". Everybody knew that one. If they didn't give me a direction
> for the "unbalanced" force, I would initially push down on the book. After
> a little moaning and groaning on their part, they would tell me to direct
> the unbalanced force in the horizontal direction. Doing this gave me an
> opportunity to show that an "unbalanced" force changed the velocity of an
> object in the direction of the "unbalanced" force (i.e. accelerated the
> object in the same direction as the unbalanced force) - laying down the
> groundwork for the Newton's 2nd Law.
>
> At this point we get into friction because the next question I asked was,
> "What happened to the book after it left my hand?". Everybody knew the
> answer to that one too - it slowed down (it decelerated).
>
> They also knew the answer to the next question: "Why?" - FRICTION!. They
> even were able to tell me which way the frictional force must have acted to
> slow the book down.
>
> So, an "unbalanced" force acting in the direction of motion accelerates an
> object and later the unbalanced force of friction acting in the direction
> opposite to the direction of motion slows it down and stopped it
> (re-inforcing the initial ideas of Newton's 2nd Law introduced earlier).
>
> It was clear to them that friction opposes motion.
>
> It was opposing the motion even as I was accelerating the book with my
> pushing force.
>
> Thus there were two forces acting during the acceleration phase of the
> demonstration. The unbalanced force was NOT the "unbalanced" force
> mentioned in the earlier paragraphs - the pushing force - but the
> difference between the pushing force and the smaller (a conclusion the
> students make - not I), frictional force.
>
> Once the book left my hand I was no longer pushing on it and only the
> frictional force remained. The unbalanced force acting on the book was now
> only the frictional force and, since it was opposing the motion of the
> book, it caused the book's velocity to change until it was reduced to zero.
>
> At that point the book was at rest. It was once again simply lying on the
> table and we were back to square one where only the force of gravity and
> the table's reaction force were acting on the book. Just as before, there
> was no frictional force. All these points would be made by the students -
> not I.
>
> That's why the "..the car wouldn't move backwards". The frictional force
> brought it to rest and, once it came to rest, the frictional force ceased
> to exist. There was no longer any unbalanced forces acting on the car so it
> just sat there.
>
>
>
>
>
> > is there still the force of the car engine acting in the forward
> > direction even when the car shuts off?
>
> One of the questions I asked during the demonstration was "Why does the
> book keep moving after I was no longer pushing it? Why didn't it just stop?"
>
> Earlier in the course, the class had considered
> Galileo's speculations involving a ball rolling
> down one inclined plane and up another in which
> the latter became less and less inclined. A series
> of demonstrations followed that confirmed the
> conclusions that they had arrived at earlier after
> reflecting on Galileo's thoughts on the matter
> and allowed me to put them in a frame of mind
> where they could see the connection between
> uniform velocity and balanced forces. At that
> point I could bring up a new concept - Newton's
> 1st Law (the Law of Inertia).
>
> Their answer to the question was immediate - INERTIA! There is NO force
> acting in the forward direction. Only the tendency of a body to maintain
> it's state of motion ("'desire' to keep moving" as you said) is responsible
> for the car's continued movement in the forward direction. That's the law.
>
>
>
>
>
>
>
> > (and even so, if the forces were balanced before, it seems the
> > net force would still be in the direction of the friction force)?
>
> When the driving force produced by the engine was balanced by the
> frictional force, the net force was zero. That's why the car moved with
> uniform velocity down the road - neither speeding up, or down, or changing
> directions. Once you turned off the driving force, only the frictional
> force was left to act on the car. Now we have a net force and, since it is
> the frictional force, the net force acts in the direction of the frictional
> force. Since this is opposite to the direction of motion, the car slows
> down.
>
>
>
>
>
>
>
> > is it the ground exerting a forward force on the car?
>
> The ground exerts an upward force on the car perpendicular to the car's
> motion so it can't contribute to a horizontal force in either direction. It
> does play a role in determining the size of the frictional force and thus
> the rate at which the car slows down.
>
> The important thing to remember is that there is NO forward force on the
> car "...when the car shuts off".
>
> Now Aristotle would vehemently argue otherwise
> - so you are in very good company when you
> expressed the idea that there should be a force
> in the forward direction to account for the
> continued motion in that direction. In fact,
> earlier in my physics course my students would
> also have agreed with you. They had been shown
> examples where motion could only be sustained
> by the application of a force - something that
> they were very familiar with. But this was
> before we considered Galileo's thought
> experiments, created a few of our own, and
> looked at a system where very little friction
> was present (an air track) - all of this part
> of a prelude to Newton's 1st Law.
>
>
>
>
> > i guess a similar example would be rolling a ball and letting go...
> > what other force is at work as the ball is being slowed down by
> > friction in the opposite direction?
>
> None. Friction is the only force acting on the ball in the horizontal
> direction. As a result, it is an unbalanced force and, because it opposes
>
=== message truncated ===
__________________________________
Do you Yahoo!?
Protect your identity with Yahoo! Mail AddressGuard
http://antispam.yahoo.com/whatsnewfree
This archive was generated by hypermail 2.1.3 : Mon Aug 02 2004 - 12:05:32 PDT